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Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, with the Front-Cover Texts being ``GNAT Reference Manual'', and with no Back-Cover Texts.  A copy of the license is included in the section entitled ``GNU Free Documentation License''.

     [file gcc-common.texi not included]

GNAT Reference Manual
*********************

GNAT Reference Manual

GNAT, The GNU Ada Compiler

GCC version version-GCC

AdaCore

About This Guide
****************

This manual contains useful information in writing programs using the GNAT compiler.  It includes information on implementation dependent characteristics of GNAT, including all the information required by Annex M of the Ada language standard.

GNAT implements Ada 95 and Ada 2005, and it may also be invoked in Ada 83 compatibility mode. By default, GNAT assumes Ada 2005, but you can override with a compiler switch to explicitly specify the language version. (Please refer to Compiling Different Versions of Ada (GNAT User's Guide), for details on these switches.) Throughout this manual, references to ``Ada'' without a year suffix apply to both the Ada 95 and Ada 2005 versions of the language.

Ada is designed to be highly portable. In general, a program will have the same effect even when compiled by different compilers on different platforms. However, since Ada is designed to be used in a wide variety of applications, it also contains a number of system dependent features to be used in interfacing to the external world.

Note: Any program that makes use of implementation-dependent features may be non-portable.  You should follow good programming practice and isolate and clearly document any sections of your program that make use of these features in a non-portable manner.

What This Reference Manual Contains
===================================

This reference manual contains the following chapters:

  * Implementation Defined Pragmas, lists GNAT implementation-dependent pragmas, which can be used to extend and enhance the functionality of the compiler.

  * Implementation Defined Attributes, lists GNAT implementation-dependent attributes which can be used to extend and enhance the functionality of the compiler.

  * Implementation Advice, provides information on generally desirable behavior which are not requirements that all compilers must follow since it cannot be provided on all systems, or which may be undesirable on some systems.

  * Implementation Defined Characteristics, provides a guide to minimizing implementation dependent features.

  * Intrinsic Subprograms, describes the intrinsic subprograms implemented by GNAT, and how they can be imported into user application programs.

  * Representation Clauses and Pragmas, describes in detail the way that GNAT represents data, and in particular the exact set of representation clauses and pragmas that is accepted.

  * Standard Library Routines, provides a listing of packages and a brief description of the functionality that is provided by Ada's extensive set of standard library routines as implemented by GNAT.

  * The Implementation of Standard I/O, details how the GNAT implementation of the input-output facilities.

  * The GNAT Library, is a catalog of packages that complement the Ada predefined library.

  * Interfacing to Other Languages, describes how programs written in Ada using GNAT can be interfaced to other programming languages.

     Specialized Needs Annexes, describes the GNAT implementation of all of the specialized needs annexes.

  * Implementation of Specific Ada Features, discusses issues related to GNAT's implementation of machine code insertions, tasking, and several other features.

  * Implementation of Ada 2012 Features, describes the status of the GNAT implementation of the Ada 2012 language standard.

  * Obsolescent Features documents implementation dependent features, including pragmas and attributes, which are considered obsolescent, since there are other preferred ways of achieving the same results. These obsolescent forms are retained for backwards compatibility.

This reference manual assumes a basic familiarity with the Ada 95 language, as described in the International Standard ANSI/ISO/IEC-8652:1995, January 1995. It does not require knowledge of the new features introduced by Ada 2005, (officially known as ISO/IEC 8652:1995 with Technical Corrigendum 1 and Amendment 1). Both reference manuals are included in the GNAT documentation package.

Conventions
===========

Following are examples of the typographical and graphic conventions used in this guide:

  * Functions, utility program names, standard names, and classes.

  * Option flags

  * File names, button names, and field names.

  * Variables, environment variables, and METASYNTACTIC VARIABLES.

  * Emphasis.

  * [optional information or parameters]

  * Examples are described by text

          and then shown this way.

Commands that are entered by the user are preceded in this manual by the characters $  (dollar sign followed by space).  If your system uses this sequence as a prompt, then the commands will appear exactly as you see them in the manual.  If your system uses some other prompt, then the command will appear with the $ replaced by whatever prompt character you are using.

Related Information
===================

See the following documents for further information on GNAT:

  * See About This Guide (GNAT User's Guide), which provides information on how to use the GNAT compiler system.

  * Ada 95 Reference Manual, which contains all reference material for the Ada 95 programming language.

  * Ada 95 Annotated Reference Manual, which is an annotated version of the Ada 95 standard.  The annotations describe detailed aspects of the design decision, and in particular contain useful sections on Ada 83 compatibility.

  * Ada 2005 Reference Manual, which contains all reference material for the Ada 2005 programming language.

  * Ada 2005 Annotated Reference Manual, which is an annotated version of the Ada 2005 standard.  The annotations describe detailed aspects of the design decision, and in particular contain useful sections on Ada 83 and Ada 95 compatibility.

  * DEC Ada, Technical Overview and Comparison on DIGITAL Platforms, which contains specific information on compatibility between GNAT and DEC Ada 83 systems.

  * DEC Ada, Language Reference Manual, part number AA-PYZAB-TK which describes in detail the pragmas and attributes provided by the DEC Ada 83 compiler system.

Implementation Defined Pragmas
******************************

Ada defines a set of pragmas that can be used to supply additional information to the compiler.  These language defined pragmas are implemented in GNAT and work as described in the Ada Reference Manual.

In addition, Ada allows implementations to define additional pragmas whose meaning is defined by the implementation.  GNAT provides a number of these implementation-defined pragmas, which can be used to extend and enhance the functionality of the compiler.  This section of the GNAT Reference Manual describes these additional pragmas.

Note that any program using these pragmas might not be portable to other compilers (although GNAT implements this set of pragmas on all platforms).  Therefore if portability to other compilers is an important consideration, the use of these pragmas should be minimized.

Pragma Abort_Defer
==================

Syntax:

     pragma Abort_Defer;

This pragma must appear at the start of the statement sequence of a handled sequence of statements (right after the begin).  It has the effect of deferring aborts for the sequence of statements (but not for the declarations or handlers, if any, associated with this statement sequence).

Pragma Ada_83
=============

Syntax:

     pragma Ada_83;

A configuration pragma that establishes Ada 83 mode for the unit to which it applies, regardless of the mode set by the command line switches.  In Ada 83 mode, GNAT attempts to be as compatible with the syntax and semantics of Ada 83, as defined in the original Ada 83 Reference Manual as possible.  In particular, the keywords added by Ada 95 and Ada 2005 are not recognized, optional package bodies are allowed, and generics may name types with unknown discriminants without using the (<>) notation.  In addition, some but not all of the additional restrictions of Ada 83 are enforced.

Ada 83 mode is intended for two purposes.  Firstly, it allows existing Ada 83 code to be compiled and adapted to GNAT with less effort. Secondly, it aids in keeping code backwards compatible with Ada 83. However, there is no guarantee that code that is processed correctly by GNAT in Ada 83 mode will in fact compile and execute with an Ada 83 compiler, since GNAT does not enforce all the additional checks required by Ada 83.

Pragma Ada_95
=============

Syntax:

     pragma Ada_95;

A configuration pragma that establishes Ada 95 mode for the unit to which it applies, regardless of the mode set by the command line switches. This mode is set automatically for the Ada and System packages and their children, so you need not specify it in these contexts.  This pragma is useful when writing a reusable component that itself uses Ada 95 features, but which is intended to be usable from either Ada 83 or Ada 95 programs.

Pragma Ada_05
=============

Syntax:

     pragma Ada_05;

A configuration pragma that establishes Ada 2005 mode for the unit to which it applies, regardless of the mode set by the command line switches. This pragma is useful when writing a reusable component that itself uses Ada 2005 features, but which is intended to be usable from either Ada 83 or Ada 95 programs.

Pragma Ada_2005
===============

Syntax:

     pragma Ada_2005;

This configuration pragma is a synonym for pragma Ada_05 and has the same syntax and effect.

Pragma Ada_12
=============

Syntax:

     pragma Ada_12;

A configuration pragma that establishes Ada 2012 mode for the unit to which it applies, regardless of the mode set by the command line switches. This mode is set automatically for the Ada and System packages and their children, so you need not specify it in these contexts.  This pragma is useful when writing a reusable component that itself uses Ada 2012 features, but which is intended to be usable from Ada 83, Ada 95, or Ada 2005 programs.

Pragma Ada_2012
===============

Syntax:

     pragma Ada_2012;

This configuration pragma is a synonym for pragma Ada_12 and has the same syntax and effect.

Pragma Annotate
===============

Syntax:

     pragma Annotate (IDENTIFIER [,IDENTIFIER {, ARG}]);

     ARG ::= NAME | EXPRESSION

This pragma is used to annotate programs.  IDENTIFIER identifies the type of annotation.  GNAT verifies that it is an identifier, but does not otherwise analyze it. The second optional identifier is also left unanalyzed, and by convention is used to control the action of the tool to which the annotation is addressed.  The remaining ARG arguments can be either string literals or more generally expressions. String literals are assumed to be either of type Standard.String or else Wide_String or Wide_Wide_String depending on the character literals they contain. All other kinds of arguments are analyzed as expressions, and must be unambiguous.

The analyzed pragma is retained in the tree, but not otherwise processed by any part of the GNAT compiler, except to generate corresponding note lines in the generated ALI file. For the format of these note lines, see the compiler source file lib-writ.ads. This pragma is intended for use by external tools, including ASIS. The use of pragma Annotate does not affect the compilation process in any way. This pragma may be used as a configuration pragma.

Pragma Assert
=============

Syntax:

     pragma Assert (
       boolean_EXPRESSION
       [, string_EXPRESSION]);

The effect of this pragma depends on whether the corresponding command line switch is set to activate assertions.  The pragma expands into code equivalent to the following:

     if assertions-enabled then
        if not boolean_EXPRESSION then
           System.Assertions.Raise_Assert_Failure
             (string_EXPRESSION);
        end if;
     end if;

The string argument, if given, is the message that will be associated with the exception occurrence if the exception is raised.  If no second argument is given, the default message is FILE:NNN, where FILE is the name of the source file containing the assert, and NNN is the line number of the assert.  A pragma is not a statement, so if a statement sequence contains nothing but a pragma assert, then a null statement is required in addition, as in:

     ...
     if J > 3 then
        pragma Assert (K > 3, "Bad value for K");
        null;
     end if;

Note that, as with the if statement to which it is equivalent, the type of the expression is either Standard.Boolean, or any type derived from this standard type.

If assertions are disabled (switch -gnata not used), then there is no run-time effect (and in particular, any side effects from the expression will not occur at run time).  (The expression is still analyzed at compile time, and may cause types to be frozen if they are mentioned here for the first time).

If assertions are enabled, then the given expression is tested, and if it is False then System.Assertions.Raise_Assert_Failure is called which results in the raising of Assert_Failure with the given message.

You should generally avoid side effects in the expression arguments of this pragma, because these side effects will turn on and off with the setting of the assertions mode, resulting in assertions that have an effect on the program.  However, the expressions are analyzed for semantic correctness whether or not assertions are enabled, so turning assertions on and off cannot affect the legality of a program.

Note that the implementation defined policy DISABLE, given in a pragma Assertion_Policy, can be used to suppress this semantic analysis.

Note: this is a standard language-defined pragma in versions of Ada from 2005 on. In GNAT, it is implemented in all versions of Ada, and the DISABLE policy is an implementation-defined addition.

Pragma Assertion_Policy
=======================

Syntax:

     pragma Assertion_Policy (CHECK | DISABLE | IGNORE);

If the argument is CHECK, then pragma Assert is enabled. If the argument is IGNORE, then pragma Assert is ignored. This pragma overrides the effect of the -gnata switch on the command line.

The implementation defined policy DISABLE is like IGNORE except that it completely disables semantic checking of the argument to pragma Assert. This may be useful when the pragma argument references subprograms in a with'ed package which is replaced by a dummy package for the final build.

Note: this is a standard language-defined pragma in versions of Ada from 2005 on. In GNAT, it is implemented in all versions of Ada, and the DISABLE policy is an implementation-defined addition.

Pragma Assume_No_Invalid_Values
===============================

Syntax:

     pragma Assume_No_Invalid_Values (On | Off);

This is a configuration pragma that controls the assumptions made by the compiler about the occurrence of invalid representations (invalid values) in the code.

The default behavior (corresponding to an Off argument for this pragma), is to assume that values may in general be invalid unless the compiler can prove they are valid. Consider the following example:

     V1 : Integer range 1 .. 10;
     V2 : Integer range 11 .. 20;
     ...
     for J in V2 .. V1 loop
        ...
     end loop;

if V1 and V2 have valid values, then the loop is known at compile time not to execute since the lower bound must be greater than the upper bound. However in default mode, no such assumption is made, and the loop may execute. If Assume_No_Invalid_Values (On) is given, the compiler will assume that any occurrence of a variable other than in an explicit 'Valid test always has a valid value, and the loop above will be optimized away.

The use of Assume_No_Invalid_Values (On) is appropriate if you know your code is free of uninitialized variables and other possible sources of invalid representations, and may result in more efficient code. A program that accesses an invalid representation with this pragma in effect is erroneous, so no guarantees can be made about its behavior.

It is peculiar though permissible to use this pragma in conjunction with validity checking (-gnatVa). In such cases, accessing invalid values will generally give an exception, though formally the program is erroneous so there are no guarantees that this will always be the case, and it is recommended that these two options not be used together.

Pragma Ast_Entry
================

Syntax:

     pragma AST_Entry (entry_IDENTIFIER);

This pragma is implemented only in the OpenVMS implementation of GNAT.  The argument is the simple name of a single entry; at most one AST_Entry pragma is allowed for any given entry.  This pragma must be used in conjunction with the AST_Entry attribute, and is only allowed after the entry declaration and in the same task type specification or single task as the entry to which it applies.  This pragma specifies that the given entry may be used to handle an OpenVMS asynchronous system trap (AST) resulting from an OpenVMS system service call.  The pragma does not affect normal use of the entry.  For further details on this pragma, see the DEC Ada Language Reference Manual, section 9.12a.

Pragma C_Pass_By_Copy
=====================

Syntax:

     pragma C_Pass_By_Copy
       ([Max_Size =>] static_integer_EXPRESSION);

Normally the default mechanism for passing C convention records to C convention subprograms is to pass them by reference, as suggested by RM B.3(69).  Use the configuration pragma C_Pass_By_Copy to change this default, by requiring that record formal parameters be passed by copy if all of the following conditions are met:

  * The size of the record type does not exceed the value specified for Max_Size.

  * The record type has Convention C.

  * The formal parameter has this record type, and the subprogram has a foreign (non-Ada) convention.

If these conditions are met the argument is passed by copy, i.e. in a manner consistent with what C expects if the corresponding formal in the C prototype is a struct (rather than a pointer to a struct).

You can also pass records by copy by specifying the convention C_Pass_By_Copy for the record type, or by using the extended Import and Export pragmas, which allow specification of passing mechanisms on a parameter by parameter basis.

Pragma Check
============

Syntax:

     pragma Check (
          [Name    =>] Identifier,
          [Check   =>] Boolean_EXPRESSION
       [, [Message =>] string_EXPRESSION] );

This pragma is similar to the predefined pragma Assert except that an extra identifier argument is present. In conjunction with pragma Check_Policy, this can be used to define groups of assertions that can be independently controlled. The identifier Assertion is special, it refers to the normal set of pragma Assert statements. The identifiers Precondition and Postcondition correspond to the pragmas of these names, so these three names would normally not be used directly in a pragma Check.

Checks introduced by this pragma are normally deactivated by default. They can be activated either by the command line option -gnata, which turns on all checks, or individually controlled using pragma Check_Policy.

Pragma Check_Name
=================

Syntax:

     pragma Check_Name (check_name_IDENTIFIER);

This is a configuration pragma that defines a new implementation defined check name (unless IDENTIFIER matches one of the predefined check names, in which case the pragma has no effect). Check names are global to a partition, so if two or more configuration pragmas are present in a partition mentioning the same name, only one new check name is introduced.

An implementation defined check name introduced with this pragma may be used in only three contexts: pragma Suppress, pragma Unsuppress, and as the prefix of a Check_Name'Enabled attribute reference. For any of these three cases, the check name must be visible. A check name is visible if it is in the configuration pragmas applying to the current unit, or if it appears at the start of any unit that is part of the dependency set of the current unit (e.g., units that are mentioned in with clauses).

Pragma Check_Policy
===================

Syntax:

     pragma Check_Policy
      ([Name   =>] Identifier,
       [Policy =>] POLICY_IDENTIFIER);

     POLICY_IDENTIFIER ::= ON | OFF | CHECK | DISABLE | IGNORE

This pragma is similar to the predefined pragma Assertion_Policy, except that it controls sets of named assertions introduced using the Check pragmas. It can be used as a configuration pragma or (unlike Assertion_Policy) can be used within a declarative part, in which case it controls the status to the end of the corresponding construct (in a manner identical to pragma Suppress).

The identifier given as the first argument corresponds to a name used in associated Check pragmas. For example, if the pragma:

     pragma Check_Policy (Critical_Error, OFF);

is given, then subsequent Check pragmas whose first argument is also Critical_Error will be disabled. The special identifier Assertion controls the behavior of normal Assert pragmas (thus a pragma Check_Policy with this identifier is similar to the normal Assertion_Policy pragma except that it can appear within a declarative part).

The special identifiers Precondition and Postcondition control the status of preconditions and postconditions. If a Precondition pragma is encountered, it is ignored if turned off by a Check_Policy specifying that Precondition checks are Off or Ignored. Similarly use of the name Postcondition controls whether Postcondition pragmas are recognized.

The check policy is OFF to turn off corresponding checks, and ON to turn on corresponding checks. The default for a set of checks for which no Check_Policy is given is OFF unless the compiler switch -gnata is given, which turns on all checks by default.

The check policy settings CHECK and IGNORE are also recognized as synonyms for ON and OFF. These synonyms are provided for compatibility with the standard Assertion_Policy pragma.

The implementation defined policy DISABLE is like OFF except that it completely disables semantic checking of the argument to the corresponding class of pragmas. This may be useful when the pragma arguments reference subprograms in a with'ed package which is replaced by a dummy package for the final build.

Pragma Comment
==============

Syntax:

     pragma Comment (static_string_EXPRESSION);

This is almost identical in effect to pragma Ident.  It allows the placement of a comment into the object file and hence into the executable file if the operating system permits such usage.  The difference is that Comment, unlike Ident, has no limitations on placement of the pragma (it can be placed anywhere in the main source unit), and if more than one pragma is used, all comments are retained.

Pragma Common_Object
====================

Syntax:

     pragma Common_Object (
          [Internal =>] LOCAL_NAME
       [, [External =>] EXTERNAL_SYMBOL]
       [, [Size     =>] EXTERNAL_SYMBOL] );

     EXTERNAL_SYMBOL ::=
       IDENTIFIER
     | static_string_EXPRESSION

This pragma enables the shared use of variables stored in overlaid linker areas corresponding to the use of COMMON in Fortran.  The single object LOCAL_NAME is assigned to the area designated by the EXTERNAL argument. You may define a record to correspond to a series of fields.  The SIZE argument is syntax checked in GNAT, but otherwise ignored.

Common_Object is not supported on all platforms.  If no support is available, then the code generator will issue a message indicating that the necessary attribute for implementation of this pragma is not available.

Pragma Compile_Time_Error
=========================

Syntax:

     pragma Compile_Time_Error
              (boolean_EXPRESSION, static_string_EXPRESSION);

This pragma can be used to generate additional compile time error messages. It is particularly useful in generics, where errors can be issued for specific problematic instantiations. The first parameter is a boolean expression. The pragma is effective only if the value of this expression is known at compile time, and has the value True. The set of expressions whose values are known at compile time includes all static boolean expressions, and also other values which the compiler can determine at compile time (e.g., the size of a record type set by an explicit size representation clause, or the value of a variable which was initialized to a constant and is known not to have been modified). If these conditions are met, an error message is generated using the value given as the second argument. This string value may contain embedded ASCII.LF characters to break the message into multiple lines.

Pragma Compile_Time_Warning
===========================

Syntax:

     pragma Compile_Time_Warning
              (boolean_EXPRESSION, static_string_EXPRESSION);

Same as pragma Compile_Time_Error, except a warning is issued instead of an error message. Note that if this pragma is used in a package that is with'ed by a client, the client will get the warning even though it is issued by a with'ed package (normally warnings in with'ed units are suppressed, but this is a special exception to that rule).

One typical use is within a generic where compile time known characteristics of formal parameters are tested, and warnings given appropriately. Another use with a first parameter of True is to warn a client about use of a package, for example that it is not fully implemented.

Pragma Compiler_Unit
====================

Syntax:

     pragma Compiler_Unit;

This pragma is intended only for internal use in the GNAT run-time library. It indicates that the unit is used as part of the compiler build. The effect is to disallow constructs (raise with message, conditional expressions etc) that would cause trouble when bootstrapping using an older version of GNAT. For the exact list of restrictions, see the compiler sources and references to Is_Compiler_Unit.

Pragma Complete_Representation
==============================

Syntax:

     pragma Complete_Representation;

This pragma must appear immediately within a record representation clause. Typical placements are before the first component clause or after the last component clause. The effect is to give an error message if any component is missing a component clause. This pragma may be used to ensure that a record representation clause is complete, and that this invariant is maintained if fields are added to the record in the future.

Pragma Complex_Representation
=============================

Syntax:

     pragma Complex_Representation
             ([Entity =>] LOCAL_NAME);

The ENTITY argument must be the name of a record type which has two fields of the same floating-point type.  The effect of this pragma is to force gcc to use the special internal complex representation form for this record, which may be more efficient.  Note that this may result in the code for this type not conforming to standard ABI (application binary interface) requirements for the handling of record types.  For example, in some environments, there is a requirement for passing records by pointer, and the use of this pragma may result in passing this type in floating-point registers.

Pragma Component_Alignment
==========================

Syntax:

     pragma Component_Alignment (
          [Form =>] ALIGNMENT_CHOICE
       [, [Name =>] type_LOCAL_NAME]);

     ALIGNMENT_CHOICE ::=
       Component_Size
     | Component_Size_4
     | Storage_Unit
     | Default

Specifies the alignment of components in array or record types. The meaning of the FORM argument is as follows:

Component_Size
     Aligns scalar components and subcomponents of the array or record type on boundaries appropriate to their inherent size (naturally aligned).  For example, 1-byte components are aligned on byte boundaries, 2-byte integer components are aligned on 2-byte boundaries, 4-byte integer components are aligned on 4-byte boundaries and so on.  These alignment rules correspond to the normal rules for C compilers on all machines except the VAX.

Component_Size_4
     Naturally aligns components with a size of four or fewer bytes.  Components that are larger than 4 bytes are placed on the next 4-byte boundary.

Storage_Unit
     Specifies that array or record components are byte aligned, i.e. aligned on boundaries determined by the value of the constant System.Storage_Unit.

Default
     Specifies that array or record components are aligned on default boundaries, appropriate to the underlying hardware or operating system or both.  For OpenVMS VAX systems, the Default choice is the same as the Storage_Unit choice (byte alignment).  For all other systems, the Default choice is the same as Component_Size (natural alignment).

If the Name parameter is present, TYPE_LOCAL_NAME must refer to a local record or array type, and the specified alignment choice applies to the specified type.  The use of Component_Alignment together with a pragma Pack causes the Component_Alignment pragma to be ignored.  The use of Component_Alignment together with a record representation clause is only effective for fields not specified by the representation clause.

If the Name parameter is absent, the pragma can be used as either a configuration pragma, in which case it applies to one or more units in accordance with the normal rules for configuration pragmas, or it can be used within a declarative part, in which case it applies to types that are declared within this declarative part, or within any nested scope within this declarative part.  In either case it specifies the alignment to be applied to any record or array type which has otherwise standard representation.

If the alignment for a record or array type is not specified (using pragma Pack, pragma Component_Alignment, or a record rep clause), the GNAT uses the default alignment as described previously.

Pragma Convention_Identifier
============================

Syntax:

     pragma Convention_Identifier (
              [Name =>]       IDENTIFIER,
              [Convention =>] convention_IDENTIFIER);

This pragma provides a mechanism for supplying synonyms for existing convention identifiers. The Name identifier can subsequently be used as a synonym for the given convention in other pragmas (including for example pragma Import or another Convention_Identifier pragma). As an example of the use of this, suppose you had legacy code which used Fortran77 as the identifier for Fortran. Then the pragma:

     pragma Convention_Identifier (Fortran77, Fortran);

would allow the use of the convention identifier Fortran77 in subsequent code, avoiding the need to modify the sources. As another example, you could use this to parameterize convention requirements according to systems. Suppose you needed to use Stdcall on windows systems, and C on some other system, then you could define a convention identifier Library and use a single Convention_Identifier pragma to specify which convention would be used system-wide.

Pragma CPP_Class
================

Syntax:

     pragma CPP_Class ([Entity =>] LOCAL_NAME);

The argument denotes an entity in the current declarative region that is declared as a record type. It indicates that the type corresponds to an externally declared C++ class type, and is to be laid out the same way that C++ would lay out the type. If the C++ class has virtual primitives then the record must be declared as a tagged record type.

Types for which CPP_Class is specified do not have assignment or equality operators defined (such operations can be imported or declared as subprograms as required). Initialization is allowed only by constructor functions (see pragma CPP_Constructor). Such types are implicitly limited if not explicitly declared as limited or derived from a limited type, and an error is issued in that case.

Pragma CPP_Class is intended primarily for automatic generation using an automatic binding generator tool. See Interfacing to C++ for related information.

Note: Pragma CPP_Class is currently obsolete. It is supported for backward compatibility but its functionality is available using pragma Import with Convention = CPP.

Pragma CPP_Constructor
======================

Syntax:

     pragma CPP_Constructor ([Entity =>] LOCAL_NAME
       [, [External_Name =>] static_string_EXPRESSION ]
       [, [Link_Name     =>] static_string_EXPRESSION ]);

This pragma identifies an imported function (imported in the usual way with pragma Import) as corresponding to a C++ constructor. If External_Name and Link_Name are not specified then the Entity argument is a name that must have been previously mentioned in a pragma Import with Convention = CPP. Such name must be of one of the following forms:

  * function FNAME return T

       * function FNAME return T'Class

       * function FNAME (...) return T

  * function FNAME (...) return T'Class

where T is a limited record type imported from C++ with pragma Import and Convention = CPP.

The first two forms import the default constructor, used when an object of type T is created on the Ada side with no explicit constructor. The latter two forms cover all the non-default constructors of the type. See the GNAT users guide for details.

If no constructors are imported, it is impossible to create any objects on the Ada side and the type is implicitly declared abstract.

Pragma CPP_Constructor is intended primarily for automatic generation using an automatic binding generator tool. See Interfacing to C++ for more related information.

Note: The use of functions returning class-wide types for constructors is currently obsolete. They are supported for backward compatibility. The use of functions returning the type T leave the Ada sources more clear because the imported C++ constructors always return an object of type T; that is, they never return an object whose type is a descendant of type T.

Pragma CPP_Virtual
==================

This pragma is now obsolete has has no effect because GNAT generates the same object layout than the G++ compiler.

See Interfacing to C++ for related information.

Pragma CPP_Vtable
=================

This pragma is now obsolete has has no effect because GNAT generates the same object layout than the G++ compiler.

See Interfacing to C++ for related information.

Pragma Debug
============

Syntax:

     pragma Debug ([CONDITION, ]PROCEDURE_CALL_WITHOUT_SEMICOLON);

     PROCEDURE_CALL_WITHOUT_SEMICOLON ::=
       PROCEDURE_NAME
     | PROCEDURE_PREFIX ACTUAL_PARAMETER_PART

The procedure call argument has the syntactic form of an expression, meeting the syntactic requirements for pragmas.

If debug pragmas are not enabled or if the condition is present and evaluates to False, this pragma has no effect. If debug pragmas are enabled, the semantics of the pragma is exactly equivalent to the procedure call statement corresponding to the argument with a terminating semicolon. Pragmas are permitted in sequences of declarations, so you can use pragma Debug to intersperse calls to debug procedures in the middle of declarations. Debug pragmas can be enabled either by use of the command line switch -gnata or by use of the configuration pragma Debug_Policy.

Pragma Debug_Policy
===================

Syntax:

     pragma Debug_Policy (CHECK | DISABLE | IGNORE);

If the argument is CHECK, then pragma DEBUG is enabled. If the argument is IGNORE, then pragma DEBUG is ignored. This pragma overrides the effect of the -gnata switch on the command line.

The implementation defined policy DISABLE is like IGNORE except that it completely disables semantic checking of the argument to pragma Debug. This may be useful when the pragma argument references subprograms in a with'ed package which is replaced by a dummy package for the final build.

Pragma Detect_Blocking
======================

Syntax:

     pragma Detect_Blocking;

This is a configuration pragma that forces the detection of potentially blocking operations within a protected operation, and to raise Program_Error if that happens.

Pragma Elaboration_Checks
=========================

Syntax:

     pragma Elaboration_Checks (Dynamic | Static);

This is a configuration pragma that provides control over the elaboration model used by the compilation affected by the pragma.  If the parameter is Dynamic, then the dynamic elaboration model described in the Ada Reference Manual is used, as though the -gnatE switch had been specified on the command line.  If the parameter is Static, then the default GNAT static model is used.  This configuration pragma overrides the setting of the command line.  For full details on the elaboration models used by the GNAT compiler, see Elaboration Order Handling in GNAT (GNAT User's Guide).

Pragma Eliminate
================

Syntax:

     pragma Eliminate ([Entity          =>] DEFINING_DESIGNATOR,
                       [Source_Location =>] STRING_LITERAL);

The string literal given for the source location is a string which specifies the line number of the occurrence of the entity, using the syntax for SOURCE_TRACE given below:

      SOURCE_TRACE     ::= SOURCE_REFERENCE [LBRACKET SOURCE_TRACE RBRACKET]

      LBRACKET         ::= [
      RBRACKET         ::= ]

      SOURCE_REFERENCE ::= FILE_NAME : LINE_NUMBER

      LINE_NUMBER      ::= DIGIT {DIGIT}

Spaces around the colon in a Source_Reference are optional.

The DEFINING_DESIGNATOR matches the defining designator used in an explicit subprogram declaration, where the entity name in this designator appears on the source line specified by the source location.

The source trace that is given as the Source_Location shall obey the following rules. The FILE_NAME is the short name (with no directory information) of an Ada source file, given using exactly the required syntax for the underlying file system (e.g. case is important if the underlying operating system is case sensitive). LINE_NUMBER gives the line number of the occurrence of the entity as a decimal literal without an exponent or point. If an entity is not declared in a generic instantiation (this includes generic subprogram instances), the source trace includes only one source reference. If an entity is declared inside a generic instantiation, its source trace (when parsing from left to right) starts with the source location of the declaration of the entity in the generic unit and ends with the source location of the instantiation (it is given in square brackets). This approach is recursively used in case of nested instantiations: the rightmost (nested most deeply in square brackets) element of the source trace is the location of the outermost instantiation, the next to left element is the location of the next (first nested) instantiation in the code of the corresponding generic unit, and so on, and the leftmost element (that is out of any square brackets) is the location of the declaration of the entity to eliminate in a generic unit.

Note that the Source_Location argument specifies which of a set of similarly named entities is being eliminated, dealing both with overloading, and also appearence of the same entity name in different scopes.

This pragma indicates that the given entity is not used in the program to be compiled and built. The effect of the pragma is to allow the compiler to eliminate the code or data associated with the named entity. Any reference to an eliminated entity causes a compile-time or link-time error.

The intention of pragma Eliminate is to allow a program to be compiled in a system-independent manner, with unused entities eliminated, without needing to modify the source text. Normally the required set of Eliminate pragmas is constructed automatically using the gnatelim tool.

Any source file change that removes, splits, or adds lines may make the set of Eliminate pragmas invalid because their Source_Location argument values may get out of date.

Pragma Eliminate may be used where the referenced entity is a dispatching operation. In this case all the subprograms to which the given operation can dispatch are considered to be unused (are never called as a result of a direct or a dispatching call).

Pragma Export_Exception
=======================

Syntax:

     pragma Export_Exception (
          [Internal =>] LOCAL_NAME
       [, [External =>] EXTERNAL_SYMBOL]
       [, [Form     =>] Ada | VMS]
       [, [Code     =>] static_integer_EXPRESSION]);

     EXTERNAL_SYMBOL ::=
       IDENTIFIER
     | static_string_EXPRESSION

This pragma is implemented only in the OpenVMS implementation of GNAT.  It causes the specified exception to be propagated outside of the Ada program, so that it can be handled by programs written in other OpenVMS languages. This pragma establishes an external name for an Ada exception and makes the name available to the OpenVMS Linker as a global symbol.  For further details on this pragma, see the DEC Ada Language Reference Manual, section 13.9a3.2.

Pragma Export_Function
======================

Syntax:

     pragma Export_Function (
          [Internal         =>] LOCAL_NAME
       [, [External         =>] EXTERNAL_SYMBOL]
       [, [Parameter_Types  =>] PARAMETER_TYPES]
       [, [Result_Type      =>] result_SUBTYPE_MARK]
       [, [Mechanism        =>] MECHANISM]
       [, [Result_Mechanism =>] MECHANISM_NAME]);

     EXTERNAL_SYMBOL ::=
       IDENTIFIER
     | static_string_EXPRESSION
     | ""

     PARAMETER_TYPES ::=
       null
     | TYPE_DESIGNATOR {, TYPE_DESIGNATOR}

     TYPE_DESIGNATOR ::=
       subtype_NAME
     | subtype_Name ' Access

     MECHANISM ::=
       MECHANISM_NAME
     | (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})

     MECHANISM_ASSOCIATION ::=
       [formal_parameter_NAME =>] MECHANISM_NAME

     MECHANISM_NAME ::=
       Value
     | Reference
     | Descriptor [([Class =>] CLASS_NAME)]
     | Short_Descriptor [([Class =>] CLASS_NAME)]

     CLASS_NAME ::= ubs | ubsb | uba | s | sb | a

Use this pragma to make a function externally callable and optionally provide information on mechanisms to be used for passing parameter and result values.  We recommend, for the purposes of improving portability, this pragma always be used in conjunction with a separate pragma Export, which must precede the pragma Export_Function. GNAT does not require a separate pragma Export, but if none is present, Convention Ada is assumed, which is usually not what is wanted, so it is usually appropriate to use this pragma in conjunction with a Export or Convention pragma that specifies the desired foreign convention. Pragma Export_Function (and Export, if present) must appear in the same declarative region as the function to which they apply.

INTERNAL_NAME must uniquely designate the function to which the pragma applies.  If more than one function name exists of this name in the declarative part you must use the Parameter_Types and Result_Type parameters is mandatory to achieve the required unique designation.  SUBTYPE_MARKs in these parameters must exactly match the subtypes in the corresponding function specification, using positional notation to match parameters with subtype marks. The form with an 'Access attribute can be used to match an anonymous access parameter.

Passing by descriptor is supported only on the OpenVMS ports of GNAT. The default behavior for Export_Function is to accept either 64bit or 32bit descriptors unless short_descriptor is specified, then only 32bit descriptors are accepted.

Special treatment is given if the EXTERNAL is an explicit null string or a static string expressions that evaluates to the null string. In this case, no external name is generated. This form still allows the specification of parameter mechanisms.

Pragma Export_Object
====================

Syntax:

     pragma Export_Object
           [Internal =>] LOCAL_NAME
        [, [External =>] EXTERNAL_SYMBOL]
        [, [Size     =>] EXTERNAL_SYMBOL]

     EXTERNAL_SYMBOL ::=
       IDENTIFIER
     | static_string_EXPRESSION

This pragma designates an object as exported, and apart from the extended rules for external symbols, is identical in effect to the use of the normal Export pragma applied to an object.  You may use a separate Export pragma (and you probably should from the point of view of portability), but it is not required.  SIZE is syntax checked, but otherwise ignored by GNAT.

Pragma Export_Procedure
=======================

Syntax:

     pragma Export_Procedure (
          [Internal        =>] LOCAL_NAME
       [, [External        =>] EXTERNAL_SYMBOL]
       [, [Parameter_Types =>] PARAMETER_TYPES]
       [, [Mechanism       =>] MECHANISM]);

     EXTERNAL_SYMBOL ::=
       IDENTIFIER
     | static_string_EXPRESSION
     | ""

     PARAMETER_TYPES ::=
       null
     | TYPE_DESIGNATOR {, TYPE_DESIGNATOR}

     TYPE_DESIGNATOR ::=
       subtype_NAME
     | subtype_Name ' Access

     MECHANISM ::=
       MECHANISM_NAME
     | (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})

     MECHANISM_ASSOCIATION ::=
       [formal_parameter_NAME =>] MECHANISM_NAME

     MECHANISM_NAME ::=
       Value
     | Reference
     | Descriptor [([Class =>] CLASS_NAME)]
     | Short_Descriptor [([Class =>] CLASS_NAME)]

     CLASS_NAME ::= ubs | ubsb | uba | s | sb | a

This pragma is identical to Export_Function except that it applies to a procedure rather than a function and the parameters Result_Type and Result_Mechanism are not permitted. GNAT does not require a separate pragma Export, but if none is present, Convention Ada is assumed, which is usually not what is wanted, so it is usually appropriate to use this pragma in conjunction with a Export or Convention pragma that specifies the desired foreign convention.

Passing by descriptor is supported only on the OpenVMS ports of GNAT. The default behavior for Export_Procedure is to accept either 64bit or 32bit descriptors unless short_descriptor is specified, then only 32bit descriptors are accepted.

Special treatment is given if the EXTERNAL is an explicit null string or a static string expressions that evaluates to the null string. In this case, no external name is generated. This form still allows the specification of parameter mechanisms.

Pragma Export_Value
===================

Syntax:

     pragma Export_Value (
       [Value     =>] static_integer_EXPRESSION,
       [Link_Name =>] static_string_EXPRESSION);

This pragma serves to export a static integer value for external use. The first argument specifies the value to be exported. The Link_Name argument specifies the symbolic name to be associated with the integer value. This pragma is useful for defining a named static value in Ada that can be referenced in assembly language units to be linked with the application. This pragma is currently supported only for the AAMP target and is ignored for other targets.

Pragma Export_Valued_Procedure
==============================

Syntax:

     pragma Export_Valued_Procedure (
          [Internal        =>] LOCAL_NAME
       [, [External        =>] EXTERNAL_SYMBOL]
       [, [Parameter_Types =>] PARAMETER_TYPES]
       [, [Mechanism       =>] MECHANISM]);

     EXTERNAL_SYMBOL ::=
       IDENTIFIER
     | static_string_EXPRESSION
     | ""

     PARAMETER_TYPES ::=
       null
     | TYPE_DESIGNATOR {, TYPE_DESIGNATOR}

     TYPE_DESIGNATOR ::=
       subtype_NAME
     | subtype_Name ' Access

     MECHANISM ::=
       MECHANISM_NAME
     | (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})

     MECHANISM_ASSOCIATION ::=
       [formal_parameter_NAME =>] MECHANISM_NAME

     MECHANISM_NAME ::=
       Value
     | Reference
     | Descriptor [([Class =>] CLASS_NAME)]
     | Short_Descriptor [([Class =>] CLASS_NAME)]

     CLASS_NAME ::= ubs | ubsb | uba | s | sb | a

This pragma is identical to Export_Procedure except that the first parameter of LOCAL_NAME, which must be present, must be of mode OUT, and externally the subprogram is treated as a function with this parameter as the result of the function.  GNAT provides for this capability to allow the use of OUT and IN OUT parameters in interfacing to external functions (which are not permitted in Ada functions). GNAT does not require a separate pragma Export, but if none is present, Convention Ada is assumed, which is almost certainly not what is wanted since the whole point of this pragma is to interface with foreign language functions, so it is usually appropriate to use this pragma in conjunction with a Export or Convention pragma that specifies the desired foreign convention.

Passing by descriptor is supported only on the OpenVMS ports of GNAT. The default behavior for Export_Valued_Procedure is to accept either 64bit or 32bit descriptors unless short_descriptor is specified, then only 32bit descriptors are accepted.

Special treatment is given if the EXTERNAL is an explicit null string or a static string expressions that evaluates to the null string. In this case, no external name is generated. This form still allows the specification of parameter mechanisms.

Pragma Extend_System
====================

Syntax:

     pragma Extend_System ([Name =>] IDENTIFIER);

This pragma is used to provide backwards compatibility with other implementations that extend the facilities of package System.  In GNAT, System contains only the definitions that are present in the Ada RM.  However, other implementations, notably the DEC Ada 83 implementation, provide many extensions to package System.

For each such implementation accommodated by this pragma, GNAT provides a package Aux_XXX, e.g. Aux_DEC for the DEC Ada 83 implementation, which provides the required additional definitions.  You can use this package in two ways.  You can with it in the normal way and access entities either by selection or using a use clause.  In this case no special processing is required.

However, if existing code contains references such as System.XXX where XXX is an entity in the extended definitions provided in package System, you may use this pragma to extend visibility in System in a non-standard way that provides greater compatibility with the existing code.  Pragma Extend_System is a configuration pragma whose single argument is the name of the package containing the extended definition (e.g. Aux_DEC for the DEC Ada case).  A unit compiled under control of this pragma will be processed using special visibility processing that looks in package System.Aux_XXX where Aux_XXX is the pragma argument for any entity referenced in package System, but not found in package System.

You can use this pragma either to access a predefined System extension supplied with the compiler, for example Aux_DEC or you can construct your own extension unit following the above definition.  Note that such a package is a child of System and thus is considered part of the implementation.  To compile it you will have to use the appropriate switch for compiling system units. See About This Guide (GNAT User's Guide), for details.

Pragma Extensions_Allowed
=========================

Syntax:

     pragma Extensions_Allowed (On | Off);

This configuration pragma enables or disables the implementation extension mode (the use of Off as a parameter cancels the effect of the -gnatX command switch).

In extension mode, the latest version of the Ada language is implemented (currently Ada 2012), and in addition a small number of GNAT specific extensions are recognized as follows:

Constrained attribute for generic objects
     The Constrained attribute is permitted for objects of generic types. The result indicates if the corresponding actual is constrained.

Pragma External
===============

Syntax:

     pragma External (
       [   Convention    =>] convention_IDENTIFIER,
       [   Entity        =>] LOCAL_NAME
       [, [External_Name =>] static_string_EXPRESSION ]
       [, [Link_Name     =>] static_string_EXPRESSION ]);

This pragma is identical in syntax and semantics to pragma Export as defined in the Ada Reference Manual.  It is provided for compatibility with some Ada 83 compilers that used this pragma for exactly the same purposes as pragma Export before the latter was standardized.

Pragma External_Name_Casing
===========================

Syntax:

     pragma External_Name_Casing (
       Uppercase | Lowercase
       [, Uppercase | Lowercase | As_Is]);

This pragma provides control over the casing of external names associated with Import and Export pragmas.  There are two cases to consider:

Implicit external names
     Implicit external names are derived from identifiers.  The most common case arises when a standard Ada Import or Export pragma is used with only two arguments, as in:

             pragma Import (C, C_Routine);

     Since Ada is a case-insensitive language, the spelling of the identifier in the Ada source program does not provide any information on the desired casing of the external name, and so a convention is needed.  In GNAT the default treatment is that such names are converted to all lower case letters.  This corresponds to the normal C style in many environments. The first argument of pragma External_Name_Casing can be used to control this treatment.  If Uppercase is specified, then the name will be forced to all uppercase letters.  If Lowercase is specified, then the normal default of all lower case letters will be used.

     This same implicit treatment is also used in the case of extended DEC Ada 83 compatible Import and Export pragmas where an external name is explicitly specified using an identifier rather than a string.

Explicit external names
     Explicit external names are given as string literals.  The most common case arises when a standard Ada Import or Export pragma is used with three arguments, as in:

          pragma Import (C, C_Routine, "C_routine");

     In this case, the string literal normally provides the exact casing required for the external name.  The second argument of pragma External_Name_Casing may be used to modify this behavior. If Uppercase is specified, then the name will be forced to all uppercase letters.  If Lowercase is specified, then the name will be forced to all lowercase letters.  A specification of As_Is provides the normal default behavior in which the casing is taken from the string provided.

This pragma may appear anywhere that a pragma is valid.  In particular, it can be used as a configuration pragma in the gnat.adc file, in which case it applies to all subsequent compilations, or it can be used as a program unit pragma, in which case it only applies to the current unit, or it can be used more locally to control individual Import/Export pragmas.

It is primarily intended for use with OpenVMS systems, where many compilers convert all symbols to upper case by default.  For interfacing to such compilers (e.g. the DEC C compiler), it may be convenient to use the pragma:

     pragma External_Name_Casing (Uppercase, Uppercase);

to enforce the upper casing of all external symbols.

Pragma Fast_Math
================

Syntax:

     pragma Fast_Math;

This is a configuration pragma which activates a mode in which speed is considered more important for floating-point operations than absolutely accurate adherence to the requirements of the standard. Currently the following operations are affected:

Complex Multiplication
     The normal simple formula for complex multiplication can result in intermediate overflows for numbers near the end of the range. The Ada standard requires that this situation be detected and corrected by scaling, but in Fast_Math mode such cases will simply result in overflow. Note that to take advantage of this you must instantiate your own version of Ada.Numerics.Generic_Complex_Types under control of the pragma, rather than use the preinstantiated versions.

Pragma Favor_Top_Level
======================

Syntax:

     pragma Favor_Top_Level (type_NAME);

The named type must be an access-to-subprogram type. This pragma is an efficiency hint to the compiler, regarding the use of 'Access or 'Unrestricted_Access on nested (non-library-level) subprograms. The pragma means that nested subprograms are not used with this type, or are rare, so that the generated code should be efficient in the top-level case. When this pragma is used, dynamically generated trampolines may be used on some targets for nested subprograms. See also the No_Implicit_Dynamic_Code restriction.

Pragma Finalize_Storage_Only
============================

Syntax:

     pragma Finalize_Storage_Only (first_subtype_LOCAL_NAME);

This pragma allows the compiler not to emit a Finalize call for objects defined at the library level.  This is mostly useful for types where finalization is only used to deal with storage reclamation since in most environments it is not necessary to reclaim memory just before terminating execution, hence the name.

Pragma Float_Representation
===========================

Syntax:

     pragma Float_Representation (FLOAT_REP[, float_type_LOCAL_NAME]);

     FLOAT_REP ::= VAX_Float | IEEE_Float

In the one argument form, this pragma is a configuration pragma which allows control over the internal representation chosen for the predefined floating point types declared in the packages Standard and System. On all systems other than OpenVMS, the argument must be IEEE_Float and the pragma has no effect. On OpenVMS, the argument may be VAX_Float to specify the use of the VAX float format for the floating-point types in Standard. This requires that the standard runtime libraries be recompiled.

The two argument form specifies the representation to be used for the specified floating-point type. On all systems other than OpenVMS, the argument must be IEEE_Float and the pragma has no effect. On OpenVMS, the argument may be VAX_Float to specify the use of the VAX float format, as follows:

  * For digits values up to 6, F float format will be used.

  * For digits values from 7 to 9, D float format will be used.

  * For digits values from 10 to 15, G float format will be used.

  * Digits values above 15 are not allowed.

Pragma Ident
============

Syntax:

     pragma Ident (static_string_EXPRESSION);

This pragma provides a string identification in the generated object file, if the system supports the concept of this kind of identification string. This pragma is allowed only in the outermost declarative part or declarative items of a compilation unit. If more than one Ident pragma is given, only the last one processed is effective. On OpenVMS systems, the effect of the pragma is identical to the effect of the DEC Ada 83 pragma of the same name. Note that in DEC Ada 83, the maximum allowed length is 31 characters, so if it is important to maintain compatibility with this compiler, you should obey this length limit.

Pragma Implemented
==================

Syntax:

     pragma Implemented (procedure_LOCAL_NAME, implementation_kind);

     implementation_kind ::= By_Entry | By_Protected_Procedure | By_Any

This is an Ada 2012 representation pragma which applies to protected, task and synchronized interface primitives. The use of pragma Implemented provides a way to impose a static requirement on the overriding operation by adhering to one of the three implementation kids: entry, protected procedure or any of the above.

     type Synch_Iface is synchronized interface;
     procedure Prim_Op (Obj : in out Iface) is abstract;
     pragma Implemented (Prim_Op, By_Protected_Procedure);

     protected type Prot_1 is new Synch_Iface with
        procedure Prim_Op;  --  Legal
     end Prot_1;

     protected type Prot_2 is new Synch_Iface with
        entry Prim_Op;      --  Illegal
     end Prot_2;

     task type Task_Typ is new Synch_Iface with
        entry Prim_Op;      --  Illegal
     end Task_Typ;

When applied to the procedure_or_entry_NAME of a requeue statement, pragma Implemented determines the runtime behavior of the requeue. Implementation kind By_Entry guarantees that the action of requeueing will proceed from an entry to another entry. Implementation kind By_Protected_Procedure transforms the requeue into a dispatching call, thus eliminating the chance of blocking. Kind By_Any shares the behavior of By_Entry and By_Protected_Procedure depending on the target's overriding subprogram kind.

Pragma Implicit_Packing
=======================

Syntax:

     pragma Implicit_Packing;

This is a configuration pragma that requests implicit packing for packed arrays for which a size clause is given but no explicit pragma Pack or specification of Component_Size is present. It also applies to records where no record representation clause is present. Consider this example:

     type R is array (0 .. 7) of Boolean;
     for R'Size use 8;

In accordance with the recommendation in the RM (RM 13.3(53)), a Size clause does not change the layout of a composite object. So the Size clause in the above example is normally rejected, since the default layout of the array uses 8-bit components, and thus the array requires a minimum of 64 bits.

If this declaration is compiled in a region of code covered by an occurrence of the configuration pragma Implicit_Packing, then the Size clause in this and similar examples will cause implicit packing and thus be accepted. For this implicit packing to occur, the type in question must be an array of small components whose size is known at compile time, and the Size clause must specify the exact size that corresponds to the length of the array multiplied by the size in bits of the component type.

Similarly, the following example shows the use in the record case

     type r is record
        a, b, c, d, e, f, g, h : boolean;
        chr                    : character;
     end record;
     for r'size use 16;

Without a pragma Pack, each Boolean field requires 8 bits, so the minimum size is 72 bits, but with a pragma Pack, 16 bits would be sufficient. The use of pragma Implicit_Packing allows this record declaration to compile without an explicit pragma Pack.

Pragma Import_Exception
=======================

Syntax:

     pragma Import_Exception (
          [Internal =>] LOCAL_NAME
       [, [External =>] EXTERNAL_SYMBOL]
       [, [Form     =>] Ada | VMS]
       [, [Code     =>] static_integer_EXPRESSION]);

     EXTERNAL_SYMBOL ::=
       IDENTIFIER
     | static_string_EXPRESSION

This pragma is implemented only in the OpenVMS implementation of GNAT. It allows OpenVMS conditions (for example, from OpenVMS system services or other OpenVMS languages) to be propagated to Ada programs as Ada exceptions. The pragma specifies that the exception associated with an exception declaration in an Ada program be defined externally (in non-Ada code). For further details on this pragma, see the DEC Ada Language Reference Manual, section 13.9a.3.1.

Pragma Import_Function
======================

Syntax:

     pragma Import_Function (
          [Internal                 =>] LOCAL_NAME,
       [, [External                 =>] EXTERNAL_SYMBOL]
       [, [Parameter_Types          =>] PARAMETER_TYPES]
       [, [Result_Type              =>] SUBTYPE_MARK]
       [, [Mechanism                =>] MECHANISM]
       [, [Result_Mechanism         =>] MECHANISM_NAME]
       [, [First_Optional_Parameter =>] IDENTIFIER]);

     EXTERNAL_SYMBOL ::=
       IDENTIFIER
     | static_string_EXPRESSION

     PARAMETER_TYPES ::=
       null
     | TYPE_DESIGNATOR {, TYPE_DESIGNATOR}

     TYPE_DESIGNATOR ::=
       subtype_NAME
     | subtype_Name ' Access

     MECHANISM ::=
       MECHANISM_NAME
     | (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})

     MECHANISM_ASSOCIATION ::=
       [formal_parameter_NAME =>] MECHANISM_NAME

     MECHANISM_NAME ::=
       Value
     | Reference
     | Descriptor [([Class =>] CLASS_NAME)]
     | Short_Descriptor [([Class =>] CLASS_NAME)]

     CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca

This pragma is used in conjunction with a pragma Import to specify additional information for an imported function.  The pragma Import (or equivalent pragma Interface) must precede the Import_Function pragma and both must appear in the same declarative part as the function specification.

The INTERNAL argument must uniquely designate the function to which the pragma applies.  If more than one function name exists of this name in the declarative part you must use the Parameter_Types and RESULT_TYPE parameters to achieve the required unique designation.  Subtype marks in these parameters must exactly match the subtypes in the corresponding function specification, using positional notation to match parameters with subtype marks. The form with an 'Access attribute can be used to match an anonymous access parameter.

You may optionally use the MECHANISM and RESULT_MECHANISM parameters to specify passing mechanisms for the parameters and result.  If you specify a single mechanism name, it applies to all parameters.  Otherwise you may specify a mechanism on a parameter by parameter basis using either positional or named notation.  If the mechanism is not specified, the default mechanism is used.

Passing by descriptor is supported only on the OpenVMS ports of GNAT. The default behavior for Import_Function is to pass a 64bit descriptor unless short_descriptor is specified, then a 32bit descriptor is passed.

First_Optional_Parameter applies only to OpenVMS ports of GNAT. It specifies that the designated parameter and all following parameters are optional, meaning that they are not passed at the generated code level (this is distinct from the notion of optional parameters in Ada where the parameters are passed anyway with the designated optional parameters).  All optional parameters must be of mode IN and have default parameter values that are either known at compile time expressions, or uses of the 'Null_Parameter attribute.

Pragma Import_Object
====================

Syntax:

     pragma Import_Object
          [Internal =>] LOCAL_NAME
       [, [External =>] EXTERNAL_SYMBOL]
       [, [Size     =>] EXTERNAL_SYMBOL]);

     EXTERNAL_SYMBOL ::=
       IDENTIFIER
     | static_string_EXPRESSION

This pragma designates an object as imported, and apart from the extended rules for external symbols, is identical in effect to the use of the normal Import pragma applied to an object.  Unlike the subprogram case, you need not use a separate Import pragma, although you may do so (and probably should do so from a portability point of view).  SIZE is syntax checked, but otherwise ignored by GNAT.

Pragma Import_Procedure
=======================

Syntax:

     pragma Import_Procedure (
          [Internal                 =>] LOCAL_NAME
       [, [External                 =>] EXTERNAL_SYMBOL]
       [, [Parameter_Types          =>] PARAMETER_TYPES]
       [, [Mechanism                =>] MECHANISM]
       [, [First_Optional_Parameter =>] IDENTIFIER]);

     EXTERNAL_SYMBOL ::=
       IDENTIFIER
     | static_string_EXPRESSION

     PARAMETER_TYPES ::=
       null
     | TYPE_DESIGNATOR {, TYPE_DESIGNATOR}

     TYPE_DESIGNATOR ::=
       subtype_NAME
     | subtype_Name ' Access

     MECHANISM ::=
       MECHANISM_NAME
     | (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})

     MECHANISM_ASSOCIATION ::=
       [formal_parameter_NAME =>] MECHANISM_NAME

     MECHANISM_NAME ::=
       Value
     | Reference
     | Descriptor [([Class =>] CLASS_NAME)]
     | Short_Descriptor [([Class =>] CLASS_NAME)]

     CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca

This pragma is identical to Import_Function except that it applies to a procedure rather than a function and the parameters Result_Type and Result_Mechanism are not permitted.

Pragma Import_Valued_Procedure
==============================

Syntax:

     pragma Import_Valued_Procedure (
          [Internal                 =>] LOCAL_NAME
       [, [External                 =>] EXTERNAL_SYMBOL]
       [, [Parameter_Types          =>] PARAMETER_TYPES]
       [, [Mechanism                =>] MECHANISM]
       [, [First_Optional_Parameter =>] IDENTIFIER]);

     EXTERNAL_SYMBOL ::=
       IDENTIFIER
     | static_string_EXPRESSION

     PARAMETER_TYPES ::=
       null
     | TYPE_DESIGNATOR {, TYPE_DESIGNATOR}

     TYPE_DESIGNATOR ::=
       subtype_NAME
     | subtype_Name ' Access

     MECHANISM ::=
       MECHANISM_NAME
     | (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})

     MECHANISM_ASSOCIATION ::=
       [formal_parameter_NAME =>] MECHANISM_NAME

     MECHANISM_NAME ::=
       Value
     | Reference
     | Descriptor [([Class =>] CLASS_NAME)]
     | Short_Descriptor [([Class =>] CLASS_NAME)]

     CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca

This pragma is identical to Import_Procedure except that the first parameter of LOCAL_NAME, which must be present, must be of mode OUT, and externally the subprogram is treated as a function with this parameter as the result of the function.  The purpose of this capability is to allow the use of OUT and IN OUT parameters in interfacing to external functions (which are not permitted in Ada functions).  You may optionally use the Mechanism parameters to specify passing mechanisms for the parameters. If you specify a single mechanism name, it applies to all parameters. Otherwise you may specify a mechanism on a parameter by parameter basis using either positional or named notation.  If the mechanism is not specified, the default mechanism is used.

Note that it is important to use this pragma in conjunction with a separate pragma Import that specifies the desired convention, since otherwise the default convention is Ada, which is almost certainly not what is required.

Pragma Initialize_Scalars
=========================

Syntax:

     pragma Initialize_Scalars;

This pragma is similar to Normalize_Scalars conceptually but has two important differences.  First, there is no requirement for the pragma to be used uniformly in all units of a partition, in particular, it is fine to use this just for some or all of the application units of a partition, without needing to recompile the run-time library.

In the case where some units are compiled with the pragma, and some without, then a declaration of a variable where the type is defined in package Standard or is locally declared will always be subject to initialization, as will any declaration of a scalar variable.  For composite variables, whether the variable is initialized may also depend on whether the package in which the type of the variable is declared is compiled with the pragma.

The other important difference is that you can control the value used for initializing scalar objects.  At bind time, you can select several options for initialization. You can initialize with invalid values (similar to Normalize_Scalars, though for Initialize_Scalars it is not always possible to determine the invalid values in complex cases like signed component fields with non-standard sizes). You can also initialize with high or low values, or with a specified bit pattern.  See the users guide for binder options for specifying these cases.

This means that you can compile a program, and then without having to recompile the program, you can run it with different values being used for initializing otherwise uninitialized values, to test if your program behavior depends on the choice.  Of course the behavior should not change, and if it does, then most likely you have an erroneous reference to an uninitialized value.

It is even possible to change the value at execution time eliminating even the need to rebind with a different switch using an environment variable. See the GNAT users guide for details.

Note that pragma Initialize_Scalars is particularly useful in conjunction with the enhanced validity checking that is now provided in GNAT, which checks for invalid values under more conditions. Using this feature (see description of the -gnatV flag in the users guide) in conjunction with pragma Initialize_Scalars provides a powerful new tool to assist in the detection of problems caused by uninitialized variables.

Note: the use of Initialize_Scalars has a fairly extensive effect on the generated code. This may cause your code to be substantially larger. It may also cause an increase in the amount of stack required, so it is probably a good idea to turn on stack checking (see description of stack checking in the GNAT users guide) when using this pragma.

Pragma Inline_Always
====================

Syntax:

     pragma Inline_Always (NAME [, NAME]);

Similar to pragma Inline except that inlining is not subject to the use of option -gnatn and the inlining happens regardless of whether this option is used.

Pragma Inline_Generic
=====================

Syntax:

     pragma Inline_Generic (generic_package_NAME);

This is implemented for compatibility with DEC Ada 83 and is recognized, but otherwise ignored, by GNAT.  All generic instantiations are inlined by default when using GNAT.

Pragma Interface
================

Syntax:

     pragma Interface (
          [Convention    =>] convention_identifier,
          [Entity        =>] local_NAME
       [, [External_Name =>] static_string_expression]
       [, [Link_Name     =>] static_string_expression]);

This pragma is identical in syntax and semantics to the standard Ada pragma Import.  It is provided for compatibility with Ada 83.  The definition is upwards compatible both with pragma Interface as defined in the Ada 83 Reference Manual, and also with some extended implementations of this pragma in certain Ada 83 implementations.  The only difference between pragma Interface and pragma Import is that there is special circuitry to allow both pragmas to appear for the same subprogram entity (normally it is illegal to have multiple Import pragmas. This is useful in maintaining Ada 83/Ada 95 compatibility and is compatible with other Ada 83 compilers.

Pragma Interface_Name
=====================

Syntax:

     pragma Interface_Name (
          [Entity        =>] LOCAL_NAME
       [, [External_Name =>] static_string_EXPRESSION]
       [, [Link_Name     =>] static_string_EXPRESSION]);

This pragma provides an alternative way of specifying the interface name for an interfaced subprogram, and is provided for compatibility with Ada 83 compilers that use the pragma for this purpose.  You must provide at least one of EXTERNAL_NAME or LINK_NAME.

Pragma Interrupt_Handler
========================

Syntax:

     pragma Interrupt_Handler (procedure_LOCAL_NAME);

This program unit pragma is supported for parameterless protected procedures as described in Annex C of the Ada Reference Manual. On the AAMP target the pragma can also be specified for nonprotected parameterless procedures that are declared at the library level (which includes procedures declared at the top level of a library package). In the case of AAMP, when this pragma is applied to a nonprotected procedure, the instruction IERET is generated for returns from the procedure, enabling maskable interrupts, in place of the normal return instruction.

Pragma Interrupt_State
======================

Syntax:

     pragma Interrupt_State
      ([Name  =>] value,
       [State =>] SYSTEM | RUNTIME | USER);

Normally certain interrupts are reserved to the implementation.  Any attempt to attach an interrupt causes Program_Error to be raised, as described in RM C.3.2(22).  A typical example is the SIGINT interrupt used in many systems for an Ctrl-C interrupt.  Normally this interrupt is reserved to the implementation, so that Ctrl-C can be used to interrupt execution.  Additionally, signals such as SIGSEGV, SIGABRT, SIGFPE and SIGILL are often mapped to specific Ada exceptions, or used to implement run-time functions such as the abort statement and stack overflow checking.

Pragma Interrupt_State provides a general mechanism for overriding such uses of interrupts.  It subsumes the functionality of pragma Unreserve_All_Interrupts.  Pragma Interrupt_State is not available on Windows or VMS.  On all other platforms than VxWorks, it applies to signals; on VxWorks, it applies to vectored hardware interrupts and may be used to mark interrupts required by the board support package as reserved.

Interrupts can be in one of three states:

  * System

     The interrupt is reserved (no Ada handler can be installed), and the Ada run-time may not install a handler. As a result you are guaranteed standard system default action if this interrupt is raised.

  * Runtime

     The interrupt is reserved (no Ada handler can be installed). The run time is allowed to install a handler for internal control purposes, but is not required to do so.

  * User

     The interrupt is unreserved.  The user may install a handler to provide some other action.

These states are the allowed values of the State parameter of the pragma.  The Name parameter is a value of the type Ada.Interrupts.Interrupt_ID.  Typically, it is a name declared in Ada.Interrupts.Names.

This is a configuration pragma, and the binder will check that there are no inconsistencies between different units in a partition in how a given interrupt is specified. It may appear anywhere a pragma is legal.

The effect is to move the interrupt to the specified state.

By declaring interrupts to be SYSTEM, you guarantee the standard system action, such as a core dump.

By declaring interrupts to be USER, you guarantee that you can install a handler.

Note that certain signals on many operating systems cannot be caught and handled by applications.  In such cases, the pragma is ignored.  See the operating system documentation, or the value of the array Reserved declared in the spec of package System.OS_Interface.

Overriding the default state of signals used by the Ada runtime may interfere with an application's runtime behavior in the cases of the synchronous signals, and in the case of the signal used to implement the abort statement.

Pragma Invariant
================

Syntax:

     pragma Invariant
       ([Entity =>]    private_type_LOCAL_NAME,
        [Check  =>]    EXPRESSION
        [,[Message =>] String_Expression]);

This pragma provides exactly the same capabilities as the Invariant aspect defined in AI05-0146-1, and in the Ada 2012 Reference Manual. The Invariant aspect is fully implemented in Ada 2012 mode, but since it requires the use of the aspect syntax, which is not available exception in 2012 mode, it is not possible to use the Invariant aspect in earlier versions of Ada. However the Invariant pragma may be used in any version of Ada.

The pragma must appear within the visible part of the package specification, after the type to which its Entity argument appears. As with the Invariant aspect, the Check expression is not analyzed until the end of the visible part of the package, so it may contain forward references. The Message argument, if present, provides the exception message used if the invariant is violated. If no Message parameter is provided, a default message that identifies the line on which the pragma appears is used.

It is permissible to have multiple Invariants for the same type entity, in which case they are and'ed together. It is permissible to use this pragma in Ada 2012 mode, but you cannot have both an invariant aspect and an invariant pragma for the same entity.

For further details on the use of this pragma, see the Ada 2012 documentation of the Invariant aspect.

Pragma Keep_Names
=================

Syntax:

     pragma Keep_Names ([On =>] enumeration_first_subtype_LOCAL_NAME);

The LOCAL_NAME argument must refer to an enumeration first subtype in the current declarative part. The effect is to retain the enumeration literal names for use by Image and Value even if a global Discard_Names pragma applies. This is useful when you want to generally suppress enumeration literal names and for example you therefore use a Discard_Names pragma in the gnat.adc file, but you want to retain the names for specific enumeration types.

Pragma License
==============

Syntax:

     pragma License (Unrestricted | GPL | Modified_GPL | Restricted);

This pragma is provided to allow automated checking for appropriate license conditions with respect to the standard and modified GPL.  A pragma License, which is a configuration pragma that typically appears at the start of a source file or in a separate gnat.adc file, specifies the licensing conditions of a unit as follows:

  * Unrestricted This is used for a unit that can be freely used with no license restrictions. Examples of such units are public domain units, and units from the Ada Reference Manual.

  * GPL This is used for a unit that is licensed under the unmodified GPL, and which therefore cannot be with'ed by a restricted unit.

  * Modified_GPL This is used for a unit licensed under the GNAT modified GPL that includes a special exception paragraph that specifically permits the inclusion of the unit in programs without requiring the entire program to be released under the GPL.

  * Restricted This is used for a unit that is restricted in that it is not permitted to depend on units that are licensed under the GPL.  Typical examples are proprietary code that is to be released under more restrictive license conditions.  Note that restricted units are permitted to with units which are licensed under the modified GPL (this is the whole point of the modified GPL).

Normally a unit with no License pragma is considered to have an unknown license, and no checking is done.  However, standard GNAT headers are recognized, and license information is derived from them as follows.

     A GNAT license header starts with a line containing 78 hyphens.  The following comment text is searched for the appearance of any of the following strings.

     If the string ``GNU General Public License'' is found, then the unit is assumed to have GPL license, unless the string ``As a special exception'' follows, in which case the license is assumed to be modified GPL.

     If one of the strings ``This specification is adapted from the Ada Semantic Interface'' or ``This specification is derived from the Ada Reference Manual'' is found then the unit is assumed to be unrestricted.

These default actions means that a program with a restricted license pragma will automatically get warnings if a GPL unit is inappropriately with'ed.  For example, the program:

     with Sem_Ch3;
     with GNAT.Sockets;
     procedure Secret_Stuff is
       ...
     end Secret_Stuff

if compiled with pragma License (Restricted) in a gnat.adc file will generate the warning:

     1.  with Sem_Ch3;
             |
        >>> license of withed unit "Sem_Ch3" is incompatible

     2.  with GNAT.Sockets;
     3.  procedure Secret_Stuff is

Here we get a warning on Sem_Ch3 since it is part of the GNAT compiler and is licensed under the GPL, but no warning for GNAT.Sockets which is part of the GNAT run time, and is therefore licensed under the modified GPL.

Pragma Link_With
================

Syntax:

     pragma Link_With (static_string_EXPRESSION {,static_string_EXPRESSION});

This pragma is provided for compatibility with certain Ada 83 compilers. It has exactly the same effect as pragma Linker_Options except that spaces occurring within one of the string expressions are treated as separators. For example, in the following case:

     pragma Link_With ("-labc -ldef");

results in passing the strings -labc and -ldef as two separate arguments to the linker. In addition pragma Link_With allows multiple arguments, with the same effect as successive pragmas.

Pragma Linker_Alias
===================

Syntax:

     pragma Linker_Alias (
       [Entity =>] LOCAL_NAME,
       [Target =>] static_string_EXPRESSION);

LOCAL_NAME must refer to an object that is declared at the library level. This pragma establishes the given entity as a linker alias for the given target. It is equivalent to __attribute__((alias)) in GNU C and causes LOCAL_NAME to be emitted as an alias for the symbol STATIC_STRING_EXPRESSION in the object file, that is to say no space is reserved for LOCAL_NAME by the assembler and it will be resolved to the same address as STATIC_STRING_EXPRESSION by the linker.

The actual linker name for the target must be used (e.g. the fully encoded name with qualification in Ada, or the mangled name in C++), or it must be declared using the C convention with pragma Import or pragma Export.

Not all target machines support this pragma. On some of them it is accepted only if pragma Weak_External has been applied to LOCAL_NAME.

     --  Example of the use of pragma Linker_Alias

     package p is
       i : Integer := 1;
       pragma Export (C, i);

       new_name_for_i : Integer;
       pragma Linker_Alias (new_name_for_i, "i");
     end p;

Pragma Linker_Constructor
=========================

Syntax:

     pragma Linker_Constructor (procedure_LOCAL_NAME);

PROCEDURE_LOCAL_NAME must refer to a parameterless procedure that is declared at the library level. A procedure to which this pragma is applied will be treated as an initialization routine by the linker. It is equivalent to __attribute__((constructor)) in GNU C and causes PROCEDURE_LOCAL_NAME to be invoked before the entry point of the executable is called (or immediately after the shared library is loaded if the procedure is linked in a shared library), in particular before the Ada run-time environment is set up.

Because of these specific contexts, the set of operations such a procedure can perform is very limited and the type of objects it can manipulate is essentially restricted to the elementary types. In particular, it must only contain code to which pragma Restrictions (No_Elaboration_Code) applies.

This pragma is used by GNAT to implement auto-initialization of shared Stand Alone Libraries, which provides a related capability without the restrictions listed above. Where possible, the use of Stand Alone Libraries is preferable to the use of this pragma.

Pragma Linker_Destructor
========================

Syntax:

     pragma Linker_Destructor (procedure_LOCAL_NAME);

PROCEDURE_LOCAL_NAME must refer to a parameterless procedure that is declared at the library level. A procedure to which this pragma is applied will be treated as a finalization routine by the linker. It is equivalent to __attribute__((destructor)) in GNU C and causes PROCEDURE_LOCAL_NAME to be invoked after the entry point of the executable has exited (or immediately before the shared library is unloaded if the procedure is linked in a shared library), in particular after the Ada run-time environment is shut down.

See pragma Linker_Constructor for the set of restrictions that apply because of these specific contexts.

Pragma Linker_Section
=====================

Syntax:

     pragma Linker_Section (
       [Entity  =>] LOCAL_NAME,
       [Section =>] static_string_EXPRESSION);

LOCAL_NAME must refer to an object that is declared at the library level. This pragma specifies the name of the linker section for the given entity. It is equivalent to __attribute__((section)) in GNU C and causes LOCAL_NAME to be placed in the STATIC_STRING_EXPRESSION section of the executable (assuming the linker doesn't rename the section).

The compiler normally places library-level objects in standard sections depending on their type: procedures and functions generally go in the .text section, initialized variables in the .data section and uninitialized variables in the .bss section.

Other, special sections may exist on given target machines to map special hardware, for example I/O ports or flash memory. This pragma is a means to defer the final layout of the executable to the linker, thus fully working at the symbolic level with the compiler.

Some file formats do not support arbitrary sections so not all target machines support this pragma. The use of this pragma may cause a program execution to be erroneous if it is used to place an entity into an inappropriate section (e.g. a modified variable into the .text section). See also pragma Persistent_BSS.

     --  Example of the use of pragma Linker_Section

     package IO_Card is
       Port_A : Integer;
       pragma Volatile (Port_A);
       pragma Linker_Section (Port_A, ".bss.port_a");

       Port_B : Integer;
       pragma Volatile (Port_B);
       pragma Linker_Section (Port_B, ".bss.port_b");
     end IO_Card;

Pragma Long_Float
=================

Syntax:

     pragma Long_Float (FLOAT_FORMAT);

     FLOAT_FORMAT ::= D_Float | G_Float

This pragma is implemented only in the OpenVMS implementation of GNAT. It allows control over the internal representation chosen for the predefined type Long_Float and for floating point type representations with digits specified in the range 7 through 15. For further details on this pragma, see the DEC Ada Language Reference Manual, section 3.5.7b.  Note that to use this pragma, the standard runtime libraries must be recompiled.

Pragma Machine_Attribute
========================

Syntax:

     pragma Machine_Attribute (
          [Entity         =>] LOCAL_NAME,
          [Attribute_Name =>] static_string_EXPRESSION
       [, [Info           =>] static_EXPRESSION] );

Machine-dependent attributes can be specified for types and/or declarations.  This pragma is semantically equivalent to __attribute__((ATTRIBUTE_NAME)) (if INFO is not specified) or __attribute__((ATTRIBUTE_NAME(INFO))) in GNU C, where ATTRIBUTE_NAME is recognized by the compiler middle-end or the TARGET_ATTRIBUTE_TABLE machine specific macro.  A string literal for the optional parameter INFO is transformed into an identifier, which may make this pragma unusable for some attributes.  See Defining target-specific uses of __attribute__ (GNU Compiler Collection (GCC) Internals), further information.

Pragma Main
===========

Syntax:

     pragma Main
      (MAIN_OPTION [, MAIN_OPTION]);

     MAIN_OPTION ::=
       [Stack_Size              =>] static_integer_EXPRESSION
     | [Task_Stack_Size_Default =>] static_integer_EXPRESSION
     | [Time_Slicing_Enabled    =>] static_boolean_EXPRESSION

This pragma is provided for compatibility with OpenVMS VAX Systems.  It has no effect in GNAT, other than being syntax checked.

Pragma Main_Storage
===================

Syntax:

     pragma Main_Storage
       (MAIN_STORAGE_OPTION [, MAIN_STORAGE_OPTION]);

     MAIN_STORAGE_OPTION ::=
       [WORKING_STORAGE =>] static_SIMPLE_EXPRESSION
     | [TOP_GUARD       =>] static_SIMPLE_EXPRESSION

This pragma is provided for compatibility with OpenVMS VAX Systems.  It has no effect in GNAT, other than being syntax checked.  Note that the pragma also has no effect in DEC Ada 83 for OpenVMS Alpha Systems.

Pragma No_Body
==============

Syntax:

     pragma No_Body;

There are a number of cases in which a package spec does not require a body, and in fact a body is not permitted. GNAT will not permit the spec to be compiled if there is a body around. The pragma No_Body allows you to provide a body file, even in a case where no body is allowed. The body file must contain only comments and a single No_Body pragma. This is recognized by the compiler as indicating that no body is logically present.

This is particularly useful during maintenance when a package is modified in such a way that a body needed before is no longer needed. The provision of a dummy body with a No_Body pragma ensures that there is no interference from earlier versions of the package body.

Pragma No_Return
================

Syntax:

     pragma No_Return (procedure_LOCAL_NAME {, procedure_LOCAL_NAME});

Each PROCEDURE_LOCAL_NAME argument must refer to one or more procedure declarations in the current declarative part.  A procedure to which this pragma is applied may not contain any explicit return statements. In addition, if the procedure contains any implicit returns from falling off the end of a statement sequence, then execution of that implicit return will cause Program_Error to be raised.

One use of this pragma is to identify procedures whose only purpose is to raise an exception. Another use of this pragma is to suppress incorrect warnings about missing returns in functions, where the last statement of a function statement sequence is a call to such a procedure.

Note that in Ada 2005 mode, this pragma is part of the language, and is identical in effect to the pragma as implemented in Ada 95 mode.

Pragma No_Strict_Aliasing
=========================

Syntax:

     pragma No_Strict_Aliasing [([Entity =>] type_LOCAL_NAME)];

TYPE_LOCAL_NAME must refer to an access type declaration in the current declarative part.  The effect is to inhibit strict aliasing optimization for the given type.  The form with no arguments is a configuration pragma which applies to all access types declared in units to which the pragma applies. For a detailed description of the strict aliasing optimization, and the situations in which it must be suppressed, see Optimization and Strict Aliasing (GNAT User's Guide).

This pragma currently has no effects on access to unconstrained array types.

Pragma Normalize_Scalars
========================

Syntax:

     pragma Normalize_Scalars;

This is a language defined pragma which is fully implemented in GNAT.  The effect is to cause all scalar objects that are not otherwise initialized to be initialized.  The initial values are implementation dependent and are as follows:

Standard.Character

     Objects whose root type is Standard.Character are initialized to Character'Last unless the subtype range excludes NUL (in which case NUL is used). This choice will always generate an invalid value if one exists.

Standard.Wide_Character

     Objects whose root type is Standard.Wide_Character are initialized to Wide_Character'Last unless the subtype range excludes NUL (in which case NUL is used). This choice will always generate an invalid value if one exists.

Standard.Wide_Wide_Character

     Objects whose root type is Standard.Wide_Wide_Character are initialized to the invalid value 16#FFFF_FFFF# unless the subtype range excludes NUL (in which case NUL is used). This choice will always generate an invalid value if one exists.

Integer types

     Objects of an integer type are treated differently depending on whether negative values are present in the subtype. If no negative values are present, then all one bits is used as the initial value except in the special case where zero is excluded from the subtype, in which case all zero bits are used. This choice will always generate an invalid value if one exists.

     For subtypes with negative values present, the largest negative number is used, except in the unusual case where this largest negative number is in the subtype, and the largest positive number is not, in which case the largest positive value is used. This choice will always generate an invalid value if one exists.

Floating-Point Types
     Objects of all floating-point types are initialized to all 1-bits. For standard IEEE format, this corresponds to a NaN (not a number) which is indeed an invalid value.

Fixed-Point Types
     Objects of all fixed-point types are treated as described above for integers, with the rules applying to the underlying integer value used to represent the fixed-point value.

Modular types
     Objects of a modular type are initialized to all one bits, except in the special case where zero is excluded from the subtype, in which case all zero bits are used. This choice will always generate an invalid value if one exists.

Enumeration types
     Objects of an enumeration type are initialized to all one-bits, i.e. to the value 2 ** typ'Size - 1 unless the subtype excludes the literal whose Pos value is zero, in which case a code of zero is used. This choice will always generate an invalid value if one exists.

Pragma Obsolescent
==================

Syntax:

     pragma Obsolescent;

     pragma Obsolescent (
       [Message =>] static_string_EXPRESSION
     [,[Version =>] Ada_05]]);

     pragma Obsolescent (
       [Entity  =>] NAME
     [,[Message =>] static_string_EXPRESSION
     [,[Version =>] Ada_05]] );

This pragma can occur immediately following a declaration of an entity, including the case of a record component. If no Entity argument is present, then this declaration is the one to which the pragma applies. If an Entity parameter is present, it must either match the name of the entity in this declaration, or alternatively, the pragma can immediately follow an enumeration type declaration, where the Entity argument names one of the enumeration literals.

This pragma is used to indicate that the named entity is considered obsolescent and should not be used. Typically this is used when an API must be modified by eventually removing or modifying existing subprograms or other entities. The pragma can be used at an intermediate stage when the entity is still present, but will be removed later.

The effect of this pragma is to output a warning message on a reference to an entity thus marked that the subprogram is obsolescent if the appropriate warning option in the compiler is activated. If the Message parameter is present, then a second warning message is given containing this text. In addition, a reference to the entity is considered to be a violation of pragma Restrictions (No_Obsolescent_Features).

This pragma can also be used as a program unit pragma for a package, in which case the entity name is the name of the package, and the pragma indicates that the entire package is considered obsolescent. In this case a client with'ing such a package violates the restriction, and the with statement is flagged with warnings if the warning option is set.

If the Version parameter is present (which must be exactly the identifier Ada_05, no other argument is allowed), then the indication of obsolescence applies only when compiling in Ada 2005 mode. This is primarily intended for dealing with the situations in the predefined library where subprograms or packages have become defined as obsolescent in Ada 2005 (e.g. in Ada.Characters.Handling), but may be used anywhere.

The following examples show typical uses of this pragma:

     package p is
        pragma Obsolescent (p, Message => "use pp instead of p");
     end p;

     package q is
        procedure q2;
        pragma Obsolescent ("use q2new instead");

        type R is new integer;
        pragma Obsolescent
          (Entity  => R,
           Message => "use RR in Ada 2005",
           Version => Ada_05);

        type M is record
           F1 : Integer;
           F2 : Integer;
           pragma Obsolescent;
           F3 : Integer;
        end record;

        type E is (a, bc, 'd', quack);
        pragma Obsolescent (Entity => bc)
        pragma Obsolescent (Entity => 'd')

        function "+"
          (a, b : character) return character;
        pragma Obsolescent (Entity => "+");
     end;

Note that, as for all pragmas, if you use a pragma argument identifier, then all subsequent parameters must also use a pragma argument identifier. So if you specify "Entity =>" for the Entity argument, and a Message argument is present, it must be preceded by "Message =>".

Pragma Optimize_Alignment
=========================

Syntax:

     pragma Optimize_Alignment (TIME | SPACE | OFF);

This is a configuration pragma which affects the choice of default alignments for types where no alignment is explicitly specified. There is a time/space trade-off in the selection of these values. Large alignments result in more efficient code, at the expense of larger data space, since sizes have to be increased to match these alignments. Smaller alignments save space, but the access code is slower. The normal choice of default alignments (which is what you get if you do not use this pragma, or if you use an argument of OFF), tries to balance these two requirements.

Specifying SPACE causes smaller default alignments to be chosen in two cases. First any packed record is given an alignment of 1. Second, if a size is given for the type, then the alignment is chosen to avoid increasing this size. For example, consider:

        type R is record
           X : Integer;
           Y : Character;
        end record;

        for R'Size use 5*8;

In the default mode, this type gets an alignment of 4, so that access to the Integer field X are efficient. But this means that objects of the type end up with a size of 8 bytes. This is a valid choice, since sizes of objects are allowed to be bigger than the size of the type, but it can waste space if for example fields of type R appear in an enclosing record. If the above type is compiled in Optimize_Alignment (Space) mode, the alignment is set to 1.

Specifying TIME causes larger default alignments to be chosen in the case of small types with sizes that are not a power of 2. For example, consider:

        type R is record
           A : Character;
           B : Character;
           C : Boolean;
        end record;

        pragma Pack (R);
        for R'Size use 17;

The default alignment for this record is normally 1, but if this type is compiled in Optimize_Alignment (Time) mode, then the alignment is set to 4, which wastes space for objects of the type, since they are now 4 bytes long, but results in more efficient access when the whole record is referenced.

As noted above, this is a configuration pragma, and there is a requirement that all units in a partition be compiled with a consistent setting of the optimization setting. This would normally be achieved by use of a configuration pragma file containing the appropriate setting. The exception to this rule is that units with an explicit configuration pragma in the same file as the source unit are excluded from the consistency check, as are all predefined units. The latter are compiled by default in pragma Optimize_Alignment (Off) mode if no pragma appears at the start of the file.

Pragma Ordered
==============

Syntax:

     pragma Ordered (enumeration_first_subtype_LOCAL_NAME);

Most enumeration types are from a conceptual point of view unordered. For example, consider:

     type Color is (Red, Blue, Green, Yellow);

By Ada semantics Blue > Red and Green > Blue, but really these relations make no sense; the enumeration type merely specifies a set of possible colors, and the order is unimportant.

For unordered enumeration types, it is generally a good idea if clients avoid comparisons (other than equality or inequality) and explicit ranges. (A client is a unit where the type is referenced, other than the unit where the type is declared, its body, and its subunits.) For example, if code buried in some client says:

     if Current_Color < Yellow then ...
     if Current_Color in Blue .. Green then ...

then the client code is relying on the order, which is undesirable. It makes the code hard to read and creates maintenance difficulties if entries have to be added to the enumeration type. Instead, the code in the client should list the possibilities, or an appropriate subtype should be declared in the unit that declares the original enumeration type. E.g., the following subtype could be declared along with the type Color:

     subtype RBG is Color range Red .. Green;

and then the client could write:

     if Current_Color in RBG then ...
     if Current_Color = Blue or Current_Color = Green then ...

However, some enumeration types are legitimately ordered from a conceptual point of view. For example, if you declare:

     type Day is (Mon, Tue, Wed, Thu, Fri, Sat, Sun);

then the ordering imposed by the language is reasonable, and clients can depend on it, writing for example:

     if D in Mon .. Fri then ...
     if D < Wed then ...

The pragma Ordered is provided to mark enumeration types that are conceptually ordered, alerting the reader that clients may depend on the ordering. GNAT provides a pragma to mark enumerations as ordered rather than one to mark them as unordered, since in our experience, the great majority of enumeration types are conceptually unordered.

The types Boolean, Character, Wide_Character, and Wide_Wide_Character are considered to be ordered types, so each is declared with a pragma Ordered in package Standard.

Normally pragma Ordered serves only as documentation and a guide for coding standards, but GNAT provides a warning switch -gnatw.u that requests warnings for inappropriate uses (comparisons and explicit subranges) for unordered types. If this switch is used, then any enumeration type not marked with pragma Ordered will be considered as unordered, and will generate warnings for inappropriate uses.

For additional information please refer to the description of the -gnatw.u switch in the GNAT User's Guide.

Pragma Passive
==============

Syntax:

     pragma Passive [(Semaphore | No)];

Syntax checked, but otherwise ignored by GNAT.  This is recognized for compatibility with DEC Ada 83 implementations, where it is used within a task definition to request that a task be made passive.  If the argument Semaphore is present, or the argument is omitted, then DEC Ada 83 treats the pragma as an assertion that the containing task is passive and that optimization of context switch with this task is permitted and desired.  If the argument No is present, the task must not be optimized.  GNAT does not attempt to optimize any tasks in this manner (since protected objects are available in place of passive tasks).

Pragma Persistent_BSS
=====================

Syntax:

     pragma Persistent_BSS [(LOCAL_NAME)]

This pragma allows selected objects to be placed in the .persistent_bss section. On some targets the linker and loader provide for special treatment of this section, allowing a program to be reloaded without affecting the contents of this data (hence the name persistent).

There are two forms of usage. If an argument is given, it must be the local name of a library level object, with no explicit initialization and whose type is potentially persistent. If no argument is given, then the pragma is a configuration pragma, and applies to all library level objects with no explicit initialization of potentially persistent types.

A potentially persistent type is a scalar type, or a non-tagged, non-discriminated record, all of whose components have no explicit initialization and are themselves of a potentially persistent type, or an array, all of whose constraints are static, and whose component type is potentially persistent.

If this pragma is used on a target where this feature is not supported, then the pragma will be ignored. See also pragma Linker_Section.

Pragma Polling
==============

Syntax:

     pragma Polling (ON | OFF);

This pragma controls the generation of polling code.  This is normally off. If pragma Polling (ON) is used then periodic calls are generated to the routine Ada.Exceptions.Poll.  This routine is a separate unit in the runtime library, and can be found in file a-excpol.adb.

Pragma Polling can appear as a configuration pragma (for example it can be placed in the gnat.adc file) to enable polling globally, or it can be used in the statement or declaration sequence to control polling more locally.

A call to the polling routine is generated at the start of every loop and at the start of every subprogram call.  This guarantees that the Poll routine is called frequently, and places an upper bound (determined by the complexity of the code) on the period between two Poll calls.

The primary purpose of the polling interface is to enable asynchronous aborts on targets that cannot otherwise support it (for example Windows NT), but it may be used for any other purpose requiring periodic polling. The standard version is null, and can be replaced by a user program.  This will require re-compilation of the Ada.Exceptions package that can be found in files a-except.ads and a-except.adb.

A standard alternative unit (in file 4wexcpol.adb in the standard GNAT distribution) is used to enable the asynchronous abort capability on targets that do not normally support the capability.  The version of Poll in this file makes a call to the appropriate runtime routine to test for an abort condition.

Note that polling can also be enabled by use of the -gnatP switch. See Switches for gcc (GNAT User's Guide), for details.

Pragma Postcondition
====================

Syntax:

     pragma Postcondition (
        [Check   =>] Boolean_Expression
      [,[Message =>] String_Expression]);

The Postcondition pragma allows specification of automatic postcondition checks for subprograms. These checks are similar to assertions, but are automatically inserted just prior to the return statements of the subprogram with which they are associated (including implicit returns at the end of procedure bodies and associated exception handlers).

In addition, the boolean expression which is the condition which must be true may contain references to function'Result in the case of a function to refer to the returned value.

Postcondition pragmas may appear either immediately following the (separate) declaration of a subprogram, or at the start of the declarations of a subprogram body. Only other pragmas may intervene (that is appear between the subprogram declaration and its postconditions, or appear before the postcondition in the declaration sequence in a subprogram body). In the case of a postcondition appearing after a subprogram declaration, the formal arguments of the subprogram are visible, and can be referenced in the postcondition expressions.

The postconditions are collected and automatically tested just before any return (implicit or explicit) in the subprogram body. A postcondition is only recognized if postconditions are active at the time the pragma is encountered. The compiler switch gnata turns on all postconditions by default, and pragma Check_Policy with an identifier of Postcondition can also be used to control whether postconditions are active.

The general approach is that postconditions are placed in the spec if they represent functional aspects which make sense to the client. For example we might have:

        function Direction return Integer;
        pragma Postcondition
         (Direction'Result = +1
            or else
          Direction'Result = -1);

which serves to document that the result must be +1 or -1, and will test that this is the case at run time if postcondition checking is active.

Postconditions within the subprogram body can be used to check that some internal aspect of the implementation, not visible to the client, is operating as expected. For instance if a square root routine keeps an internal counter of the number of times it is called, then we might have the following postcondition:

        Sqrt_Calls : Natural := 0;

        function Sqrt (Arg : Float) return Float is
          pragma Postcondition
            (Sqrt_Calls = Sqrt_Calls'Old + 1);
          ...
        end Sqrt

As this example, shows, the use of the Old attribute is often useful in postconditions to refer to the state on entry to the subprogram.

Note that postconditions are only checked on normal returns from the subprogram. If an abnormal return results from raising an exception, then the postconditions are not checked.

If a postcondition fails, then the exception System.Assertions.Assert_Failure is raised. If a message argument was supplied, then the given string will be used as the exception message. If no message argument was supplied, then the default message has the form "Postcondition failed at file:line". The exception is raised in the context of the subprogram body, so it is possible to catch postcondition failures within the subprogram body itself.

Within a package spec, normal visibility rules in Ada would prevent forward references within a postcondition pragma to functions defined later in the same package. This would introduce undesirable ordering constraints. To avoid this problem, all postcondition pragmas are analyzed at the end of the package spec, allowing forward references.

The following example shows that this even allows mutually recursive postconditions as in:

     package Parity_Functions is
        function Odd  (X : Natural) return Boolean;
        pragma Postcondition
          (Odd'Result =
             (x = 1
               or else
             (x /= 0 and then Even (X - 1))));

        function Even (X : Natural) return Boolean;
        pragma Postcondition
          (Even'Result =
             (x = 0
               or else
             (x /= 1 and then Odd (X - 1))));

     end Parity_Functions;

There are no restrictions on the complexity or form of conditions used within Postcondition pragmas. The following example shows that it is even possible to verify performance behavior.

     package Sort is

        Performance : constant Float;
        --  Performance constant set by implementation
        --  to match target architecture behavior.

        procedure Treesort (Arg : String);
        --  Sorts characters of argument using N*logN sort
        pragma Postcondition
          (Float (Clock - Clock'Old) <=
             Float (Arg'Length) *
             log (Float (Arg'Length)) *
             Performance);
     end Sort;

Note: postcondition pragmas associated with subprograms that are marked as Inline_Always, or those marked as Inline with front-end inlining (-gnatN option set) are accepted and legality-checked by the compiler, but are ignored at run-time even if postcondition checking is enabled.

Pragma Precondition
===================

Syntax:

     pragma Precondition (
        [Check   =>] Boolean_Expression
      [,[Message =>] String_Expression]);

The Precondition pragma is similar to Postcondition except that the corresponding checks take place immediately upon entry to the subprogram, and if a precondition fails, the exception is raised in the context of the caller, and the attribute 'Result cannot be used within the precondition expression.

Otherwise, the placement and visibility rules are identical to those described for postconditions. The following is an example of use within a package spec:

     package Math_Functions is
        ...
        function Sqrt (Arg : Float) return Float;
        pragma Precondition (Arg >= 0.0)
        ...
     end Math_Functions;

Precondition pragmas may appear either immediately following the (separate) declaration of a subprogram, or at the start of the declarations of a subprogram body. Only other pragmas may intervene (that is appear between the subprogram declaration and its postconditions, or appear before the postcondition in the declaration sequence in a subprogram body).

Note: postcondition pragmas associated with subprograms that are marked as Inline_Always, or those marked as Inline with front-end inlining (-gnatN option set) are accepted and legality-checked by the compiler, but are ignored at run-time even if postcondition checking is enabled.

Pragma Profile (Ravenscar)
==========================

Syntax:

     pragma Profile (Ravenscar);

A configuration pragma that establishes the following set of configuration pragmas:

Task_Dispatching_Policy (FIFO_Within_Priorities)
     [RM D.2.2] Tasks are dispatched following a preemptive priority-ordered scheduling policy.

Locking_Policy (Ceiling_Locking)
     [RM D.3] While tasks and interrupts execute a protected action, they inherit the ceiling priority of the corresponding protected object.

plus the following set of restrictions:

Max_Entry_Queue_Length => 1
     Defines the maximum number of calls that are queued on a (protected) entry. Note that this restrictions is checked at run time. Violation of this restriction results in the raising of Program_Error exception at the point of the call. For the Profile (Ravenscar) the value of Max_Entry_Queue_Length is always 1 and hence no task can be queued on a protected entry.

Max_Protected_Entries => 1
     [RM D.7] Specifies the maximum number of entries per protected type. The bounds of every entry family of a protected unit shall be static, or shall be defined by a discriminant of a subtype whose corresponding bound is static. For the Profile (Ravenscar) the value of Max_Protected_Entries is always 1.

Max_Task_Entries => 0
     [RM D.7] Specifies the maximum number of entries per task.  The bounds of every entry family of a task unit shall be static, or shall be defined by a discriminant of a subtype whose corresponding bound is static.  A value of zero indicates that no rendezvous are possible.  For the Profile (Ravenscar), the value of Max_Task_Entries is always 0 (zero).

No_Abort_Statements
     [RM D.7] There are no abort_statements, and there are no calls to Task_Identification.Abort_Task.

No_Dynamic_Attachment
     There is no call to any of the operations defined in package Ada.Interrupts (Is_Reserved, Is_Attached, Current_Handler, Attach_Handler, Exchange_Handler, Detach_Handler, and Reference).

No_Dynamic_Priorities
     [RM D.7] There are no semantic dependencies on the package Dynamic_Priorities.

No_Implicit_Heap_Allocations
     [RM D.7] No constructs are allowed to cause implicit heap allocation.

No_Local_Protected_Objects
     Protected objects and access types that designate such objects shall be declared only at library level.

No_Local_Timing_Events
     [RM D.7] All objects of type Ada.Timing_Events.Timing_Event are declared at the library level.

No_Protected_Type_Allocators
     There are no allocators for protected types or types containing protected subcomponents.

No_Relative_Delay
     There are no delay_relative statements.

No_Requeue_Statements
     Requeue statements are not allowed.

No_Select_Statements
     There are no select_statements.

No_Specific_Termination_Handlers
     [RM D.7] There are no calls to Ada.Task_Termination.Set_Specific_Handler or to Ada.Task_Termination.Specific_Handler.

No_Task_Allocators
     [RM D.7] There are no allocators for task types or types containing task subcomponents.

No_Task_Hierarchy
     [RM D.7] All (non-environment) tasks depend directly on the environment task of the partition.

No_Task_Termination
     Tasks which terminate are erroneous.

Simple_Barriers
     Entry barrier condition expressions shall be either static boolean expressions or boolean objects which are declared in the protected type which contains the entry.

The Ravenscar profile also includes the following restrictions that specify that there are no semantic dependences on the corresponding predefined packages:

No_Dependence => Ada.Asynchronous_Task_Control

No_Dependence => Ada.Calendar

No_Dependence => Ada.Execution_Time.Group_Budget

No_Dependence => Ada.Execution_Time.Timers

No_Dependence => Ada.Task_Attributes

No_Dependence => System.Multiprocessors.Dispatching_Domains

This set of configuration pragmas and restrictions correspond to the definition of the ``Ravenscar Profile'' for limited tasking, devised and published by the International Real-Time Ada Workshop, 1997, and whose most recent description is available at http://www-users.cs.york.ac.uk/~burns/ravenscar.ps.

The original definition of the profile was revised at subsequent IRTAW meetings. It has been included in the ISO Guide for the Use of the Ada Programming Language in High Integrity Systems, and has been approved by ISO/IEC/SC22/WG9 for inclusion in the next revision of the standard. The formal definition given by the Ada Rapporteur Group (ARG) can be found in two Ada Issues (AI-249 and AI-305) available at http://www.ada-auth.org/cgi-bin/cvsweb.cgi/ais/ai-00249.txt and http://www.ada-auth.org/cgi-bin/cvsweb.cgi/ais/ai-00305.txt.

The above set is a superset of the restrictions provided by pragma Profile (Restricted), it includes six additional restrictions (Simple_Barriers, No_Select_Statements, No_Calendar, No_Implicit_Heap_Allocations, No_Relative_Delay and No_Task_Termination).  This means that pragma Profile (Ravenscar), like the pragma Profile (Restricted), automatically causes the use of a simplified, more efficient version of the tasking run-time system.

Pragma Profile (Restricted)
===========================

Syntax:

     pragma Profile (Restricted);

A configuration pragma that establishes the following set of restrictions:

  * No_Abort_Statements

  * No_Entry_Queue

  * No_Task_Hierarchy

  * No_Task_Allocators

  * No_Dynamic_Priorities

  * No_Terminate_Alternatives

  * No_Dynamic_Attachment

  * No_Protected_Type_Allocators

  * No_Local_Protected_Objects

  * No_Requeue_Statements

  * No_Task_Attributes_Package

  * Max_Asynchronous_Select_Nesting =  0

  * Max_Task_Entries =  0

  * Max_Protected_Entries = 1

  * Max_Select_Alternatives = 0

This set of restrictions causes the automatic selection of a simplified version of the run time that provides improved performance for the limited set of tasking functionality permitted by this set of restrictions.

Pragma Psect_Object
===================

Syntax:

     pragma Psect_Object (
          [Internal =>] LOCAL_NAME,
       [, [External =>] EXTERNAL_SYMBOL]
       [, [Size     =>] EXTERNAL_SYMBOL]);

     EXTERNAL_SYMBOL ::=
       IDENTIFIER
     | static_string_EXPRESSION

This pragma is identical in effect to pragma Common_Object.

Pragma Pure_Function
====================

Syntax:

     pragma Pure_Function ([Entity =>] function_LOCAL_NAME);

This pragma appears in the same declarative part as a function declaration (or a set of function declarations if more than one overloaded declaration exists, in which case the pragma applies to all entities).  It specifies that the function Entity is to be considered pure for the purposes of code generation.  This means that the compiler can assume that there are no side effects, and in particular that two calls with identical arguments produce the same result.  It also means that the function can be used in an address clause.

Note that, quite deliberately, there are no static checks to try to ensure that this promise is met, so Pure_Function can be used with functions that are conceptually pure, even if they do modify global variables.  For example, a square root function that is instrumented to count the number of times it is called is still conceptually pure, and can still be optimized, even though it modifies a global variable (the count).  Memo functions are another example (where a table of previous calls is kept and consulted to avoid re-computation).

Note also that the normal rules excluding optimization of subprograms in pure units (when parameter types are descended from System.Address, or when the full view of a parameter type is limited), do not apply for the Pure_Function case. If you explicitly specify Pure_Function, the compiler may optimize away calls with identical arguments, and if that results in unexpected behavior, the proper action is not to use the pragma for subprograms that are not (conceptually) pure.

Note: Most functions in a Pure package are automatically pure, and there is no need to use pragma Pure_Function for such functions.  One exception is any function that has at least one formal of type System.Address or a type derived from it.  Such functions are not considered pure by default, since the compiler assumes that the Address parameter may be functioning as a pointer and that the referenced data may change even if the address value does not. Similarly, imported functions are not considered to be pure by default, since there is no way of checking that they are in fact pure.  The use of pragma Pure_Function for such a function will override these default assumption, and cause the compiler to treat a designated subprogram as pure in these cases.

Note: If pragma Pure_Function is applied to a renamed function, it applies to the underlying renamed function.  This can be used to disambiguate cases of overloading where some but not all functions in a set of overloaded functions are to be designated as pure.

If pragma Pure_Function is applied to a library level function, the function is also considered pure from an optimization point of view, but the unit is not a Pure unit in the categorization sense. So for example, a function thus marked is free to with non-pure units.

Pragma Restriction_Warnings
===========================

Syntax:

     pragma Restriction_Warnings
       (restriction_IDENTIFIER {, restriction_IDENTIFIER});

This pragma allows a series of restriction identifiers to be specified (the list of allowed identifiers is the same as for pragma Restrictions). For each of these identifiers the compiler checks for violations of the restriction, but generates a warning message rather than an error message if the restriction is violated.

Pragma Shared
=============

This pragma is provided for compatibility with Ada 83. The syntax and semantics are identical to pragma Atomic.

Pragma Short_Circuit_And_Or
===========================

This configuration pragma causes any occurrence of the AND operator applied to operands of type Standard.Boolean to be short-circuited (i.e. the AND operator is treated as if it were AND THEN). Or is similarly treated as OR ELSE. This may be useful in the context of certification protocols requiring the use of short-circuited logical operators. If this configuration pragma occurs locally within the file being compiled, it applies only to the file being compiled. There is no requirement that all units in a partition use this option.

Pragma Short_Descriptors
========================

Syntax:

     pragma Short_Descriptors

In VMS versions of the compiler, this configuration pragma causes all occurrences of the mechanism types Descriptor[_xxx] to be treated as Short_Descriptor[_xxx]. This is helpful in porting legacy applications from a 32-bit environment to a 64-bit environment. This pragma is ignored for non-VMS versions.

Pragma Source_File_Name
=======================

Syntax:

     pragma Source_File_Name (
       [Unit_Name   =>] unit_NAME,
       Spec_File_Name =>  STRING_LITERAL,
       [Index => INTEGER_LITERAL]);

     pragma Source_File_Name (
       [Unit_Name   =>] unit_NAME,
       Body_File_Name =>  STRING_LITERAL,
       [Index => INTEGER_LITERAL]);

Use this to override the normal naming convention.  It is a configuration pragma, and so has the usual applicability of configuration pragmas (i.e. it applies to either an entire partition, or to all units in a compilation, or to a single unit, depending on how it is used. UNIT_NAME is mapped to FILE_NAME_LITERAL.  The identifier for the second argument is required, and indicates whether this is the file name for the spec or for the body.

The optional Index argument should be used when a file contains multiple units, and when you do not want to use gnatchop to separate then into multiple files (which is the recommended procedure to limit the number of recompilations that are needed when some sources change). For instance, if the source file source.ada contains

     package B is
     ...
     end B;

     with B;
     procedure A is
     begin
        ..
     end A;

you could use the following configuration pragmas:

     pragma Source_File_Name
       (B, Spec_File_Name => "source.ada", Index => 1);
     pragma Source_File_Name
       (A, Body_File_Name => "source.ada", Index => 2);

Note that the gnatname utility can also be used to generate those configuration pragmas.

Another form of the Source_File_Name pragma allows the specification of patterns defining alternative file naming schemes to apply to all files.

     pragma Source_File_Name
       (  [Spec_File_Name  =>] STRING_LITERAL
        [,[Casing          =>] CASING_SPEC]
        [,[Dot_Replacement =>] STRING_LITERAL]);

     pragma Source_File_Name
       (  [Body_File_Name  =>] STRING_LITERAL
        [,[Casing          =>] CASING_SPEC]
        [,[Dot_Replacement =>] STRING_LITERAL]);

     pragma Source_File_Name
       (  [Subunit_File_Name =>] STRING_LITERAL
        [,[Casing            =>] CASING_SPEC]
        [,[Dot_Replacement   =>] STRING_LITERAL]);

     CASING_SPEC ::= Lowercase | Uppercase | Mixedcase

The first argument is a pattern that contains a single asterisk indicating the point at which the unit name is to be inserted in the pattern string to form the file name.  The second argument is optional.  If present it specifies the casing of the unit name in the resulting file name string. The default is lower case.  Finally the third argument allows for systematic replacement of any dots in the unit name by the specified string literal.

Note that Source_File_Name pragmas should not be used if you are using project files. The reason for this rule is that the project manager is not aware of these pragmas, and so other tools that use the projet file would not be aware of the intended naming conventions. If you are using project files, file naming is controlled by Source_File_Name_Project pragmas, which are usually supplied automatically by the project manager. A pragma Source_File_Name cannot appear after a Pragma Source_File_Name_Project.

For more details on the use of the Source_File_Name pragma, see Using Other File Names (GNAT User's Guide), and Alternative File Naming Schemes (GNAT User's Guide).

Pragma Source_File_Name_Project
===============================

This pragma has the same syntax and semantics as pragma Source_File_Name. It is only allowed as a stand alone configuration pragma. It cannot appear after a Pragma Source_File_Name, and most importantly, once pragma Source_File_Name_Project appears, no further Source_File_Name pragmas are allowed.

The intention is that Source_File_Name_Project pragmas are always generated by the Project Manager in a manner consistent with the naming specified in a project file, and when naming is controlled in this manner, it is not permissible to attempt to modify this naming scheme using Source_File_Name or Source_File_Name_Project pragmas (which would not be known to the project manager).

Pragma Source_Reference
=======================

Syntax:

     pragma Source_Reference (INTEGER_LITERAL, STRING_LITERAL);

This pragma must appear as the first line of a source file. INTEGER_LITERAL is the logical line number of the line following the pragma line (for use in error messages and debugging information).  STRING_LITERAL is a static string constant that specifies the file name to be used in error messages and debugging information.  This is most notably used for the output of gnatchop with the -r switch, to make sure that the original unchopped source file is the one referred to.

The second argument must be a string literal, it cannot be a static string expression other than a string literal.  This is because its value is needed for error messages issued by all phases of the compiler.

Pragma Static_Elaboration_Desired
=================================

Syntax:

     pragma Static_Elaboration_Desired;

This pragma is used to indicate that the compiler should attempt to initialize statically the objects declared in the library unit to which the pragma applies, when these objects are initialized (explicitly or implicitly) by an aggregate. In the absence of this pragma, aggregates in object declarations are expanded into assignments and loops, even when the aggregate components are static constants. When the aggregate is present the compiler builds a static expression that requires no run-time code, so that the initialized object can be placed in read-only data space. If the components are not static, or the aggregate has more that 100 components, the compiler emits a warning that the pragma cannot be obeyed. (See also the restriction No_Implicit_Loops, which supports static construction of larger aggregates with static components that include an others choice.)

Pragma Stream_Convert
=====================

Syntax:

     pragma Stream_Convert (
       [Entity =>] type_LOCAL_NAME,
       [Read   =>] function_NAME,
       [Write  =>] function_NAME);

This pragma provides an efficient way of providing stream functions for types defined in packages.  Not only is it simpler to use than declaring the necessary functions with attribute representation clauses, but more significantly, it allows the declaration to made in such a way that the stream packages are not loaded unless they are needed.  The use of the Stream_Convert pragma adds no overhead at all, unless the stream attributes are actually used on the designated type.

The first argument specifies the type for which stream functions are provided.  The second parameter provides a function used to read values of this type.  It must name a function whose argument type may be any subtype, and whose returned type must be the type given as the first argument to the pragma.

The meaning of the READ parameter is that if a stream attribute directly or indirectly specifies reading of the type given as the first parameter, then a value of the type given as the argument to the Read function is read from the stream, and then the Read function is used to convert this to the required target type.

Similarly the WRITE parameter specifies how to treat write attributes that directly or indirectly apply to the type given as the first parameter. It must have an input parameter of the type specified by the first parameter, and the return type must be the same as the input type of the Read function. The effect is to first call the Write function to convert to the given stream type, and then write the result type to the stream.

The Read and Write functions must not be overloaded subprograms.  If necessary renamings can be supplied to meet this requirement. The usage of this attribute is best illustrated by a simple example, taken from the GNAT implementation of package Ada.Strings.Unbounded:

     function To_Unbounded (S : String)
                return Unbounded_String
       renames To_Unbounded_String;

     pragma Stream_Convert
       (Unbounded_String, To_Unbounded, To_String);

The specifications of the referenced functions, as given in the Ada Reference Manual are:

     function To_Unbounded_String (Source : String)
       return Unbounded_String;

     function To_String (Source : Unbounded_String)
       return String;

The effect is that if the value of an unbounded string is written to a stream, then the representation of the item in the stream is in the same format that would be used for Standard.String'Output, and this same representation is expected when a value of this type is read from the stream. Note that the value written always includes the bounds, even for Unbounded_String'Write, since Unbounded_String is not an array type.

Pragma Style_Checks
===================

Syntax:

     pragma Style_Checks (string_LITERAL | ALL_CHECKS |
                          On | Off [, LOCAL_NAME]);

This pragma is used in conjunction with compiler switches to control the built in style checking provided by GNAT.  The compiler switches, if set, provide an initial setting for the switches, and this pragma may be used to modify these settings, or the settings may be provided entirely by the use of the pragma.  This pragma can be used anywhere that a pragma is legal, including use as a configuration pragma (including use in the gnat.adc file).

The form with a string literal specifies which style options are to be activated.  These are additive, so they apply in addition to any previously set style check options.  The codes for the options are the same as those used in the -gnaty switch to gcc or gnatmake. For example the following two methods can be used to enable layout checking:

  * pragma Style_Checks ("l");

  * gcc -c -gnatyl ...

The form ALL_CHECKS activates all standard checks (its use is equivalent to the use of the gnaty switch with no options.  See About This Guide (GNAT User's Guide), for details.)

Note: the behavior is slightly different in GNAT mode (-gnatg used). In this case, ALL_CHECKS implies the standard set of GNAT mode style check options (i.e. equivalent to -gnatyg).

The forms with Off and On can be used to temporarily disable style checks as shown in the following example:

     pragma Style_Checks ("k"); -- requires keywords in lower case
     pragma Style_Checks (Off); -- turn off style checks
     NULL;                      -- this will not generate an error message
     pragma Style_Checks (On);  -- turn style checks back on
     NULL;                      -- this will generate an error message

Finally the two argument form is allowed only if the first argument is On or Off.  The effect is to turn of semantic style checks for the specified entity, as shown in the following example:

     pragma Style_Checks ("r"); -- require consistency of identifier casing
     Arg : Integer;
     Rf1 : Integer := ARG;      -- incorrect, wrong case
     pragma Style_Checks (Off, Arg);
     Rf2 : Integer := ARG;      -- OK, no error

Pragma Subtitle
===============

Syntax:

     pragma Subtitle ([Subtitle =>] STRING_LITERAL);

This pragma is recognized for compatibility with other Ada compilers but is ignored by GNAT.

Pragma Suppress
===============

Syntax:

     pragma Suppress (Identifier [, [On =>] Name]);

This is a standard pragma, and supports all the check names required in the RM. It is included here because GNAT recognizes one additional check name: Alignment_Check which can be used to suppress alignment checks on addresses used in address clauses. Such checks can also be suppressed by suppressing range checks, but the specific use of Alignment_Check allows suppression of alignment checks without suppressing other range checks.

Note that pragma Suppress gives the compiler permission to omit checks, but does not require the compiler to omit checks. The compiler will generate checks if they are essentially free, even when they are suppressed. In particular, if the compiler can prove that a certain check will necessarily fail, it will generate code to do an unconditional ``raise'', even if checks are suppressed. The compiler warns in this case.

Of course, run-time checks are omitted whenever the compiler can prove that they will not fail, whether or not checks are suppressed.

Pragma Suppress_All
===================

Syntax:

     pragma Suppress_All;

This pragma can appear anywhere within a unit. The effect is to apply Suppress (All_Checks) to the unit in which it appears.  This pragma is implemented for compatibility with DEC Ada 83 usage where it appears at the end of a unit, and for compatibility with Rational Ada, where it appears as a program unit pragma. The use of the standard Ada pragma Suppress (All_Checks) as a normal configuration pragma is the preferred usage in GNAT.

Pragma Suppress_Exception_Locations
===================================

Syntax:

     pragma Suppress_Exception_Locations;

In normal mode, a raise statement for an exception by default generates an exception message giving the file name and line number for the location of the raise. This is useful for debugging and logging purposes, but this entails extra space for the strings for the messages. The configuration pragma Suppress_Exception_Locations can be used to suppress the generation of these strings, with the result that space is saved, but the exception message for such raises is null. This configuration pragma may appear in a global configuration pragma file, or in a specific unit as usual. It is not required that this pragma be used consistently within a partition, so it is fine to have some units within a partition compiled with this pragma and others compiled in normal mode without it.

Pragma Suppress_Initialization
==============================

Syntax:

     pragma Suppress_Initialization ([Entity =>] type_Name);

This pragma suppresses any implicit or explicit initialization associated with the given type name for all variables of this type, including initialization resulting from the use of pragmas Normalize_Scalars or Initialize_Scalars.

This is considered a representation item, so it cannot be given after the type is frozen. It applies to all subsequent object declarations, and also any allocator that creates objects of the type.

If the pragma is given for the first subtype, then it is considered to apply to the base type and all its subtypes. If the pragma is given for other than a first subtype, then it applies only to the given subtype. The pragma may not be given after the type is frozen.

Pragma Task_Info
================

Syntax

     pragma Task_Info (EXPRESSION);

This pragma appears within a task definition (like pragma Priority) and applies to the task in which it appears.  The argument must be of type System.Task_Info.Task_Info_Type. The Task_Info pragma provides system dependent control over aspects of tasking implementation, for example, the ability to map tasks to specific processors.  For details on the facilities available for the version of GNAT that you are using, see the documentation in the spec of package System.Task_Info in the runtime library.

Pragma Task_Name
================

Syntax

     pragma Task_Name (string_EXPRESSION);

This pragma appears within a task definition (like pragma Priority) and applies to the task in which it appears.  The argument must be of type String, and provides a name to be used for the task instance when the task is created.  Note that this expression is not required to be static, and in particular, it can contain references to task discriminants.  This facility can be used to provide different names for different tasks as they are created, as illustrated in the example below.

The task name is recorded internally in the run-time structures and is accessible to tools like the debugger.  In addition the routine Ada.Task_Identification.Image will return this string, with a unique task address appended.

     --  Example of the use of pragma Task_Name

     with Ada.Task_Identification;
     use Ada.Task_Identification;
     with Text_IO; use Text_IO;
     procedure t3 is

        type Astring is access String;

        task type Task_Typ (Name : access String) is
           pragma Task_Name (Name.all);
        end Task_Typ;

        task body Task_Typ is
           Nam : constant String := Image (Current_Task);
        begin
           Put_Line ("-->" & Nam (1 .. 14) & "<--");
        end Task_Typ;

        type Ptr_Task is access Task_Typ;
        Task_Var : Ptr_Task;

     begin
        Task_Var :=
          new Task_Typ (new String'("This is task 1"));
        Task_Var :=
          new Task_Typ (new String'("This is task 2"));
     end;

Pragma Task_Storage
===================

Syntax:

     pragma Task_Storage (
       [Task_Type =>] LOCAL_NAME,
       [Top_Guard =>] static_integer_EXPRESSION);

This pragma specifies the length of the guard area for tasks.  The guard area is an additional storage area allocated to a task.  A value of zero means that either no guard area is created or a minimal guard area is created, depending on the target.  This pragma can appear anywhere a Storage_Size attribute definition clause is allowed for a task type.

Pragma Test_Case
================

Syntax:

     pragma Test_Case (
        [Name     =>] static_string_Expression
       ,[Mode     =>] (Nominal | Robustness)
      [, Requires =>  Boolean_Expression]
      [, Ensures  =>  Boolean_Expression]);

The Test_Case pragma allows defining fine-grain specifications for use by testing and verification tools. The compiler checks its validity but the presence of pragma Test_Case does not lead to any modification of the code generated by the compiler.

Test_Case pragmas may only appear immediately following the (separate) declaration of a subprogram in a package declaration, inside a package spec unit. Only other pragmas may intervene (that is appear between the subprogram declaration and a test case).

The compiler checks that boolean expressions given in Requires and Ensures are valid, where the rules for Requires are the same as the rule for an expression in Precondition and the rules for Ensures are the same as the rule for an expression in Postcondition. In particular, attributes 'Old and 'Result can only be used within the Ensures expression. The following is an example of use within a package spec:

     package Math_Functions is
        ...
        function Sqrt (Arg : Float) return Float;
        pragma Test_Case (Name     => "Test 1",
                          Mode     => Nominal,
                          Requires => Arg < 100,
                          Ensures  => Sqrt'Result < 10);
        ...
     end Math_Functions;

The meaning of a test case is that, if the associated subprogram is executed in a context where Requires holds, then Ensures should hold when the subprogram returns. Mode Nominal indicates that the input context should satisfy the precondition of the subprogram, and the output context should then satisfy its postcondition. More Robustness indicates that the pre- and postcondition of the subprogram should be ignored for this test case.

Pragma Thread_Local_Storage
===========================

Syntax:

     pragma Thread_Local_Storage ([Entity =>] LOCAL_NAME);

This pragma specifies that the specified entity, which must be a variable declared in a library level package, is to be marked as "Thread Local Storage" (TLS). On systems supporting this (which include Solaris, GNU/Linux and VxWorks 6), this causes each thread (and hence each Ada task) to see a distinct copy of the variable.

The variable may not have default initialization, and if there is an explicit initialization, it must be either null for an access variable, or a static expression for a scalar variable. This provides a low level mechanism similar to that provided by the Ada.Task_Attributes package, but much more efficient and is also useful in writing interface code that will interact with foreign threads.

If this pragma is used on a system where TLS is not supported, then an error message will be generated and the program will be rejected.

Pragma Time_Slice
=================

Syntax:

     pragma Time_Slice (static_duration_EXPRESSION);

For implementations of GNAT on operating systems where it is possible to supply a time slice value, this pragma may be used for this purpose. It is ignored if it is used in a system that does not allow this control, or if it appears in other than the main program unit. Note that the effect of this pragma is identical to the effect of the DEC Ada 83 pragma of the same name when operating under OpenVMS systems.

Pragma Title
============

Syntax:

     pragma Title (TITLING_OPTION [, TITLING OPTION]);

     TITLING_OPTION ::=
       [Title    =>] STRING_LITERAL,
     | [Subtitle =>] STRING_LITERAL

Syntax checked but otherwise ignored by GNAT.  This is a listing control pragma used in DEC Ada 83 implementations to provide a title and/or subtitle for the program listing.  The program listing generated by GNAT does not have titles or subtitles.

Unlike other pragmas, the full flexibility of named notation is allowed for this pragma, i.e. the parameters may be given in any order if named notation is used, and named and positional notation can be mixed following the normal rules for procedure calls in Ada.

Pragma Unchecked_Union
======================

Syntax:

     pragma Unchecked_Union (first_subtype_LOCAL_NAME);

This pragma is used to specify a representation of a record type that is equivalent to a C union. It was introduced as a GNAT implementation defined pragma in the GNAT Ada 95 mode. Ada 2005 includes an extended version of this pragma, making it language defined, and GNAT fully implements this extended version in all language modes (Ada 83, Ada 95, and Ada 2005). For full details, consult the Ada 2005 Reference Manual, section B.3.3.

Pragma Unimplemented_Unit
=========================

Syntax:

     pragma Unimplemented_Unit;

If this pragma occurs in a unit that is processed by the compiler, GNAT aborts with the message XXX not implemented, where XXX is the name of the current compilation unit.  This pragma is intended to allow the compiler to handle unimplemented library units in a clean manner.

The abort only happens if code is being generated.  Thus you can use specs of unimplemented packages in syntax or semantic checking mode.

Pragma Universal_Aliasing
=========================

Syntax:

     pragma Universal_Aliasing [([Entity =>] type_LOCAL_NAME)];

TYPE_LOCAL_NAME must refer to a type declaration in the current declarative part.  The effect is to inhibit strict type-based aliasing optimization for the given type.  In other words, the effect is as though access types designating this type were subject to pragma No_Strict_Aliasing. For a detailed description of the strict aliasing optimization, and the situations in which it must be suppressed, see Optimization and Strict Aliasing (GNAT User's Guide).

Pragma Universal_Data
=====================

Syntax:

     pragma Universal_Data [(library_unit_Name)];

This pragma is supported only for the AAMP target and is ignored for other targets. The pragma specifies that all library-level objects (Counter 0 data) associated with the library unit are to be accessed and updated using universal addressing (24-bit addresses for AAMP5) rather than the default of 16-bit Data Environment (DENV) addressing. Use of this pragma will generally result in less efficient code for references to global data associated with the library unit, but allows such data to be located anywhere in memory. This pragma is a library unit pragma, but can also be used as a configuration pragma (including use in the gnat.adc file). The functionality of this pragma is also available by applying the -univ switch on the compilations of units where universal addressing of the data is desired.

Pragma Unmodified
=================

Syntax:

     pragma Unmodified (LOCAL_NAME {, LOCAL_NAME});

This pragma signals that the assignable entities (variables, out parameters, in out parameters) whose names are listed are deliberately not assigned in the current source unit. This suppresses warnings about the entities being referenced but not assigned, and in addition a warning will be generated if one of these entities is in fact assigned in the same unit as the pragma (or in the corresponding body, or one of its subunits).

This is particularly useful for clearly signaling that a particular parameter is not modified, even though the spec suggests that it might be.

Pragma Unreferenced
===================

Syntax:

     pragma Unreferenced (LOCAL_NAME {, LOCAL_NAME});
     pragma Unreferenced (library_unit_NAME {, library_unit_NAME});

This pragma signals that the entities whose names are listed are deliberately not referenced in the current source unit. This suppresses warnings about the entities being unreferenced, and in addition a warning will be generated if one of these entities is in fact subsequently referenced in the same unit as the pragma (or in the corresponding body, or one of its subunits).

This is particularly useful for clearly signaling that a particular parameter is not referenced in some particular subprogram implementation and that this is deliberate. It can also be useful in the case of objects declared only for their initialization or finalization side effects.

If LOCAL_NAME identifies more than one matching homonym in the current scope, then the entity most recently declared is the one to which the pragma applies. Note that in the case of accept formals, the pragma Unreferenced may appear immediately after the keyword do which allows the indication of whether or not accept formals are referenced or not to be given individually for each accept statement.

The left hand side of an assignment does not count as a reference for the purpose of this pragma. Thus it is fine to assign to an entity for which pragma Unreferenced is given.

Note that if a warning is desired for all calls to a given subprogram, regardless of whether they occur in the same unit as the subprogram declaration, then this pragma should not be used (calls from another unit would not be flagged); pragma Obsolescent can be used instead for this purpose, see Pragma Obsolescent.

The second form of pragma Unreferenced is used within a context clause. In this case the arguments must be unit names of units previously mentioned in with clauses (similar to the usage of pragma Elaborate_All. The effect is to suppress warnings about unreferenced units and unreferenced entities within these units.

Pragma Unreferenced_Objects
===========================

Syntax:

     pragma Unreferenced_Objects (local_subtype_NAME {, local_subtype_NAME});

This pragma signals that for the types or subtypes whose names are listed, objects which are declared with one of these types or subtypes may not be referenced, and if no references appear, no warnings are given.

This is particularly useful for objects which are declared solely for their initialization and finalization effect. Such variables are sometimes referred to as RAII variables (Resource Acquisition Is Initialization). Using this pragma on the relevant type (most typically a limited controlled type), the compiler will automatically suppress unwanted warnings about these variables not being referenced.

Pragma Unreserve_All_Interrupts
===============================

Syntax:

     pragma Unreserve_All_Interrupts;

Normally certain interrupts are reserved to the implementation.  Any attempt to attach an interrupt causes Program_Error to be raised, as described in RM C.3.2(22).  A typical example is the SIGINT interrupt used in many systems for a Ctrl-C interrupt.  Normally this interrupt is reserved to the implementation, so that Ctrl-C can be used to interrupt execution.

If the pragma Unreserve_All_Interrupts appears anywhere in any unit in a program, then all such interrupts are unreserved.  This allows the program to handle these interrupts, but disables their standard functions.  For example, if this pragma is used, then pressing Ctrl-C will not automatically interrupt execution.  However, a program can then handle the SIGINT interrupt as it chooses.

For a full list of the interrupts handled in a specific implementation, see the source code for the spec of Ada.Interrupts.Names in file a-intnam.ads.  This is a target dependent file that contains the list of interrupts recognized for a given target.  The documentation in this file also specifies what interrupts are affected by the use of the Unreserve_All_Interrupts pragma.

For a more general facility for controlling what interrupts can be handled, see pragma Interrupt_State, which subsumes the functionality of the Unreserve_All_Interrupts pragma.

Pragma Unsuppress
=================

Syntax:

     pragma Unsuppress (IDENTIFIER [, [On =>] NAME]);

This pragma undoes the effect of a previous pragma Suppress.  If there is no corresponding pragma Suppress in effect, it has no effect.  The range of the effect is the same as for pragma Suppress.  The meaning of the arguments is identical to that used in pragma Suppress.

One important application is to ensure that checks are on in cases where code depends on the checks for its correct functioning, so that the code will compile correctly even if the compiler switches are set to suppress checks.

Pragma Use_VADS_Size
====================

Syntax:

     pragma Use_VADS_Size;

This is a configuration pragma.  In a unit to which it applies, any use of the 'Size attribute is automatically interpreted as a use of the 'VADS_Size attribute.  Note that this may result in incorrect semantic processing of valid Ada 95 or Ada 2005 programs.  This is intended to aid in the handling of existing code which depends on the interpretation of Size as implemented in the VADS compiler.  See description of the VADS_Size attribute for further details.

Pragma Validity_Checks
======================

Syntax:

     pragma Validity_Checks (string_LITERAL | ALL_CHECKS | On | Off);

This pragma is used in conjunction with compiler switches to control the built-in validity checking provided by GNAT.  The compiler switches, if set provide an initial setting for the switches, and this pragma may be used to modify these settings, or the settings may be provided entirely by the use of the pragma.  This pragma can be used anywhere that a pragma is legal, including use as a configuration pragma (including use in the gnat.adc file).

The form with a string literal specifies which validity options are to be activated.  The validity checks are first set to include only the default reference manual settings, and then a string of letters in the string specifies the exact set of options required.  The form of this string is exactly as described for the -gnatVx compiler switch (see the GNAT users guide for details).  For example the following two methods can be used to enable validity checking for mode in and in out subprogram parameters:

  * pragma Validity_Checks ("im");

  * gcc -c -gnatVim ...

The form ALL_CHECKS activates all standard checks (its use is equivalent to the use of the gnatva switch.

The forms with Off and On can be used to temporarily disable validity checks as shown in the following example:

     pragma Validity_Checks ("c"); -- validity checks for copies
     pragma Validity_Checks (Off); -- turn off validity checks
     A := B;                       -- B will not be validity checked
     pragma Validity_Checks (On);  -- turn validity checks back on
     A := C;                       -- C will be validity checked

Pragma Volatile
===============

Syntax:

     pragma Volatile (LOCAL_NAME);

This pragma is defined by the Ada Reference Manual, and the GNAT implementation is fully conformant with this definition.  The reason it is mentioned in this section is that a pragma of the same name was supplied in some Ada 83 compilers, including DEC Ada 83.  The Ada 95 / Ada 2005 implementation of pragma Volatile is upwards compatible with the implementation in DEC Ada 83.

Pragma Warnings
===============

Syntax:

     pragma Warnings (On | Off);
     pragma Warnings (On | Off, LOCAL_NAME);
     pragma Warnings (static_string_EXPRESSION);
     pragma Warnings (On | Off, static_string_EXPRESSION);

Normally warnings are enabled, with the output being controlled by the command line switch.  Warnings (Off) turns off generation of warnings until a Warnings (On) is encountered or the end of the current unit.  If generation of warnings is turned off using this pragma, then no warning messages are output, regardless of the setting of the command line switches.

The form with a single argument may be used as a configuration pragma.

If the LOCAL_NAME parameter is present, warnings are suppressed for the specified entity.  This suppression is effective from the point where it occurs till the end of the extended scope of the variable (similar to the scope of Suppress).

The form with a single static_string_EXPRESSION argument provides more precise control over which warnings are active. The string is a list of letters specifying which warnings are to be activated and which deactivated. The code for these letters is the same as the string used in the command line switch controlling warnings. For a brief summary, use the gnatmake command with no arguments, which will generate usage information containing the list of warnings switches supported. For full details see Warning Message Control (GNAT User's Guide).

The specified warnings will be in effect until the end of the program or another pragma Warnings is encountered. The effect of the pragma is cumulative. Initially the set of warnings is the standard default set as possibly modified by compiler switches. Then each pragma Warning modifies this set of warnings as specified. This form of the pragma may also be used as a configuration pragma.

The fourth form, with an On|Off parameter and a string, is used to control individual messages, based on their text. The string argument is a pattern that is used to match against the text of individual warning messages (not including the initial "warning: " tag).

The pattern may contain asterisks, which match zero or more characters in the message. For example, you can use pragma Warnings (Off, "*bits of*unused") to suppress the warning message warning: 960 bits of "a" unused. No other regular expression notations are permitted. All characters other than asterisk in these three specific cases are treated as literal characters in the match.

There are two ways to use the pragma in this form. The OFF form can be used as a configuration pragma. The effect is to suppress all warnings (if any) that match the pattern string throughout the compilation.

The second usage is to suppress a warning locally, and in this case, two pragmas must appear in sequence:

     pragma Warnings (Off, Pattern);
     ... code where given warning is to be suppressed
     pragma Warnings (On, Pattern);

In this usage, the pattern string must match in the Off and On pragmas, and at least one matching warning must be suppressed.

Note: to write a string that will match any warning, use the string "***". It will not work to use a single asterisk or two asterisks since this looks like an operator name. This form with three asterisks is similar in effect to specifying pragma Warnings (Off) except that a matching pragma Warnings (On, "***") will be required. This can be helpful in avoiding forgetting to turn warnings back on.

Note: the debug flag -gnatd.i (/NOWARNINGS_PRAGMAS in VMS) can be used to cause the compiler to entirely ignore all WARNINGS pragmas. This can be useful in checking whether obsolete pragmas in existing programs are hiding real problems.

Note: pragma Warnings does not affect the processing of style messages. See separate entry for pragma Style_Checks for control of style messages.

Pragma Weak_External
====================

Syntax:

     pragma Weak_External ([Entity =>] LOCAL_NAME);

LOCAL_NAME must refer to an object that is declared at the library level. This pragma specifies that the given entity should be marked as a weak symbol for the linker. It is equivalent to __attribute__((weak)) in GNU C and causes LOCAL_NAME to be emitted as a weak symbol instead of a regular symbol, that is to say a symbol that does not have to be resolved by the linker if used in conjunction with a pragma Import.

When a weak symbol is not resolved by the linker, its address is set to zero. This is useful in writing interfaces to external modules that may or may not be linked in the final executable, for example depending on configuration settings.

If a program references at run time an entity to which this pragma has been applied, and the corresponding symbol was not resolved at link time, then the execution of the program is erroneous. It is not erroneous to take the Address of such an entity, for example to guard potential references, as shown in the example below.

Some file formats do not support weak symbols so not all target machines support this pragma.

     --  Example of the use of pragma Weak_External

     package External_Module is
       key : Integer;
       pragma Import (C, key);
       pragma Weak_External (key);
       function Present return boolean;
     end External_Module;

     with System; use System;
     package body External_Module is
       function Present return boolean is
       begin
         return key'Address /= System.Null_Address;
       end Present;
     end External_Module;

Pragma Wide_Character_Encoding
==============================

Syntax:

     pragma Wide_Character_Encoding (IDENTIFIER | CHARACTER_LITERAL);

This pragma specifies the wide character encoding to be used in program source text appearing subsequently. It is a configuration pragma, but may also be used at any point that a pragma is allowed, and it is permissible to have more than one such pragma in a file, allowing multiple encodings to appear within the same file.

The argument can be an identifier or a character literal. In the identifier case, it is one of HEX, UPPER, SHIFT_JIS, EUC, UTF8, or BRACKETS. In the character literal case it is correspondingly one of the characters h, u, s, e, 8, or b.

Note that when the pragma is used within a file, it affects only the encoding within that file, and does not affect withed units, specs, or subunits.

Implementation Defined Attributes
*********************************

Ada defines (throughout the Ada reference manual, summarized in Annex K), a set of attributes that provide useful additional functionality in all areas of the language.  These language defined attributes are implemented in GNAT and work as described in the Ada Reference Manual.

In addition, Ada allows implementations to define additional attributes whose meaning is defined by the implementation.  GNAT provides a number of these implementation-dependent attributes which can be used to extend and enhance the functionality of the compiler.  This section of the GNAT reference manual describes these additional attributes.

Note that any program using these attributes may not be portable to other compilers (although GNAT implements this set of attributes on all platforms).  Therefore if portability to other compilers is an important consideration, you should minimize the use of these attributes.

Abort_Signal
============

Standard'Abort_Signal (Standard is the only allowed prefix) provides the entity for the special exception used to signal task abort or asynchronous transfer of control.  Normally this attribute should only be used in the tasking runtime (it is highly peculiar, and completely outside the normal semantics of Ada, for a user program to intercept the abort exception).

Address_Size
============

Standard'Address_Size (Standard is the only allowed prefix) is a static constant giving the number of bits in an Address. It is the same value as System.Address'Size, but has the advantage of being static, while a direct reference to System.Address'Size is non-static because Address is a private type.

Asm_Input
=========

The Asm_Input attribute denotes a function that takes two parameters.  The first is a string, the second is an expression of the type designated by the prefix.  The first (string) argument is required to be a static expression, and is the constraint for the parameter, (e.g. what kind of register is required).  The second argument is the value to be used as the input argument.  The possible values for the constant are the same as those used in the RTL, and are dependent on the configuration file used to built the GCC back end. Machine Code Insertions

Asm_Output
==========

The Asm_Output attribute denotes a function that takes two parameters.  The first is a string, the second is the name of a variable of the type designated by the attribute prefix.  The first (string) argument is required to be a static expression and designates the constraint for the parameter (e.g. what kind of register is required).  The second argument is the variable to be updated with the result.  The possible values for constraint are the same as those used in the RTL, and are dependent on the configuration file used to build the GCC back end.  If there are no output operands, then this argument may either be omitted, or explicitly given as No_Output_Operands. Machine Code Insertions

AST_Entry
=========

This attribute is implemented only in OpenVMS versions of GNAT.  Applied to the name of an entry, it yields a value of the predefined type AST_Handler (declared in the predefined package System, as extended by the use of pragma Extend_System (Aux_DEC)).  This value enables the given entry to be called when an AST occurs.  For further details, refer to the DEC Ada Language Reference Manual, section 9.12a.

Bit
===

OBJ'Bit, where OBJ is any object, yields the bit offset within the storage unit (byte) that contains the first bit of storage allocated for the object.  The value of this attribute is of the type Universal_Integer, and is always a non-negative number not exceeding the value of System.Storage_Unit.

For an object that is a variable or a constant allocated in a register, the value is zero.  (The use of this attribute does not force the allocation of a variable to memory).

For an object that is a formal parameter, this attribute applies to either the matching actual parameter or to a copy of the matching actual parameter.

For an access object the value is zero.  Note that OBJ.all'Bit is subject to an Access_Check for the designated object.  Similarly for a record component X.C'Bit is subject to a discriminant check and X(I).Bit and X(I1..I2)'Bit are subject to index checks.

This attribute is designed to be compatible with the DEC Ada 83 definition and implementation of the Bit attribute.

Bit_Position
============

R.C'Bit_Position, where R is a record object and C is one of the fields of the record type, yields the bit offset within the record contains the first bit of storage allocated for the object.  The value of this attribute is of the type Universal_Integer.  The value depends only on the field C and is independent of the alignment of the containing record R.

Compiler_Version
================

Standard'Compiler_Version (Standard is the only allowed prefix) yields a static string identifying the version of the compiler being used to compile the unit containing the attribute reference. A typical result would be something like "GNAT version (20090221)".

Code_Address
============

The 'Address attribute may be applied to subprograms in Ada 95 and Ada 2005, but the intended effect seems to be to provide an address value which can be used to call the subprogram by means of an address clause as in the following example:

     procedure K is ...

     procedure L;
     for L'Address use K'Address;
     pragma Import (Ada, L);

A call to L is then expected to result in a call to K. In Ada 83, where there were no access-to-subprogram values, this was a common work-around for getting the effect of an indirect call. GNAT implements the above use of Address and the technique illustrated by the example code works correctly.

However, for some purposes, it is useful to have the address of the start of the generated code for the subprogram.  On some architectures, this is not necessarily the same as the Address value described above. For example, the Address value may reference a subprogram descriptor rather than the subprogram itself.

The 'Code_Address attribute, which can only be applied to subprogram entities, always returns the address of the start of the generated code of the specified subprogram, which may or may not be the same value as is returned by the corresponding 'Address attribute.

Default_Bit_Order
=================

Standard'Default_Bit_Order (Standard is the only permissible prefix), provides the value System.Default_Bit_Order as a Pos value (0 for High_Order_First, 1 for Low_Order_First).  This is used to construct the definition of Default_Bit_Order in package System.

Descriptor_Size
===============

Non-static attribute Descriptor_Size returns the size in bits of the descriptor allocated for a type.  The result is non-zero only for unconstrained array types and the returned value is of type universal integer.  In GNAT, an array descriptor contains bounds information and is located immediately before the first element of the array.

     type Unconstr_Array is array (Positive range <>) of Boolean;
     Put_Line ("Descriptor size = " & Unconstr_Array'Descriptor_Size'Img);

The attribute takes into account any additional padding due to type alignment. In the example above, the descriptor contains two values of type Positive representing the low and high bound.  Since Positive has a size of 31 bits and an alignment of 4, the descriptor size is 2 * Positive'Size + 2 or 64 bits.

Elaborated
==========

The prefix of the 'Elaborated attribute must be a unit name.  The value is a Boolean which indicates whether or not the given unit has been elaborated.  This attribute is primarily intended for internal use by the generated code for dynamic elaboration checking, but it can also be used in user programs.  The value will always be True once elaboration of all units has been completed.  An exception is for units which need no elaboration, the value is always False for such units.

Elab_Body
=========

This attribute can only be applied to a program unit name.  It returns the entity for the corresponding elaboration procedure for elaborating the body of the referenced unit.  This is used in the main generated elaboration procedure by the binder and is not normally used in any other context.  However, there may be specialized situations in which it is useful to be able to call this elaboration procedure from Ada code, e.g. if it is necessary to do selective re-elaboration to fix some error.

Elab_Spec
=========

This attribute can only be applied to a program unit name.  It returns the entity for the corresponding elaboration procedure for elaborating the spec of the referenced unit.  This is used in the main generated elaboration procedure by the binder and is not normally used in any other context.  However, there may be specialized situations in which it is useful to be able to call this elaboration procedure from Ada code, e.g. if it is necessary to do selective re-elaboration to fix some error.

Elab_Subp_Body
==============

This attribute can only be applied to a library level subprogram name and is only allowed in CodePeer mode. It returns the entity for the corresponding elaboration procedure for elaborating the body of the referenced subprogram unit. This is used in the main generated elaboration procedure by the binder in CodePeer mode only and is unrecognized otherwise.

Emax
====

The Emax attribute is provided for compatibility with Ada 83.  See the Ada 83 reference manual for an exact description of the semantics of this attribute.

Enabled
=======

The Enabled attribute allows an application program to check at compile time to see if the designated check is currently enabled. The prefix is a simple identifier, referencing any predefined check name (other than All_Checks) or a check name introduced by pragma Check_Name. If no argument is given for the attribute, the check is for the general state of the check, if an argument is given, then it is an entity name, and the check indicates whether an Suppress or Unsuppress has been given naming the entity (if not, then the argument is ignored).

Note that instantiations inherit the check status at the point of the instantiation, so a useful idiom is to have a library package that introduces a check name with pragma Check_Name, and then contains generic packages or subprograms which use the Enabled attribute to see if the check is enabled. A user of this package can then issue a pragma Suppress or pragma Unsuppress before instantiating the package or subprogram, controlling whether the check will be present.

Enum_Rep
========

For every enumeration subtype S, S'Enum_Rep denotes a function with the following spec:

     function S'Enum_Rep (Arg : S'Base)
       return Universal_Integer;

It is also allowable to apply Enum_Rep directly to an object of an enumeration type or to a non-overloaded enumeration literal.  In this case S'Enum_Rep is equivalent to TYP'Enum_Rep(S) where TYP is the type of the enumeration literal or object.

The function returns the representation value for the given enumeration value.  This will be equal to value of the Pos attribute in the absence of an enumeration representation clause.  This is a static attribute (i.e. the result is static if the argument is static).

S'Enum_Rep can also be used with integer types and objects, in which case it simply returns the integer value.  The reason for this is to allow it to be used for (<>) discrete formal arguments in a generic unit that can be instantiated with either enumeration types or integer types.  Note that if Enum_Rep is used on a modular type whose upper bound exceeds the upper bound of the largest signed integer type, and the argument is a variable, so that the universal integer calculation is done at run time, then the call to Enum_Rep may raise Constraint_Error.

Enum_Val
========

For every enumeration subtype S, S'Enum_Val denotes a function with the following spec:

     function S'Enum_Val (Arg : Universal_Integer)
       return S'Base;

The function returns the enumeration value whose representation matches the argument, or raises Constraint_Error if no enumeration literal of the type has the matching value. This will be equal to value of the Val attribute in the absence of an enumeration representation clause.  This is a static attribute (i.e. the result is static if the argument is static).

Epsilon
=======

The Epsilon attribute is provided for compatibility with Ada 83.  See the Ada 83 reference manual for an exact description of the semantics of this attribute.

Fixed_Value
===========

For every fixed-point type S, S'Fixed_Value denotes a function with the following specification:

     function S'Fixed_Value (Arg : Universal_Integer)
       return S;

The value returned is the fixed-point value V such that

     V = Arg * S'Small

The effect is thus similar to first converting the argument to the integer type used to represent S, and then doing an unchecked conversion to the fixed-point type.  The difference is that there are full range checks, to ensure that the result is in range. This attribute is primarily intended for use in implementation of the input-output functions for fixed-point values.

Has_Access_Values
=================

The prefix of the Has_Access_Values attribute is a type.  The result is a Boolean value which is True if the is an access type, or is a composite type with a component (at any nesting depth) that is an access type, and is False otherwise. The intended use of this attribute is in conjunction with generic definitions.  If the attribute is applied to a generic private type, it indicates whether or not the corresponding actual type has access values.

Has_Discriminants
=================

The prefix of the Has_Discriminants attribute is a type.  The result is a Boolean value which is True if the type has discriminants, and False otherwise.  The intended use of this attribute is in conjunction with generic definitions.  If the attribute is applied to a generic private type, it indicates whether or not the corresponding actual type has discriminants.

Img
===

The Img attribute differs from Image in that it may be applied to objects as well as types, in which case it gives the Image for the subtype of the object.  This is convenient for debugging:

     Put_Line ("X = " & X'Img);

has the same meaning as the more verbose:

     Put_Line ("X = " & T'Image (X));

where T is the (sub)type of the object X.

Integer_Value
=============

For every integer type S, S'Integer_Value denotes a function with the following spec:

     function S'Integer_Value (Arg : Universal_Fixed)
       return S;

The value returned is the integer value V, such that

     Arg = V * T'Small

where T is the type of Arg. The effect is thus similar to first doing an unchecked conversion from the fixed-point type to its corresponding implementation type, and then converting the result to the target integer type.  The difference is that there are full range checks, to ensure that the result is in range. This attribute is primarily intended for use in implementation of the standard input-output functions for fixed-point values.

Invalid_Value
=============

For every scalar type S, S'Invalid_Value returns an undefined value of the type. If possible this value is an invalid representation for the type. The value returned is identical to the value used to initialize an otherwise uninitialized value of the type if pragma Initialize_Scalars is used, including the ability to modify the value with the binder -Sxx flag and relevant environment variables at run time.

Large
=====

The Large attribute is provided for compatibility with Ada 83.  See the Ada 83 reference manual for an exact description of the semantics of this attribute.

Machine_Size
============

This attribute is identical to the Object_Size attribute.  It is provided for compatibility with the DEC Ada 83 attribute of this name.

Mantissa
========

The Mantissa attribute is provided for compatibility with Ada 83.  See the Ada 83 reference manual for an exact description of the semantics of this attribute.

Max_Interrupt_Priority
======================

Standard'Max_Interrupt_Priority (Standard is the only permissible prefix), provides the same value as System.Max_Interrupt_Priority.

Max_Priority
============

Standard'Max_Priority (Standard is the only permissible prefix) provides the same value as System.Max_Priority.

Maximum_Alignment
=================

Standard'Maximum_Alignment (Standard is the only permissible prefix) provides the maximum useful alignment value for the target.  This is a static value that can be used to specify the alignment for an object, guaranteeing that it is properly aligned in all cases.

Mechanism_Code
==============

FUNCTION'Mechanism_Code yields an integer code for the mechanism used for the result of function, and SUBPROGRAM'Mechanism_Code (N) yields the mechanism used for formal parameter number N (a static integer value with 1 meaning the first parameter) of SUBPROGRAM.  The code returned is:

1
     by copy (value)

2
     by reference

3
     by descriptor (default descriptor class)

4
     by descriptor (UBS: unaligned bit string)

5
     by descriptor (UBSB: aligned bit string with arbitrary bounds)

6
     by descriptor (UBA: unaligned bit array)

7
     by descriptor (S: string, also scalar access type parameter)

8
     by descriptor (SB: string with arbitrary bounds)

9
     by descriptor (A: contiguous array)

10
     by descriptor (NCA: non-contiguous array)

Values from 3 through 10 are only relevant to Digital OpenVMS implementations.

Null_Parameter
==============

A reference T'Null_Parameter denotes an imaginary object of type or subtype T allocated at machine address zero.  The attribute is allowed only as the default expression of a formal parameter, or as an actual expression of a subprogram call.  In either case, the subprogram must be imported.

The identity of the object is represented by the address zero in the argument list, independent of the passing mechanism (explicit or default).

This capability is needed to specify that a zero address should be passed for a record or other composite object passed by reference. There is no way of indicating this without the Null_Parameter attribute.

Object_Size
===========

The size of an object is not necessarily the same as the size of the type of an object.  This is because by default object sizes are increased to be a multiple of the alignment of the object.  For example, Natural'Size is 31, but by default objects of type Natural will have a size of 32 bits. Similarly, a record containing an integer and a character:

     type Rec is record
        I : Integer;
        C : Character;
     end record;

will have a size of 40 (that is Rec'Size will be 40).  The alignment will be 4, because of the integer field, and so the default size of record objects for this type will be 64 (8 bytes).

Old
===

The attribute Prefix'Old can be used within a subprogram body or within a precondition or postcondition pragma. The effect is to refer to the value of the prefix on entry. So for example if you have an argument of a record type X called Arg1, you can refer to Arg1.Field'Old which yields the value of Arg1.Field on entry. The implementation simply involves generating an object declaration which captures the value on entry. Any prefix is allowed except one of a limited type (since limited types cannot be copied to capture their values) or an expression which references a local variable (since local variables do not exist at subprogram entry time).

The following example shows the use of 'Old to implement a test of a postcondition:

     with Old_Pkg;
     procedure Old is
     begin
        Old_Pkg.Incr;
     end Old;

     package Old_Pkg is
        procedure Incr;
     end Old_Pkg;

     package body Old_Pkg is
        Count : Natural := 0;

        procedure Incr is
        begin
           ... code manipulating the value of Count

           pragma Assert (Count = Count'Old + 1);
        end Incr;
     end Old_Pkg;

Note that it is allowed to apply 'Old to a constant entity, but this will result in a warning, since the old and new values will always be the same.

Passed_By_Reference
===================

TYPE'Passed_By_Reference for any subtype TYPE returns a value of type Boolean value that is True if the type is normally passed by reference and False if the type is normally passed by copy in calls.  For scalar types, the result is always False and is static.  For non-scalar types, the result is non-static.

Pool_Address
============

X'Pool_Address for any object X returns the address of X within its storage pool. This is the same as X'Address, except that for an unconstrained array whose bounds are allocated just before the first component, X'Pool_Address returns the address of those bounds, whereas X'Address returns the address of the first component.

Here, we are interpreting ``storage pool'' broadly to mean ``wherever the object is allocated'', which could be a user-defined storage pool, the global heap, on the stack, or in a static memory area. For an object created by new, PTR.ALL'Pool_Address is what is passed to Allocate and returned from Deallocate.

Range_Length
============

TYPE'Range_Length for any discrete type TYPE yields the number of values represented by the subtype (zero for a null range).  The result is static for static subtypes.  Range_Length applied to the index subtype of a one dimensional array always gives the same result as Range applied to the array itself.

Ref
===

The System.Address'Ref (System.Address is the only permissible prefix) denotes a function identical to System.Storage_Elements.To_Address except that it is a static attribute.  See To_Address for more details.

Result
======

FUNCTION'Result can only be used with in a Postcondition pragma for a function. The prefix must be the name of the corresponding function. This is used to refer to the result of the function in the postcondition expression. For a further discussion of the use of this attribute and examples of its use, see the description of pragma Postcondition.

Safe_Emax
=========

The Safe_Emax attribute is provided for compatibility with Ada 83.  See the Ada 83 reference manual for an exact description of the semantics of this attribute.

Safe_Large
==========

The Safe_Large attribute is provided for compatibility with Ada 83.  See the Ada 83 reference manual for an exact description of the semantics of this attribute.

Small
=====

The Small attribute is defined in Ada 95 (and Ada 2005) only for fixed-point types. GNAT also allows this attribute to be applied to floating-point types for compatibility with Ada 83.  See the Ada 83 reference manual for an exact description of the semantics of this attribute when applied to floating-point types.

Storage_Unit
============

Standard'Storage_Unit (Standard is the only permissible prefix) provides the same value as System.Storage_Unit.

Stub_Type
=========

The GNAT implementation of remote access-to-classwide types is organized as described in AARM section E.4 (20.t): a value of an RACW type (designating a remote object) is represented as a normal access value, pointing to a "stub" object which in turn contains the necessary information to contact the designated remote object. A call on any dispatching operation of such a stub object does the remote call, if necessary, using the information in the stub object to locate the target partition, etc.

For a prefix T that denotes a remote access-to-classwide type, T'Stub_Type denotes the type of the corresponding stub objects.

By construction, the layout of T'Stub_Type is identical to that of type RACW_Stub_Type declared in the internal implementation-defined unit System.Partition_Interface. Use of this attribute will create an implicit dependency on this unit.

System_Allocator_Alignment
==========================

Standard'System_Allocator_Alignment (Standard is the only permissible prefix) provides the observable guaranted to be honored by the system allocator (malloc). This is a static value that can be used in user storage pools based on malloc either to reject allocation with alignment too large or to enable a realignment circuitry if the alignment request is larger than this value.

Target_Name
===========

Standard'Target_Name (Standard is the only permissible prefix) provides a static string value that identifies the target for the current compilation. For GCC implementations, this is the standard gcc target name without the terminating slash (for example, GNAT 5.0 on windows yields "i586-pc-mingw32msv").

Tick
====

Standard'Tick (Standard is the only permissible prefix) provides the same value as System.Tick,

To_Address
==========

The System'To_Address (System is the only permissible prefix) denotes a function identical to System.Storage_Elements.To_Address except that it is a static attribute.  This means that if its argument is a static expression, then the result of the attribute is a static expression.  The result is that such an expression can be used in contexts (e.g. preelaborable packages) which require a static expression and where the function call could not be used (since the function call is always non-static, even if its argument is static).

Type_Class
==========

TYPE'Type_Class for any type or subtype TYPE yields the value of the type class for the full type of TYPE.  If TYPE is a generic formal type, the value is the value for the corresponding actual subtype.  The value of this attribute is of type System.Aux_DEC.Type_Class, which has the following definition:

       type Type_Class is
         (Type_Class_Enumeration,
          Type_Class_Integer,
          Type_Class_Fixed_Point,
          Type_Class_Floating_Point,
          Type_Class_Array,
          Type_Class_Record,
          Type_Class_Access,
          Type_Class_Task,
          Type_Class_Address);

Protected types yield the value Type_Class_Task, which thus applies to all concurrent types.  This attribute is designed to be compatible with the DEC Ada 83 attribute of the same name.

UET_Address
===========

The UET_Address attribute can only be used for a prefix which denotes a library package.  It yields the address of the unit exception table when zero cost exception handling is used.  This attribute is intended only for use within the GNAT implementation.  See the unit Ada.Exceptions in files a-except.ads and a-except.adb for details on how this attribute is used in the implementation.

Unconstrained_Array
===================

The Unconstrained_Array attribute can be used with a prefix that denotes any type or subtype. It is a static attribute that yields True if the prefix designates an unconstrained array, and False otherwise. In a generic instance, the result is still static, and yields the result of applying this test to the generic actual.

Universal_Literal_String
========================

The prefix of Universal_Literal_String must be a named number.  The static result is the string consisting of the characters of the number as defined in the original source.  This allows the user program to access the actual text of named numbers without intermediate conversions and without the need to enclose the strings in quotes (which would preclude their use as numbers).

For example, the following program prints the first 50 digits of pi:

     with Text_IO; use Text_IO;
     with Ada.Numerics;
     procedure Pi is
     begin
        Put (Ada.Numerics.Pi'Universal_Literal_String);
     end;

Unrestricted_Access
===================

The Unrestricted_Access attribute is similar to Access except that all accessibility and aliased view checks are omitted.  This is a user-beware attribute.  It is similar to Address, for which it is a desirable replacement where the value desired is an access type.  In other words, its effect is identical to first applying the Address attribute and then doing an unchecked conversion to a desired access type.  In GNAT, but not necessarily in other implementations, the use of static chains for inner level subprograms means that Unrestricted_Access applied to a subprogram yields a value that can be called as long as the subprogram is in scope (normal Ada accessibility rules restrict this usage).

It is possible to use Unrestricted_Access for any type, but care must be exercised if it is used to create pointers to unconstrained objects. In this case, the resulting pointer has the same scope as the context of the attribute, and may not be returned to some enclosing scope. For instance, a function cannot use Unrestricted_Access to create a unconstrained pointer and then return that value to the caller.

VADS_Size
=========

The 'VADS_Size attribute is intended to make it easier to port legacy code which relies on the semantics of 'Size as implemented by the VADS Ada 83 compiler.  GNAT makes a best effort at duplicating the same semantic interpretation.  In particular, 'VADS_Size applied to a predefined or other primitive type with no Size clause yields the Object_Size (for example, Natural'Size is 32 rather than 31 on typical machines).  In addition 'VADS_Size applied to an object gives the result that would be obtained by applying the attribute to the corresponding type.

Value_Size
==========

TYPE'Value_Size is the number of bits required to represent a value of the given subtype.  It is the same as TYPE'Size, but, unlike Size, may be set for non-first subtypes.

Wchar_T_Size
============

Standard'Wchar_T_Size (Standard is the only permissible prefix) provides the size in bits of the C wchar_t type primarily for constructing the definition of this type in package Interfaces.C.

Word_Size
=========

Standard'Word_Size (Standard is the only permissible prefix) provides the value System.Word_Size.

Implementation Advice
*********************

The main text of the Ada Reference Manual describes the required behavior of all Ada compilers, and the GNAT compiler conforms to these requirements.

In addition, there are sections throughout the Ada Reference Manual headed by the phrase ``Implementation advice''.  These sections are not normative, i.e., they do not specify requirements that all compilers must follow.  Rather they provide advice on generally desirable behavior.  You may wonder why they are not requirements.  The most typical answer is that they describe behavior that seems generally desirable, but cannot be provided on all systems, or which may be undesirable on some systems.

As far as practical, GNAT follows the implementation advice sections in the Ada Reference Manual.  This chapter contains a table giving the reference manual section number, paragraph number and several keywords for each advice.  Each entry consists of the text of the advice followed by the GNAT interpretation of this advice.  Most often, this simply says ``followed'', which means that GNAT follows the advice.  However, in a number of cases, GNAT deliberately deviates from this advice, in which case the text describes what GNAT does and why.

1.1.3(20): Error Detection
==========================

If an implementation detects the use of an unsupported Specialized Needs Annex feature at run time, it should raise Program_Error if feasible.

Not relevant.  All specialized needs annex features are either supported, or diagnosed at compile time.

1.1.3(31): Child Units
======================

If an implementation wishes to provide implementation-defined extensions to the functionality of a language-defined library unit, it should normally do so by adding children to the library unit.

Followed.

1.1.5(12): Bounded Errors
=========================

If an implementation detects a bounded error or erroneous execution, it should raise Program_Error.

Followed in all cases in which the implementation detects a bounded error or erroneous execution.  Not all such situations are detected at runtime.

2.8(16): Pragmas
================

Normally, implementation-defined pragmas should have no semantic effect for error-free programs; that is, if the implementation-defined pragmas are removed from a working program, the program should still be legal, and should still have the same semantics.

The following implementation defined pragmas are exceptions to this rule:

Abort_Defer
     Affects semantics

Ada_83
     Affects legality

Assert
     Affects semantics

CPP_Class
     Affects semantics

CPP_Constructor
     Affects semantics

Debug
     Affects semantics

Interface_Name
     Affects semantics

Machine_Attribute
     Affects semantics

Unimplemented_Unit
     Affects legality

Unchecked_Union
     Affects semantics

In each of the above cases, it is essential to the purpose of the pragma that this advice not be followed.  For details see the separate section on implementation defined pragmas.

2.8(17-19): Pragmas
===================

Normally, an implementation should not define pragmas that can make an illegal program legal, except as follows:

A pragma used to complete a declaration, such as a pragma Import;

A pragma used to configure the environment by adding, removing, or replacing library_items.

See response to paragraph 16 of this same section.

3.5.2(5): Alternative Character Sets
====================================

If an implementation supports a mode with alternative interpretations for Character and Wide_Character, the set of graphic characters of Character should nevertheless remain a proper subset of the set of graphic characters of Wide_Character.  Any character set ``localizations'' should be reflected in the results of the subprograms defined in the language-defined package Characters.Handling (see A.3) available in such a mode.  In a mode with an alternative interpretation of Character, the implementation should also support a corresponding change in what is a legal identifier_letter.

Not all wide character modes follow this advice, in particular the JIS and IEC modes reflect standard usage in Japan, and in these encoding, the upper half of the Latin-1 set is not part of the wide-character subset, since the most significant bit is used for wide character encoding.  However, this only applies to the external forms.  Internally there is no such restriction.

3.5.4(28): Integer Types
========================

An implementation should support Long_Integer in addition to Integer if the target machine supports 32-bit (or longer) arithmetic.  No other named integer subtypes are recommended for package Standard.  Instead, appropriate named integer subtypes should be provided in the library package Interfaces (see B.2).

Long_Integer is supported.  Other standard integer types are supported so this advice is not fully followed.  These types are supported for convenient interface to C, and so that all hardware types of the machine are easily available.

3.5.4(29): Integer Types
========================

An implementation for a two's complement machine should support modular types with a binary modulus up to System.Max_Int*2+2.  An implementation should support a non-binary modules up to Integer'Last.

Followed.

3.5.5(8): Enumeration Values
============================

For the evaluation of a call on S'Pos for an enumeration subtype, if the value of the operand does not correspond to the internal code for any enumeration literal of its type (perhaps due to an un-initialized variable), then the implementation should raise Program_Error.  This is particularly important for enumeration types with noncontiguous internal codes specified by an enumeration_representation_clause.

Followed.

3.5.7(17): Float Types
======================

An implementation should support Long_Float in addition to Float if the target machine supports 11 or more digits of precision.  No other named floating point subtypes are recommended for package Standard.  Instead, appropriate named floating point subtypes should be provided in the library package Interfaces (see B.2).

Short_Float and Long_Long_Float are also provided.  The former provides improved compatibility with other implementations supporting this type.  The latter corresponds to the highest precision floating-point type supported by the hardware.  On most machines, this will be the same as Long_Float, but on some machines, it will correspond to the IEEE extended form.  The notable case is all ia32 (x86) implementations, where Long_Long_Float corresponds to the 80-bit extended precision format supported in hardware on this processor.  Note that the 128-bit format on SPARC is not supported, since this is a software rather than a hardware format.

3.6.2(11): Multidimensional Arrays
==================================

An implementation should normally represent multidimensional arrays in row-major order, consistent with the notation used for multidimensional array aggregates (see 4.3.3).  However, if a pragma Convention (Fortran, ...) applies to a multidimensional array type, then column-major order should be used instead (see B.5, ``Interfacing with Fortran'').

Followed.

9.6(30-31): Duration'Small
==========================

Whenever possible in an implementation, the value of Duration'Small should be no greater than 100 microseconds.

Followed.  (Duration'Small = 10**(-9)).

The time base for delay_relative_statements should be monotonic; it need not be the same time base as used for Calendar.Clock.

Followed.

10.2.1(12): Consistent Representation
=====================================

In an implementation, a type declared in a pre-elaborated package should have the same representation in every elaboration of a given version of the package, whether the elaborations occur in distinct executions of the same program, or in executions of distinct programs or partitions that include the given version.

Followed, except in the case of tagged types.  Tagged types involve implicit pointers to a local copy of a dispatch table, and these pointers have representations which thus depend on a particular elaboration of the package.  It is not easy to see how it would be possible to follow this advice without severely impacting efficiency of execution.

11.4.1(19): Exception Information
=================================

Exception_Message by default and Exception_Information should produce information useful for debugging.  Exception_Message should be short, about one line.  Exception_Information can be long.  Exception_Message should not include the Exception_Name.  Exception_Information should include both the Exception_Name and the Exception_Message.

Followed.  For each exception that doesn't have a specified Exception_Message, the compiler generates one containing the location of the raise statement.  This location has the form ``file:line'', where file is the short file name (without path information) and line is the line number in the file.  Note that in the case of the Zero Cost Exception mechanism, these messages become redundant with the Exception_Information that contains a full backtrace of the calling sequence, so they are disabled. To disable explicitly the generation of the source location message, use the Pragma Discard_Names.

11.5(28): Suppression of Checks
===============================

The implementation should minimize the code executed for checks that have been suppressed.

Followed.

13.1 (21-24): Representation Clauses
====================================

The recommended level of support for all representation items is qualified as follows:

An implementation need not support representation items containing non-static expressions, except that an implementation should support a representation item for a given entity if each non-static expression in the representation item is a name that statically denotes a constant declared before the entity.

Followed.  In fact, GNAT goes beyond the recommended level of support by allowing nonstatic expressions in some representation clauses even without the need to declare constants initialized with the values of such expressions. For example:

       X : Integer;
       Y : Float;
       for Y'Address use X'Address;>>

An implementation need not support a specification for the Size for a given composite subtype, nor the size or storage place for an object (including a component) of a given composite subtype, unless the constraints on the subtype and its composite subcomponents (if any) are all static constraints.

Followed.  Size Clauses are not permitted on non-static components, as described above.

An aliased component, or a component whose type is by-reference, should always be allocated at an addressable location.

Followed.

13.2(6-8): Packed Types
=======================

If a type is packed, then the implementation should try to minimize storage allocated to objects of the type, possibly at the expense of speed of accessing components, subject to reasonable complexity in addressing calculations.

The recommended level of support pragma Pack is:

For a packed record type, the components should be packed as tightly as possible subject to the Sizes of the component subtypes, and subject to any record_representation_clause that applies to the type; the implementation may, but need not, reorder components or cross aligned word boundaries to improve the packing.  A component whose Size is greater than the word size may be allocated an integral number of words.

Followed.  Tight packing of arrays is supported for all component sizes up to 64-bits. If the array component size is 1 (that is to say, if the component is a boolean type or an enumeration type with two values) then values of the type are implicitly initialized to zero. This happens both for objects of the packed type, and for objects that have a subcomponent of the packed type.

An implementation should support Address clauses for imported subprograms.

Followed.

13.3(14-19): Address Clauses
============================

For an array X, X'Address should point at the first component of the array, and not at the array bounds.

Followed.

The recommended level of support for the Address attribute is:

X'Address should produce a useful result if X is an object that is aliased or of a by-reference type, or is an entity whose Address has been specified.

Followed.  A valid address will be produced even if none of those conditions have been met.  If necessary, the object is forced into memory to ensure the address is valid.

An implementation should support Address clauses for imported subprograms.

Followed.

Objects (including subcomponents) that are aliased or of a by-reference type should be allocated on storage element boundaries.

Followed.

If the Address of an object is specified, or it is imported or exported, then the implementation should not perform optimizations based on assumptions of no aliases.

Followed.

13.3(29-35): Alignment Clauses
==============================

The recommended level of support for the Alignment attribute for subtypes is:

An implementation should support specified Alignments that are factors and multiples of the number of storage elements per word, subject to the following:

Followed.

An implementation need not support specified Alignments for combinations of Sizes and Alignments that cannot be easily loaded and stored by available machine instructions.

Followed.

An implementation need not support specified Alignments that are greater than the maximum Alignment the implementation ever returns by default.

Followed.

The recommended level of support for the Alignment attribute for objects is:

Same as above, for subtypes, but in addition:

Followed.

For stand-alone library-level objects of statically constrained subtypes, the implementation should support all Alignments supported by the target linker.  For example, page alignment is likely to be supported for such objects, but not for subtypes.

Followed.

13.3(42-43): Size Clauses
=========================

The recommended level of support for the Size attribute of objects is:

A Size clause should be supported for an object if the specified Size is at least as large as its subtype's Size, and corresponds to a size in storage elements that is a multiple of the object's Alignment (if the Alignment is nonzero).

Followed.

13.3(50-56): Size Clauses
=========================

If the Size of a subtype is specified, and allows for efficient independent addressability (see 9.10) on the target architecture, then the Size of the following objects of the subtype should equal the Size of the subtype:

Aliased objects (including components).

Followed.

Size clause on a composite subtype should not affect the internal layout of components.

Followed. But note that this can be overridden by use of the implementation pragma Implicit_Packing in the case of packed arrays.

The recommended level of support for the Size attribute of subtypes is:

The Size (if not specified) of a static discrete or fixed point subtype should be the number of bits needed to represent each value belonging to the subtype using an unbiased representation, leaving space for a sign bit only if the subtype contains negative values.  If such a subtype is a first subtype, then an implementation should support a specified Size for it that reflects this representation.

Followed.

For a subtype implemented with levels of indirection, the Size should include the size of the pointers, but not the size of what they point at.

Followed.

13.3(71-73): Component Size Clauses
===================================

The recommended level of support for the Component_Size attribute is:

An implementation need not support specified Component_Sizes that are less than the Size of the component subtype.

Followed.

An implementation should support specified Component_Sizes that are factors and multiples of the word size.  For such Component_Sizes, the array should contain no gaps between components.  For other Component_Sizes (if supported), the array should contain no gaps between components when packing is also specified; the implementation should forbid this combination in cases where it cannot support a no-gaps representation.

Followed.

13.4(9-10): Enumeration Representation Clauses
==============================================

The recommended level of support for enumeration representation clauses is:

An implementation need not support enumeration representation clauses for boolean types, but should at minimum support the internal codes in the range System.Min_Int.System.Max_Int.

Followed.

13.5.1(17-22): Record Representation Clauses
============================================

The recommended level of support for

record_representation_clauses is:

An implementation should support storage places that can be extracted with a load, mask, shift sequence of machine code, and set with a load, shift, mask, store sequence, given the available machine instructions and run-time model.

Followed.

A storage place should be supported if its size is equal to the Size of the component subtype, and it starts and ends on a boundary that obeys the Alignment of the component subtype.

Followed.

If the default bit ordering applies to the declaration of a given type, then for a component whose subtype's Size is less than the word size, any storage place that does not cross an aligned word boundary should be supported.

Followed.

An implementation may reserve a storage place for the tag field of a tagged type, and disallow other components from overlapping that place.

Followed.  The storage place for the tag field is the beginning of the tagged record, and its size is Address'Size.  GNAT will reject an explicit component clause for the tag field.

An implementation need not support a component_clause for a component of an extension part if the storage place is not after the storage places of all components of the parent type, whether or not those storage places had been specified.

Followed.  The above advice on record representation clauses is followed, and all mentioned features are implemented.

13.5.2(5): Storage Place Attributes
===================================

If a component is represented using some form of pointer (such as an offset) to the actual data of the component, and this data is contiguous with the rest of the object, then the storage place attributes should reflect the place of the actual data, not the pointer.  If a component is allocated discontinuously from the rest of the object, then a warning should be generated upon reference to one of its storage place attributes.

Followed.  There are no such components in GNAT.

13.5.3(7-8): Bit Ordering
=========================

The recommended level of support for the non-default bit ordering is:

If Word_Size = Storage_Unit, then the implementation should support the non-default bit ordering in addition to the default bit ordering.

Followed.  Word size does not equal storage size in this implementation. Thus non-default bit ordering is not supported.

13.7(37): Address as Private
============================

Address should be of a private type.

Followed.

13.7.1(16): Address Operations
==============================

Operations in System and its children should reflect the target environment semantics as closely as is reasonable.  For example, on most machines, it makes sense for address arithmetic to ``wrap around''. Operations that do not make sense should raise Program_Error.

Followed.  Address arithmetic is modular arithmetic that wraps around.  No operation raises Program_Error, since all operations make sense.

13.9(14-17): Unchecked Conversion
=================================

The Size of an array object should not include its bounds; hence, the bounds should not be part of the converted data.

Followed.

The implementation should not generate unnecessary run-time checks to ensure that the representation of S is a representation of the target type.  It should take advantage of the permission to return by reference when possible.  Restrictions on unchecked conversions should be avoided unless required by the target environment.

Followed.  There are no restrictions on unchecked conversion.  A warning is generated if the source and target types do not have the same size since the semantics in this case may be target dependent.

The recommended level of support for unchecked conversions is:

Unchecked conversions should be supported and should be reversible in the cases where this clause defines the result.  To enable meaningful use of unchecked conversion, a contiguous representation should be used for elementary subtypes, for statically constrained array subtypes whose component subtype is one of the subtypes described in this paragraph, and for record subtypes without discriminants whose component subtypes are described in this paragraph.

Followed.

13.11(23-25): Implicit Heap Usage
=================================

An implementation should document any cases in which it dynamically allocates heap storage for a purpose other than the evaluation of an allocator.

Followed, the only other points at which heap storage is dynamically allocated are as follows:

  * At initial elaboration time, to allocate dynamically sized global objects.

  * To allocate space for a task when a task is created.

  * To extend the secondary stack dynamically when needed.  The secondary stack is used for returning variable length results.

A default (implementation-provided) storage pool for an access-to-constant type should not have overhead to support deallocation of individual objects.

Followed.

A storage pool for an anonymous access type should be created at the point of an allocator for the type, and be reclaimed when the designated object becomes inaccessible.

Followed.

13.11.2(17): Unchecked De-allocation
====================================

For a standard storage pool, Free should actually reclaim the storage.

Followed.

13.13.2(17): Stream Oriented Attributes
=======================================

If a stream element is the same size as a storage element, then the normal in-memory representation should be used by Read and Write for scalar objects.  Otherwise, Read and Write should use the smallest number of stream elements needed to represent all values in the base range of the scalar type.

Followed.  By default, GNAT uses the interpretation suggested by AI-195, which specifies using the size of the first subtype. However, such an implementation is based on direct binary representations and is therefore target- and endianness-dependent. To address this issue, GNAT also supplies an alternate implementation of the stream attributes Read and Write, which uses the target-independent XDR standard representation for scalar types. The XDR implementation is provided as an alternative body of the System.Stream_Attributes package, in the file s-stratt-xdr.adb in the GNAT library. There is no s-stratt-xdr.ads file. In order to install the XDR implementation, do the following:

 1. Replace the default implementation of the System.Stream_Attributes package with the XDR implementation. For example on a Unix platform issue the commands:

          $ mv s-stratt.adb s-stratt-default.adb
          $ mv s-stratt-xdr.adb s-stratt.adb

 2. Rebuild the GNAT run-time library as documented in GNAT and Libraries (GNAT User's Guide).

A.1(52): Names of Predefined Numeric Types
==========================================

If an implementation provides additional named predefined integer types, then the names should end with Integer as in Long_Integer.  If an implementation provides additional named predefined floating point types, then the names should end with Float as in Long_Float.

Followed.

A.3.2(49): Ada.Characters.Handling
==================================

If an implementation provides a localized definition of Character or Wide_Character, then the effects of the subprograms in Characters.Handling should reflect the localizations.  See also 3.5.2.

Followed.  GNAT provides no such localized definitions.

A.4.4(106): Bounded-Length String Handling
==========================================

Bounded string objects should not be implemented by implicit pointers and dynamic allocation.

Followed.  No implicit pointers or dynamic allocation are used.

A.5.2(46-47): Random Number Generation
======================================

Any storage associated with an object of type Generator should be reclaimed on exit from the scope of the object.

Followed.

If the generator period is sufficiently long in relation to the number of distinct initiator values, then each possible value of Initiator passed to Reset should initiate a sequence of random numbers that does not, in a practical sense, overlap the sequence initiated by any other value.  If this is not possible, then the mapping between initiator values and generator states should be a rapidly varying function of the initiator value.

Followed.  The generator period is sufficiently long for the first condition here to hold true.

A.10.7(23): Get_Immediate
=========================

The Get_Immediate procedures should be implemented with unbuffered input.  For a device such as a keyboard, input should be available if a key has already been typed, whereas for a disk file, input should always be available except at end of file.  For a file associated with a keyboard-like device, any line-editing features of the underlying operating system should be disabled during the execution of Get_Immediate.

Followed on all targets except VxWorks. For VxWorks, there is no way to provide this functionality that does not result in the input buffer being flushed before the Get_Immediate call. A special unit Interfaces.Vxworks.IO is provided that contains routines to enable this functionality.

B.1(39-41): Pragma Export
=========================

If an implementation supports pragma Export to a given language, then it should also allow the main subprogram to be written in that language.  It should support some mechanism for invoking the elaboration of the Ada library units included in the system, and for invoking the finalization of the environment task.  On typical systems, the recommended mechanism is to provide two subprograms whose link names are adainit and adafinal.  adainit should contain the elaboration code for library units.  adafinal should contain the finalization code.  These subprograms should have no effect the second and subsequent time they are called.

Followed.

Automatic elaboration of pre-elaborated packages should be provided when pragma Export is supported.

Followed when the main program is in Ada.  If the main program is in a foreign language, then adainit must be called to elaborate pre-elaborated packages.

For each supported convention L other than Intrinsic, an implementation should support Import and Export pragmas for objects of L-compatible types and for subprograms, and pragma Convention for L-eligible types and for subprograms, presuming the other language has corresponding features.  Pragma Convention need not be supported for scalar types.

Followed.

B.2(12-13): Package Interfaces
==============================

For each implementation-defined convention identifier, there should be a child package of package Interfaces with the corresponding name.  This package should contain any declarations that would be useful for interfacing to the language (implementation) represented by the convention.  Any declarations useful for interfacing to any language on the given hardware architecture should be provided directly in Interfaces.

Followed. An additional package not defined in the Ada Reference Manual is Interfaces.CPP, used for interfacing to C++.

An implementation supporting an interface to C, COBOL, or Fortran should provide the corresponding package or packages described in the following clauses.

Followed.  GNAT provides all the packages described in this section.

B.3(63-71): Interfacing with C
==============================

An implementation should support the following interface correspondences between Ada and C.

Followed.

An Ada procedure corresponds to a void-returning C function.

Followed.

An Ada function corresponds to a non-void C function.

Followed.

An Ada in scalar parameter is passed as a scalar argument to a C function.

Followed.

An Ada in parameter of an access-to-object type with designated type T is passed as a T* argument to a C function, where T is the C type corresponding to the Ada type T.

Followed.

An Ada access T parameter, or an Ada out or in out parameter of an elementary type T, is passed as a T* argument to a C function, where T is the C type corresponding to the Ada type T.  In the case of an elementary out or in out parameter, a pointer to a temporary copy is used to preserve by-copy semantics.

Followed.

An Ada parameter of a record type T, of any mode, is passed as a T* argument to a C function, where T is the C structure corresponding to the Ada type T.

Followed.  This convention may be overridden by the use of the C_Pass_By_Copy pragma, or Convention, or by explicitly specifying the mechanism for a given call using an extended import or export pragma.

An Ada parameter of an array type with component type T, of any mode, is passed as a T* argument to a C function, where T is the C type corresponding to the Ada type T.

Followed.

An Ada parameter of an access-to-subprogram type is passed as a pointer to a C function whose prototype corresponds to the designated subprogram's specification.

Followed.

B.4(95-98): Interfacing with COBOL
==================================

An Ada implementation should support the following interface correspondences between Ada and COBOL.

Followed.

An Ada access T parameter is passed as a BY REFERENCE data item of the COBOL type corresponding to T.

Followed.

An Ada in scalar parameter is passed as a BY CONTENT data item of the corresponding COBOL type.

Followed.

Any other Ada parameter is passed as a BY REFERENCE data item of the COBOL type corresponding to the Ada parameter type; for scalars, a local copy is used if necessary to ensure by-copy semantics.

Followed.

B.5(22-26): Interfacing with Fortran
====================================

An Ada implementation should support the following interface correspondences between Ada and Fortran:

Followed.

An Ada procedure corresponds to a Fortran subroutine.

Followed.

An Ada function corresponds to a Fortran function.

Followed.

An Ada parameter of an elementary, array, or record type T is passed as a T argument to a Fortran procedure, where T is the Fortran type corresponding to the Ada type T, and where the INTENT attribute of the corresponding dummy argument matches the Ada formal parameter mode; the Fortran implementation's parameter passing conventions are used.  For elementary types, a local copy is used if necessary to ensure by-copy semantics.

Followed.

An Ada parameter of an access-to-subprogram type is passed as a reference to a Fortran procedure whose interface corresponds to the designated subprogram's specification.

Followed.

C.1(3-5): Access to Machine Operations
======================================

The machine code or intrinsic support should allow access to all operations normally available to assembly language programmers for the target environment, including privileged instructions, if any.

Followed.

The interfacing pragmas (see Annex B) should support interface to assembler; the default assembler should be associated with the convention identifier Assembler.

Followed.

If an entity is exported to assembly language, then the implementation should allocate it at an addressable location, and should ensure that it is retained by the linking process, even if not otherwise referenced from the Ada code.  The implementation should assume that any call to a machine code or assembler subprogram is allowed to read or update every object that is specified as exported.

Followed.

C.1(10-16): Access to Machine Operations
========================================

The implementation should ensure that little or no overhead is associated with calling intrinsic and machine-code subprograms.

Followed for both intrinsics and machine-code subprograms.

It is recommended that intrinsic subprograms be provided for convenient access to any machine operations that provide special capabilities or efficiency and that are not otherwise available through the language constructs.

Followed.  A full set of machine operation intrinsic subprograms is provided.

Atomic read-modify-write operations---e.g., test and set, compare and swap, decrement and test, enqueue/dequeue.

Followed on any target supporting such operations.

Standard numeric functions---e.g., sin, log.

Followed on any target supporting such operations.

String manipulation operations---e.g., translate and test.

Followed on any target supporting such operations.

Vector operations---e.g., compare vector against thresholds.

Followed on any target supporting such operations.

Direct operations on I/O ports.

Followed on any target supporting such operations.

C.3(28): Interrupt Support
==========================

If the Ceiling_Locking policy is not in effect, the implementation should provide means for the application to specify which interrupts are to be blocked during protected actions, if the underlying system allows for a finer-grain control of interrupt blocking.

Followed.  The underlying system does not allow for finer-grain control of interrupt blocking.

C.3.1(20-21): Protected Procedure Handlers
==========================================

Whenever possible, the implementation should allow interrupt handlers to be called directly by the hardware.

Followed on any target where the underlying operating system permits such direct calls.

Whenever practical, violations of any implementation-defined restrictions should be detected before run time.

Followed.  Compile time warnings are given when possible.

C.3.2(25): Package Interrupts
=============================

If implementation-defined forms of interrupt handler procedures are supported, such as protected procedures with parameters, then for each such form of a handler, a type analogous to Parameterless_Handler should be specified in a child package of Interrupts, with the same operations as in the predefined package Interrupts.

Followed.

C.4(14): Pre-elaboration Requirements
=====================================

It is recommended that pre-elaborated packages be implemented in such a way that there should be little or no code executed at run time for the elaboration of entities not already covered by the Implementation Requirements.

Followed.  Executable code is generated in some cases, e.g. loops to initialize large arrays.

C.5(8): Pragma Discard_Names
============================

If the pragma applies to an entity, then the implementation should reduce the amount of storage used for storing names associated with that entity.

Followed.

C.7.2(30): The Package Task_Attributes
======================================

Some implementations are targeted to domains in which memory use at run time must be completely deterministic.  For such implementations, it is recommended that the storage for task attributes will be pre-allocated statically and not from the heap.  This can be accomplished by either placing restrictions on the number and the size of the task's attributes, or by using the pre-allocated storage for the first N attribute objects, and the heap for the others.  In the latter case, N should be documented.

Not followed.  This implementation is not targeted to such a domain.

D.3(17): Locking Policies
=========================

The implementation should use names that end with _Locking for locking policies defined by the implementation.

Followed.  Two implementation-defined locking policies are defined, whose names (Inheritance_Locking and Concurrent_Readers_Locking) follow this suggestion.

D.4(16): Entry Queuing Policies
===============================

Names that end with _Queuing should be used for all implementation-defined queuing policies.

Followed.  No such implementation-defined queuing policies exist.

D.6(9-10): Preemptive Abort
===========================

Even though the abort_statement is included in the list of potentially blocking operations (see 9.5.1), it is recommended that this statement be implemented in a way that never requires the task executing the abort_statement to block.

Followed.

On a multi-processor, the delay associated with aborting a task on another processor should be bounded; the implementation should use periodic polling, if necessary, to achieve this.

Followed.

D.7(21): Tasking Restrictions
=============================

When feasible, the implementation should take advantage of the specified restrictions to produce a more efficient implementation.

GNAT currently takes advantage of these restrictions by providing an optimized run time when the Ravenscar profile and the GNAT restricted run time set of restrictions are specified.  See pragma Profile (Ravenscar) and pragma Profile (Restricted) for more details.

D.8(47-49): Monotonic Time
==========================

When appropriate, implementations should provide configuration mechanisms to change the value of Tick.

Such configuration mechanisms are not appropriate to this implementation and are thus not supported.

It is recommended that Calendar.Clock and Real_Time.Clock be implemented as transformations of the same time base.

Followed.

It is recommended that the best time base which exists in the underlying system be available to the application through Clock.  Best may mean highest accuracy or largest range.

Followed.

E.5(28-29): Partition Communication Subsystem
=============================================

Whenever possible, the PCS on the called partition should allow for multiple tasks to call the RPC-receiver with different messages and should allow them to block until the corresponding subprogram body returns.

Followed by GLADE, a separately supplied PCS that can be used with GNAT.

The Write operation on a stream of type Params_Stream_Type should raise Storage_Error if it runs out of space trying to write the Item into the stream.

Followed by GLADE, a separately supplied PCS that can be used with GNAT.

F(7): COBOL Support
===================

If COBOL (respectively, C) is widely supported in the target environment, implementations supporting the Information Systems Annex should provide the child package Interfaces.COBOL (respectively, Interfaces.C) specified in Annex B and should support a convention_identifier of COBOL (respectively, C) in the interfacing pragmas (see Annex B), thus allowing Ada programs to interface with programs written in that language.

Followed.

F.1(2): Decimal Radix Support
=============================

Packed decimal should be used as the internal representation for objects of subtype S when S'Machine_Radix = 10.

Not followed.  GNAT ignores S'Machine_Radix and always uses binary representations.

G: Numerics
===========

If Fortran (respectively, C) is widely supported in the target environment, implementations supporting the Numerics Annex should provide the child package Interfaces.Fortran (respectively, Interfaces.C) specified in Annex B and should support a convention_identifier of Fortran (respectively, C) in the interfacing pragmas (see Annex B), thus allowing Ada programs to interface with programs written in that language.

Followed.

G.1.1(56-58): Complex Types
===========================

Because the usual mathematical meaning of multiplication of a complex operand and a real operand is that of the scaling of both components of the former by the latter, an implementation should not perform this operation by first promoting the real operand to complex type and then performing a full complex multiplication.  In systems that, in the future, support an Ada binding to IEC 559:1989, the latter technique will not generate the required result when one of the components of the complex operand is infinite.  (Explicit multiplication of the infinite component by the zero component obtained during promotion yields a NaN that propagates into the final result.) Analogous advice applies in the case of multiplication of a complex operand and a pure-imaginary operand, and in the case of division of a complex operand by a real or pure-imaginary operand.

Not followed.

Similarly, because the usual mathematical meaning of addition of a complex operand and a real operand is that the imaginary operand remains unchanged, an implementation should not perform this operation by first promoting the real operand to complex type and then performing a full complex addition.  In implementations in which the Signed_Zeros attribute of the component type is True (and which therefore conform to IEC 559:1989 in regard to the handling of the sign of zero in predefined arithmetic operations), the latter technique will not generate the required result when the imaginary component of the complex operand is a negatively signed zero.  (Explicit addition of the negative zero to the zero obtained during promotion yields a positive zero.) Analogous advice applies in the case of addition of a complex operand and a pure-imaginary operand, and in the case of subtraction of a complex operand and a real or pure-imaginary operand.

Not followed.

Implementations in which Real'Signed_Zeros is True should attempt to provide a rational treatment of the signs of zero results and result components.  As one example, the result of the Argument function should have the sign of the imaginary component of the parameter X when the point represented by that parameter lies on the positive real axis; as another, the sign of the imaginary component of the Compose_From_Polar function should be the same as (respectively, the opposite of) that of the Argument parameter when that parameter has a value of zero and the Modulus parameter has a nonnegative (respectively, negative) value.

Followed.

G.1.2(49): Complex Elementary Functions
=======================================

Implementations in which Complex_Types.Real'Signed_Zeros is True should attempt to provide a rational treatment of the signs of zero results and result components.  For example, many of the complex elementary functions have components that are odd functions of one of the parameter components; in these cases, the result component should have the sign of the parameter component at the origin.  Other complex elementary functions have zero components whose sign is opposite that of a parameter component at the origin, or is always positive or always negative.

Followed.

G.2.4(19): Accuracy Requirements
================================

The versions of the forward trigonometric functions without a Cycle parameter should not be implemented by calling the corresponding version with a Cycle parameter of 2.0*Numerics.Pi, since this will not provide the required accuracy in some portions of the domain.  For the same reason, the version of Log without a Base parameter should not be implemented by calling the corresponding version with a Base parameter of Numerics.e.

Followed.

G.2.6(15): Complex Arithmetic Accuracy
======================================

The version of the Compose_From_Polar function without a Cycle parameter should not be implemented by calling the corresponding version with a Cycle parameter of 2.0*Numerics.Pi, since this will not provide the required accuracy in some portions of the domain.

Followed.

Implementation Defined Characteristics
**************************************

In addition to the implementation dependent pragmas and attributes, and the implementation advice, there are a number of other Ada features that are potentially implementation dependent and are designated as implementation-defined. These are mentioned throughout the Ada Reference Manual, and are summarized in Annex M.

A requirement for conforming Ada compilers is that they provide documentation describing how the implementation deals with each of these issues.  In this chapter, you will find each point in Annex M listed followed by a description in italic font of how GNAT handles the implementation dependence.

You can use this chapter as a guide to minimizing implementation dependent features in your programs if portability to other compilers and other operating systems is an important consideration.  The numbers in each section below correspond to the paragraph number in the Ada Reference Manual.

2.  Whether or not each recommendation given in Implementation Advice is followed.  See 1.1.2(37).

See Implementation Advice.

3.  Capacity limitations of the implementation.  See 1.1.3(3).

The complexity of programs that can be processed is limited only by the total amount of available virtual memory, and disk space for the generated object files.

4.  Variations from the standard that are impractical to avoid given the implementation's execution environment.  See 1.1.3(6).

There are no variations from the standard.

5.  Which code_statements cause external interactions.  See 1.1.3(10).

Any code_statement can potentially cause external interactions.

6.  The coded representation for the text of an Ada program.  See 2.1(4).

See separate section on source representation.

7.  The control functions allowed in comments.  See 2.1(14).

See separate section on source representation.

8.  The representation for an end of line.  See 2.2(2).

See separate section on source representation.

9.  Maximum supported line length and lexical element length.  See 2.2(15).

The maximum line length is 255 characters and the maximum length of a lexical element is also 255 characters.

10.  Implementation defined pragmas.  See 2.8(14).

See Implementation Defined Pragmas.

11.  Effect of pragma Optimize.  See 2.8(27).

Pragma Optimize, if given with a Time or Space parameter, checks that the optimization flag is set, and aborts if it is not.

12.  The sequence of characters of the value returned by S'Image when some of the graphic characters of S'Wide_Image are not defined in Character.  See 3.5(37).

The sequence of characters is as defined by the wide character encoding method used for the source.  See section on source representation for further details.

13.  The predefined integer types declared in Standard.  See 3.5.4(25).

Short_Short_Integer
     8 bit signed

Short_Integer
     (Short) 16 bit signed

Integer
     32 bit signed

Long_Integer
     64 bit signed (on most 64 bit targets, depending on the C definition of long). 32 bit signed (all other targets)

Long_Long_Integer
     64 bit signed

14.  Any nonstandard integer types and the operators defined for them.  See 3.5.4(26).

There are no nonstandard integer types.

15.  Any nonstandard real types and the operators defined for them.  See 3.5.6(8).

There are no nonstandard real types.

16.  What combinations of requested decimal precision and range are supported for floating point types.  See 3.5.7(7).

The precision and range is as defined by the IEEE standard.

17.  The predefined floating point types declared in Standard.  See 3.5.7(16).

Short_Float
     32 bit IEEE short

Float
     (Short) 32 bit IEEE short

Long_Float
     64 bit IEEE long

Long_Long_Float
     64 bit IEEE long (80 bit IEEE long on x86 processors)

18.  The small of an ordinary fixed point type.  See 3.5.9(8).

Fine_Delta is 2**(-63)

19.  What combinations of small, range, and digits are supported for fixed point types.  See 3.5.9(10).

Any combinations are permitted that do not result in a small less than Fine_Delta and do not result in a mantissa larger than 63 bits. If the mantissa is larger than 53 bits on machines where Long_Long_Float is 64 bits (true of all architectures except ia32), then the output from Text_IO is accurate to only 53 bits, rather than the full mantissa.  This is because floating-point conversions are used to convert fixed point.

20.  The result of Tags.Expanded_Name for types declared within an unnamed block_statement.  See 3.9(10).

Block numbers of the form BNNN, where NNN is a decimal integer are allocated.

21.  Implementation-defined attributes.  See 4.1.4(12).

See Implementation Defined Attributes.

22.  Any implementation-defined time types.  See 9.6(6).

There are no implementation-defined time types.

23.  The time base associated with relative delays.

See 9.6(20).  The time base used is that provided by the C library function gettimeofday.

24.  The time base of the type Calendar.Time.  See 9.6(23).

The time base used is that provided by the C library function gettimeofday.

25.  The time zone used for package Calendar operations.  See 9.6(24).

The time zone used by package Calendar is the current system time zone setting for local time, as accessed by the C library function localtime.

26.  Any limit on delay_until_statements of select_statements.  See 9.6(29).

There are no such limits.

27.  Whether or not two non-overlapping parts of a composite object are independently addressable, in the case where packing, record layout, or Component_Size is specified for the object.  See 9.10(1).

Separate components are independently addressable if they do not share overlapping storage units.

28.  The representation for a compilation.  See 10.1(2).

A compilation is represented by a sequence of files presented to the compiler in a single invocation of the gcc command.

29.  Any restrictions on compilations that contain multiple compilation_units.  See 10.1(4).

No single file can contain more than one compilation unit, but any sequence of files can be presented to the compiler as a single compilation.

30.  The mechanisms for creating an environment and for adding and replacing compilation units.  See 10.1.4(3).

See separate section on compilation model.

31.  The manner of explicitly assigning library units to a partition.  See 10.2(2).

If a unit contains an Ada main program, then the Ada units for the partition are determined by recursive application of the rules in the Ada Reference Manual section 10.2(2-6).  In other words, the Ada units will be those that are needed by the main program, and then this definition of need is applied recursively to those units, and the partition contains the transitive closure determined by this relationship.  In short, all the necessary units are included, with no need to explicitly specify the list.  If additional units are required, e.g. by foreign language units, then all units must be mentioned in the context clause of one of the needed Ada units.

If the partition contains no main program, or if the main program is in a language other than Ada, then GNAT provides the binder options -z and -n respectively, and in this case a list of units can be explicitly supplied to the binder for inclusion in the partition (all units needed by these units will also be included automatically).  For full details on the use of these options, refer to The GNAT Make Program gnatmake (GNAT User's Guide).

32.  The implementation-defined means, if any, of specifying which compilation units are needed by a given compilation unit.  See 10.2(2).

The units needed by a given compilation unit are as defined in the Ada Reference Manual section 10.2(2-6).  There are no implementation-defined pragmas or other implementation-defined means for specifying needed units.

33.  The manner of designating the main subprogram of a partition.  See 10.2(7).

The main program is designated by providing the name of the corresponding ALI file as the input parameter to the binder.

34.  The order of elaboration of library_items.  See 10.2(18).

The first constraint on ordering is that it meets the requirements of Chapter 10 of the Ada Reference Manual.  This still leaves some implementation dependent choices, which are resolved by first elaborating bodies as early as possible (i.e., in preference to specs where there is a choice), and second by evaluating the immediate with clauses of a unit to determine the probably best choice, and third by elaborating in alphabetical order of unit names where a choice still remains.

35.  Parameter passing and function return for the main subprogram.  See 10.2(21).

The main program has no parameters.  It may be a procedure, or a function returning an integer type.  In the latter case, the returned integer value is the return code of the program (overriding any value that may have been set by a call to Ada.Command_Line.Set_Exit_Status).

36.  The mechanisms for building and running partitions.  See 10.2(24).

GNAT itself supports programs with only a single partition.  The GNATDIST tool provided with the GLADE package (which also includes an implementation of the PCS) provides a completely flexible method for building and running programs consisting of multiple partitions.  See the separate GLADE manual for details.

37.  The details of program execution, including program termination.  See 10.2(25).

See separate section on compilation model.

38.  The semantics of any non-active partitions supported by the implementation.  See 10.2(28).

Passive partitions are supported on targets where shared memory is provided by the operating system.  See the GLADE reference manual for further details.

39.  The information returned by Exception_Message.  See 11.4.1(10).

Exception message returns the null string unless a specific message has been passed by the program.

40.  The result of Exceptions.Exception_Name for types declared within an unnamed block_statement.  See 11.4.1(12).

Blocks have implementation defined names of the form BNNN where NNN is an integer.

41.  The information returned by Exception_Information.  See 11.4.1(13).

Exception_Information returns a string in the following format:

     Exception_Name: nnnnn
     Message: mmmmm
     PID: ppp
     Call stack traceback locations:
     0xhhhh 0xhhhh 0xhhhh ... 0xhhh

where

  * nnnn is the fully qualified name of the exception in all upper case letters. This line is always present.

  * mmmm is the message (this line present only if message is non-null)

  * ppp is the Process Id value as a decimal integer (this line is present only if the Process Id is nonzero). Currently we are not making use of this field.

  * The Call stack traceback locations line and the following values are present only if at least one traceback location was recorded. The values are given in C style format, with lower case letters for a-f, and only as many digits present as are necessary.

The line terminator sequence at the end of each line, including the last line is a single LF character (16#0A#).

42.  Implementation-defined check names.  See 11.5(27).

The implementation defined check name Alignment_Check controls checking of address clause values for proper alignment (that is, the address supplied must be consistent with the alignment of the type).

In addition, a user program can add implementation-defined check names by means of the pragma Check_Name.

43.  The interpretation of each aspect of representation.  See 13.1(20).

See separate section on data representations.

44.  Any restrictions placed upon representation items.  See 13.1(20).

See separate section on data representations.

45.  The meaning of Size for indefinite subtypes.  See 13.3(48).

Size for an indefinite subtype is the maximum possible size, except that for the case of a subprogram parameter, the size of the parameter object is the actual size.

46.  The default external representation for a type tag.  See 13.3(75).

The default external representation for a type tag is the fully expanded name of the type in upper case letters.

47.  What determines whether a compilation unit is the same in two different partitions.  See 13.3(76).

A compilation unit is the same in two different partitions if and only if it derives from the same source file.

48.  Implementation-defined components.  See 13.5.1(15).

The only implementation defined component is the tag for a tagged type, which contains a pointer to the dispatching table.

49.  If Word_Size = Storage_Unit, the default bit ordering.  See 13.5.3(5).

Word_Size (32) is not the same as Storage_Unit (8) for this implementation, so no non-default bit ordering is supported.  The default bit ordering corresponds to the natural endianness of the target architecture.

50.  The contents of the visible part of package System and its language-defined children.  See 13.7(2).

See the definition of these packages in files system.ads and s-stoele.ads.

51.  The contents of the visible part of package System.Machine_Code, and the meaning of code_statements.  See 13.8(7).

See the definition and documentation in file s-maccod.ads.

52.  The effect of unchecked conversion.  See 13.9(11).

Unchecked conversion between types of the same size results in an uninterpreted transmission of the bits from one type to the other.  If the types are of unequal sizes, then in the case of discrete types, a shorter source is first zero or sign extended as necessary, and a shorter target is simply truncated on the left. For all non-discrete types, the source is first copied if necessary to ensure that the alignment requirements of the target are met, then a pointer is constructed to the source value, and the result is obtained by dereferencing this pointer after converting it to be a pointer to the target type. Unchecked conversions where the target subtype is an unconstrained array are not permitted. If the target alignment is greater than the source alignment, then a copy of the result is made with appropriate alignment

53. The semantics of operations on invalid representations. See 13.9.2(10-11).

For assignments and other operations where the use of invalid values cannot result in erroneous behavior, the compiler ignores the possibility of invalid values. An exception is raised at the point where an invalid value would result in erroneous behavior. For example executing:

     procedure invalidvals is
        X : Integer := -1;
        Y : Natural range 1 .. 10;
        for Y'Address use X'Address;
        Z : Natural range 1 .. 10;
        A : array (Natural range 1 .. 10) of Integer;
     begin
        Z := Y;     -- no exception
        A (Z) := 3; -- exception raised;
     end;

As indicated, an exception is raised on the array assignment, but not on the simple assignment of the invalid negative value from Y to Z.

53.  The manner of choosing a storage pool for an access type when Storage_Pool is not specified for the type.  See 13.11(17).

There are 3 different standard pools used by the compiler when Storage_Pool is not specified depending whether the type is local to a subprogram or defined at the library level and whether Storage_Sizeis specified or not.  See documentation in the runtime library units System.Pool_Global, System.Pool_Size and System.Pool_Local in files s-poosiz.ads, s-pooglo.ads and s-pooloc.ads for full details on the default pools used.

54.  Whether or not the implementation provides user-accessible names for the standard pool type(s).  See 13.11(17).

See documentation in the sources of the run time mentioned in paragraph 53 .  All these pools are accessible by means of with'ing these units.

55.  The meaning of Storage_Size.  See 13.11(18).

Storage_Size is measured in storage units, and refers to the total space available for an access type collection, or to the primary stack space for a task.

56.  Implementation-defined aspects of storage pools.  See 13.11(22).

See documentation in the sources of the run time mentioned in paragraph 53 for details on GNAT-defined aspects of storage pools.

57.  The set of restrictions allowed in a pragma Restrictions.  See 13.12(7).

All RM defined Restriction identifiers are implemented.  The following additional restriction identifiers are provided.  There are two separate lists of implementation dependent restriction identifiers.  The first set requires consistency throughout a partition (in other words, if the restriction identifier is used for any compilation unit in the partition, then all compilation units in the partition must obey the restriction.

Simple_Barriers
     This restriction ensures at compile time that barriers in entry declarations for protected types are restricted to either static boolean expressions or references to simple boolean variables defined in the private part of the protected type.  No other form of entry barriers is permitted.  This is one of the restrictions of the Ravenscar profile for limited tasking (see also pragma Profile (Ravenscar)).

Max_Entry_Queue_Length => Expr
     This restriction is a declaration that any protected entry compiled in the scope of the restriction has at most the specified number of tasks waiting on the entry at any one time, and so no queue is required.  This restriction is not checked at compile time.  A program execution is erroneous if an attempt is made to queue more than the specified number of tasks on such an entry.

No_Calendar
     This restriction ensures at compile time that there is no implicit or explicit dependence on the package Ada.Calendar.

No_Default_Initialization

     This restriction prohibits any instance of default initialization of variables. The binder implements a consistency rule which prevents any unit compiled without the restriction from with'ing a unit with the restriction (this allows the generation of initialization procedures to be skipped, since you can be sure that no call is ever generated to an initialization procedure in a unit with the restriction active). If used in conjunction with Initialize_Scalars or Normalize_Scalars, the effect is to prohibit all cases of variables declared without a specific initializer (including the case of OUT scalar parameters).

No_Direct_Boolean_Operators
     This restriction ensures that no logical (and/or/xor) are used on operands of type Boolean (or any type derived from Boolean). This is intended for use in safety critical programs where the certification protocol requires the use of short-circuit (and then, or else) forms for all composite boolean operations.

No_Dispatching_Calls
     This restriction ensures at compile time that the code generated by the compiler involves no dispatching calls. The use of this restriction allows the safe use of record extensions, classwide membership tests and other classwide features not involving implicit dispatching. This restriction ensures that the code contains no indirect calls through a dispatching mechanism. Note that this includes internally-generated calls created by the compiler, for example in the implementation of class-wide objects assignments. The membership test is allowed in the presence of this restriction, because its implementation requires no dispatching. This restriction is comparable to the official Ada restriction No_Dispatch except that it is a bit less restrictive in that it allows all classwide constructs that do not imply dispatching. The following example indicates constructs that violate this restriction.

          package Pkg is
            type T is tagged record
              Data : Natural;
            end record;
            procedure P (X : T);

            type DT is new T with record
              More_Data : Natural;
            end record;
            procedure Q (X : DT);
          end Pkg;

          with Pkg; use Pkg;
          procedure Example is
            procedure Test (O : T'Class) is
              N : Natural  := O'Size;--  Error: Dispatching call
              C : T'Class := O;      --  Error: implicit Dispatching Call
            begin
              if O in DT'Class then  --  OK   : Membership test
                 Q (DT (O));         --  OK   : Type conversion plus direct call
              else
                 P (O);              --  Error: Dispatching call
              end if;
            end Test;

            Obj : DT;
          begin
            P (Obj);                 --  OK   : Direct call
            P (T (Obj));             --  OK   : Type conversion plus direct call
            P (T'Class (Obj));       --  Error: Dispatching call

            Test (Obj);              --  OK   : Type conversion

            if Obj in T'Class then   --  OK   : Membership test
               null;
            end if;
          end Example;

No_Dynamic_Attachment
     This restriction ensures that there is no call to any of the operations defined in package Ada.Interrupts.

No_Enumeration_Maps
     This restriction ensures at compile time that no operations requiring enumeration maps are used (that is Image and Value attributes applied to enumeration types).

No_Entry_Calls_In_Elaboration_Code
     This restriction ensures at compile time that no task or protected entry calls are made during elaboration code.  As a result of the use of this restriction, the compiler can assume that no code past an accept statement in a task can be executed at elaboration time.

No_Exception_Handlers
     This restriction ensures at compile time that there are no explicit exception handlers. It also indicates that no exception propagation will be provided. In this mode, exceptions may be raised but will result in an immediate call to the last chance handler, a routine that the user must define with the following profile:

          procedure Last_Chance_Handler
            (Source_Location : System.Address; Line : Integer);
          pragma Export (C, Last_Chance_Handler,
                         "__gnat_last_chance_handler");

     The parameter is a C null-terminated string representing a message to be associated with the exception (typically the source location of the raise statement generated by the compiler). The Line parameter when nonzero represents the line number in the source program where the raise occurs.

No_Exception_Propagation
     This restriction guarantees that exceptions are never propagated to an outer subprogram scope). The only case in which an exception may be raised is when the handler is statically in the same subprogram, so that the effect of a raise is essentially like a goto statement. Any other raise statement (implicit or explicit) will be considered unhandled. Exception handlers are allowed, but may not contain an exception occurrence identifier (exception choice). In addition use of the package GNAT.Current_Exception is not permitted, and reraise statements (raise with no operand) are not permitted.

No_Exception_Registration
     This restriction ensures at compile time that no stream operations for types Exception_Id or Exception_Occurrence are used. This also makes it impossible to pass exceptions to or from a partition with this restriction in a distributed environment. If this exception is active, then the generated code is simplified by omitting the otherwise-required global registration of exceptions when they are declared.

No_Finalization
     This restriction disables the language features described in chapter 7.6 of the Ada 2005 RM as well as all form of code generation performed by the compiler to support these features. The following types are no longer considered controlled when this restriction is in effect:

       * Ada.Finalization.Controlled

       * Ada.Finalization.Limited_Controlled

       * Derivations from Controlled or Limited_Controlled

       * Class-wide types

       * Protected types

       * Task types

       * Array and record types with controlled components

     The compiler no longer generates code to initialize, finalize or adjust an object or a nested component, either declared on the stack or on the heap. The deallocation of a controlled object no longer finalizes its contents.

No_Implicit_Aliasing

     This restriction, which is not required to be partition-wide consistent, requires an explicit aliased keyword for an object to which 'Access, 'Unchecked_Access, or 'Address is applied, and forbids entirely the use of the 'Unrestricted_Access attribute for objects. Note: the reason that Unrestricted_Access is forbidden is that it would require the prefix to be aliased, and in such cases, it can always be replaced by the standard attribute Unchecked_Access which is preferable.

No_Implicit_Conditionals
     This restriction ensures that the generated code does not contain any implicit conditionals, either by modifying the generated code where possible, or by rejecting any construct that would otherwise generate an implicit conditional. Note that this check does not include run time constraint checks, which on some targets may generate implicit conditionals as well. To control the latter, constraint checks can be suppressed in the normal manner. Constructs generating implicit conditionals include comparisons of composite objects and the Max/Min attributes.

No_Implicit_Dynamic_Code
     This restriction prevents the compiler from building ``trampolines''. This is a structure that is built on the stack and contains dynamic code to be executed at run time. On some targets, a trampoline is built for the following features: Access, Unrestricted_Access, or Address of a nested subprogram; nested task bodies; primitive operations of nested tagged types. Trampolines do not work on machines that prevent execution of stack data. For example, on windows systems, enabling DEP (data execution protection) will cause trampolines to raise an exception. Trampolines are also quite slow at run time.

     On many targets, trampolines have been largely eliminated. Look at the version of system.ads for your target --- if it has Always_Compatible_Rep equal to False, then trampolines are largely eliminated. In particular, a trampoline is built for the following features: Address of a nested subprogram; Access or Unrestricted_Access of a nested subprogram, but only if pragma Favor_Top_Level applies, or the access type has a foreign-language convention; primitive operations of nested tagged types.

No_Implicit_Loops
     This restriction ensures that the generated code does not contain any implicit for loops, either by modifying the generated code where possible, or by rejecting any construct that would otherwise generate an implicit for loop. If this restriction is active, it is possible to build large array aggregates with all static components without generating an intermediate temporary, and without generating a loop to initialize individual components. Otherwise, a loop is created for arrays larger than about 5000 scalar components.

No_Initialize_Scalars
     This restriction ensures that no unit in the partition is compiled with pragma Initialize_Scalars. This allows the generation of more efficient code, and in particular eliminates dummy null initialization routines that are otherwise generated for some record and array types.

No_Local_Protected_Objects
     This restriction ensures at compile time that protected objects are only declared at the library level.

No_Protected_Type_Allocators
     This restriction ensures at compile time that there are no allocator expressions that attempt to allocate protected objects.

No_Relative_Delay
     This restriction ensures at compile time that there are no delay relative statements and prevents expressions such as delay 1.23; from appearing in source code.

No_Requeue_Statements
     This restriction ensures at compile time that no requeue statements are permitted and prevents keyword requeue from being used in source code.

No_Secondary_Stack
     This restriction ensures at compile time that the generated code does not contain any reference to the secondary stack.  The secondary stack is used to implement functions returning unconstrained objects (arrays or records) on some targets.

No_Select_Statements
     This restriction ensures at compile time no select statements of any kind are permitted, that is the keyword select may not appear. This is one of the restrictions of the Ravenscar profile for limited tasking (see also pragma Profile (Ravenscar)).

No_Standard_Storage_Pools
     This restriction ensures at compile time that no access types use the standard default storage pool.  Any access type declared must have an explicit Storage_Pool attribute defined specifying a user-defined storage pool.

No_Stream_Optimizations
     This restriction affects the performance of stream operations on types String, Wide_String and Wide_Wide_String. By default, the compiler uses block reads and writes when manipulating String objects due to their supperior performance. When this restriction is in effect, the compiler performs all IO operations on a per-character basis.

No_Streams
     This restriction ensures at compile/bind time that there are no stream objects created and no use of stream attributes. This restriction does not forbid dependences on the package Ada.Streams. So it is permissible to with Ada.Streams (or another package that does so itself) as long as no actual stream objects are created and no stream attributes are used.

     Note that the use of restriction allows optimization of tagged types, since they do not need to worry about dispatching stream operations. To take maximum advantage of this space-saving optimization, any unit declaring a tagged type should be compiled with the restriction, though this is not required.

No_Task_Attributes_Package
     This restriction ensures at compile time that there are no implicit or explicit dependencies on the package Ada.Task_Attributes.

No_Task_Termination
     This restriction ensures at compile time that no terminate alternatives appear in any task body.

No_Tasking
     This restriction prevents the declaration of tasks or task types throughout the partition.  It is similar in effect to the use of Max_Tasks => 0 except that violations are caught at compile time and cause an error message to be output either by the compiler or binder.

Static_Priorities
     This restriction ensures at compile time that all priority expressions are static, and that there are no dependencies on the package Ada.Dynamic_Priorities.

Static_Storage_Size
     This restriction ensures at compile time that any expression appearing in a Storage_Size pragma or attribute definition clause is static.

The second set of implementation dependent restriction identifiers does not require partition-wide consistency. The restriction may be enforced for a single compilation unit without any effect on any of the other compilation units in the partition.

No_Elaboration_Code
     This restriction ensures at compile time that no elaboration code is generated.  Note that this is not the same condition as is enforced by pragma Preelaborate.  There are cases in which pragma Preelaborate still permits code to be generated (e.g. code to initialize a large array to all zeroes), and there are cases of units which do not meet the requirements for pragma Preelaborate, but for which no elaboration code is generated.  Generally, it is the case that preelaborable units will meet the restrictions, with the exception of large aggregates initialized with an others_clause, and exception declarations (which generate calls to a run-time registry procedure).  This restriction is enforced on a unit by unit basis, it need not be obeyed consistently throughout a partition.

     In the case of aggregates with others, if the aggregate has a dynamic size, there is no way to eliminate the elaboration code (such dynamic bounds would be incompatible with Preelaborate in any case). If the bounds are static, then use of this restriction actually modifies the code choice of the compiler to avoid generating a loop, and instead generate the aggregate statically if possible, no matter how many times the data for the others clause must be repeatedly generated.

     It is not possible to precisely document the constructs which are compatible with this restriction, since, unlike most other restrictions, this is not a restriction on the source code, but a restriction on the generated object code. For example, if the source contains a declaration:

             Val : constant Integer := X;

     where X is not a static constant, it may be possible, depending on complex optimization circuitry, for the compiler to figure out the value of X at compile time, in which case this initialization can be done by the loader, and requires no initialization code. It is not possible to document the precise conditions under which the optimizer can figure this out.

     Note that this the implementation of this restriction requires full code generation. If it is used in conjunction with "semantics only" checking, then some cases of violations may be missed.

No_Entry_Queue
     This restriction is a declaration that any protected entry compiled in the scope of the restriction has at most one task waiting on the entry at any one time, and so no queue is required.  This restriction is not checked at compile time.  A program execution is erroneous if an attempt is made to queue a second task on such an entry.

No_Implementation_Attributes
     This restriction checks at compile time that no GNAT-defined attributes are present.  With this restriction, the only attributes that can be used are those defined in the Ada Reference Manual.

No_Implementation_Pragmas
     This restriction checks at compile time that no GNAT-defined pragmas are present.  With this restriction, the only pragmas that can be used are those defined in the Ada Reference Manual.

No_Implementation_Restrictions
     This restriction checks at compile time that no GNAT-defined restriction identifiers (other than No_Implementation_Restrictions itself) are present.  With this restriction, the only other restriction identifiers that can be used are those defined in the Ada Reference Manual.

No_Wide_Characters
     This restriction ensures at compile time that no uses of the types Wide_Character or Wide_String or corresponding wide wide types appear, and that no wide or wide wide string or character literals appear in the program (that is literals representing characters not in type Character.

SPARK
     This restriction checks at compile time that some constructs forbidden in SPARK are not present. The SPARK version used as a reference is the same as the Ada mode for the unit, so a unit compiled in Ada 95 mode with SPARK restrictions will be checked for constructs forbidden in SPARK 95. Error messages related to SPARK restriction have the form:

          violation of restriction "Spark" at <file>
           <error message>

     This is not a replacement for the semantic checks performed by the SPARK Examiner tool, as the compiler only deals currently with code, not at all with SPARK annotations and does not guarantee catching all cases of constructs forbidden by SPARK.

     Thus it may well be the case that code which passes the compiler in SPARK mode is rejected by the SPARK Examiner, e.g. due to the different visibility rules of the Examiner based on SPARK inherit annotations.

     This restriction can be useful in providing an initial filter for code developed using SPARK, or in examining legacy code to see how far it is from meeting SPARK restrictions.

58.  The consequences of violating limitations on Restrictions pragmas.  See 13.12(9).

Restrictions that can be checked at compile time result in illegalities if violated.  Currently there are no other consequences of violating restrictions.

59.  The representation used by the Read and Write attributes of elementary types in terms of stream elements.  See 13.13.2(9).

The representation is the in-memory representation of the base type of the type, using the number of bits corresponding to the TYPE'Size value, and the natural ordering of the machine.

60.  The names and characteristics of the numeric subtypes declared in the visible part of package Standard.  See A.1(3).

See items describing the integer and floating-point types supported.

61.  The accuracy actually achieved by the elementary functions.  See A.5.1(1).

The elementary functions correspond to the functions available in the C library.  Only fast math mode is implemented.

62.  The sign of a zero result from some of the operators or functions in Numerics.Generic_Elementary_Functions, when Float_Type'Signed_Zeros is True.  See A.5.1(46).

The sign of zeroes follows the requirements of the IEEE 754 standard on floating-point.

63.  The value of Numerics.Float_Random.Max_Image_Width.  See A.5.2(27).

Maximum image width is 6864, see library file s-rannum.ads.

64.  The value of Numerics.Discrete_Random.Max_Image_Width.  See A.5.2(27).

Maximum image width is 6864, see library file s-rannum.ads.

65.  The algorithms for random number generation.  See A.5.2(32).

The algorithm is the Mersenne Twister, as documented in the source file s-rannum.adb. This version of the algorithm has a period of 2**19937-1.

66.  The string representation of a random number generator's state.  See A.5.2(38).

The value returned by the Image function is the concatenation of the fixed-width decimal representations of the 624 32-bit integers of the state vector.

67.  The minimum time interval between calls to the time-dependent Reset procedure that are guaranteed to initiate different random number sequences.  See A.5.2(45).

The minimum period between reset calls to guarantee distinct series of random numbers is one microsecond.

68.  The values of the Model_Mantissa, Model_Emin, Model_Epsilon, Model, Safe_First, and Safe_Last attributes, if the Numerics Annex is not supported.  See A.5.3(72).

Run the compiler with -gnatS to produce a listing of package Standard, has the values of all numeric attributes.

69.  Any implementation-defined characteristics of the input-output packages.  See A.7(14).

There are no special implementation defined characteristics for these packages.

70.  The value of Buffer_Size in Storage_IO.  See A.9(10).

All type representations are contiguous, and the Buffer_Size is the value of TYPE'Size rounded up to the next storage unit boundary.

71.  External files for standard input, standard output, and standard error See A.10(5).

These files are mapped onto the files provided by the C streams libraries.  See source file i-cstrea.ads for further details.

72.  The accuracy of the value produced by Put.  See A.10.9(36).

If more digits are requested in the output than are represented by the precision of the value, zeroes are output in the corresponding least significant digit positions.

73.  The meaning of Argument_Count, Argument, and Command_Name.  See A.15(1).

These are mapped onto the argv and argc parameters of the main program in the natural manner.

74.  The interpretation of the Form parameter in procedure Create_Directory.  See A.16(56).

The Form parameter is not used.

75.  The interpretation of the Form parameter in procedure Create_Path.  See A.16(60).

The Form parameter is not used.

76.  The interpretation of the Form parameter in procedure Copy_File.  See A.16(68).

The Form parameter is case-insensitive.

Two fields are recognized in the Form parameter:

preserve=<value>

mode=<value>

<value> starts immediately after the character '=' and ends with the character immediately preceding the next comma (',') or with the last character of the parameter.

The only possible values for preserve= are:

no_attributes
     Do not try to preserve any file attributes. This is the default if no preserve= is found in Form.

all_attributes
     Try to preserve all file attributes (timestamps, access rights).

timestamps
     Preserve the timestamp of the copied file, but not the other file attributes.

The only possible values for mode= are:

copy
     Only do the copy if the destination file does not already exist. If it already exists, Copy_File fails.

overwrite
     Copy the file in all cases. Overwrite an already existing destination file.

append
     Append the original file to the destination file. If the destination file does not exist, the destination file is a copy of the source file. When mode=append, the field preserve=, if it exists, is not taken into account.

If the Form parameter includes one or both of the fields and the value or values are incorrect, Copy_file fails with Use_Error.

Examples of correct Forms:

     Form => "preserve=no_attributes,mode=overwrite" (the default)
     Form => "mode=append"
     Form => "mode=copy, preserve=all_attributes"

Examples of incorrect Forms

     Form => "preserve=junk"
     Form => "mode=internal, preserve=timestamps"

77.  Implementation-defined convention names.  See B.1(11).

The following convention names are supported

Ada
     Ada

Assembler
     Assembly language

Asm
     Synonym for Assembler

Assembly
     Synonym for Assembler

C
     C

C_Pass_By_Copy
     Allowed only for record types, like C, but also notes that record is to be passed by copy rather than reference.

COBOL
     COBOL

C_Plus_Plus (or CPP)
     C++

Default
     Treated the same as C

External
     Treated the same as C

Fortran
     Fortran

Intrinsic
     For support of pragma Import with convention Intrinsic, see separate section on Intrinsic Subprograms.

Stdcall
     Stdcall (used for Windows implementations only).  This convention correspond to the WINAPI (previously called Pascal convention) C/C++ convention under Windows.  A function with this convention cleans the stack before exit.

DLL
     Synonym for Stdcall

Win32
     Synonym for Stdcall

Stubbed
     Stubbed is a special convention used to indicate that the body of the subprogram will be entirely ignored.  Any call to the subprogram is converted into a raise of the Program_Error exception.  If a pragma Import specifies convention stubbed then no body need be present at all.  This convention is useful during development for the inclusion of subprograms whose body has not yet been written.

In addition, all otherwise unrecognized convention names are also treated as being synonymous with convention C.  In all implementations except for VMS, use of such other names results in a warning.  In VMS implementations, these names are accepted silently.

78.  The meaning of link names.  See B.1(36).

Link names are the actual names used by the linker.

79.  The manner of choosing link names when neither the link name nor the address of an imported or exported entity is specified.  See B.1(36).

The default linker name is that which would be assigned by the relevant external language, interpreting the Ada name as being in all lower case letters.

80.  The effect of pragma Linker_Options.  See B.1(37).

The string passed to Linker_Options is presented uninterpreted as an argument to the link command, unless it contains ASCII.NUL characters. NUL characters if they appear act as argument separators, so for example

     pragma Linker_Options ("-labc" & ASCII.NUL & "-ldef");

causes two separate arguments -labc and -ldef to be passed to the linker. The order of linker options is preserved for a given unit. The final list of options passed to the linker is in reverse order of the elaboration order. For example, linker options for a body always appear before the options from the corresponding package spec.

81.  The contents of the visible part of package Interfaces and its language-defined descendants.  See B.2(1).

See files with prefix i- in the distributed library.

82.  Implementation-defined children of package Interfaces.  The contents of the visible part of package Interfaces.  See B.2(11).

See files with prefix i- in the distributed library.

83.  The types Floating, Long_Floating, Binary, Long_Binary, Decimal_ Element, and COBOL_Character; and the initialization of the variables Ada_To_COBOL and COBOL_To_Ada, in Interfaces.COBOL.  See B.4(50).

Floating
     Float

Long_Floating
     (Floating) Long_Float

Binary
     Integer

Long_Binary
     Long_Long_Integer

Decimal_Element
     Character

COBOL_Character
     Character

For initialization, see the file i-cobol.ads in the distributed library.

84.  Support for access to machine instructions.  See C.1(1).

See documentation in file s-maccod.ads in the distributed library.

85.  Implementation-defined aspects of access to machine operations.  See C.1(9).

See documentation in file s-maccod.ads in the distributed library.

86.  Implementation-defined aspects of interrupts.  See C.3(2).

Interrupts are mapped to signals or conditions as appropriate.  See definition of unit Ada.Interrupt_Names in source file a-intnam.ads for details on the interrupts supported on a particular target.

87.  Implementation-defined aspects of pre-elaboration.  See C.4(13).

GNAT does not permit a partition to be restarted without reloading, except under control of the debugger.

88.  The semantics of pragma Discard_Names.  See C.5(7).

Pragma Discard_Names causes names of enumeration literals to be suppressed.  In the presence of this pragma, the Image attribute provides the image of the Pos of the literal, and Value accepts Pos values.

89.  The result of the Task_Identification.Image attribute.  See C.7.1(7).

The result of this attribute is a string that identifies the object or component that denotes a given task. If a variable Var has a task type, the image for this task will have the form Var_XXXXXXXX, where the suffix is the hexadecimal representation of the virtual address of the corresponding task control block. If the variable is an array of tasks, the image of each task will have the form of an indexed component indicating the position of a given task in the array, e.g. Group(5)_XXXXXXX. If the task is a component of a record, the image of the task will have the form of a selected component. These rules are fully recursive, so that the image of a task that is a subcomponent of a composite object corresponds to the expression that designates this task.

If a task is created by an allocator, its image depends on the context. If the allocator is part of an object declaration, the rules described above are used to construct its image, and this image is not affected by subsequent assignments. If the allocator appears within an expression, the image includes only the name of the task type.

If the configuration pragma Discard_Names is present, or if the restriction No_Implicit_Heap_Allocation is in effect,  the image reduces to the numeric suffix, that is to say the hexadecimal representation of the virtual address of the control block of the task.

90.  The value of Current_Task when in a protected entry or interrupt handler.  See C.7.1(17).

Protected entries or interrupt handlers can be executed by any convenient thread, so the value of Current_Task is undefined.

91.  The effect of calling Current_Task from an entry body or interrupt handler.  See C.7.1(19).

The effect of calling Current_Task from an entry body or interrupt handler is to return the identification of the task currently executing the code.

92.  Implementation-defined aspects of Task_Attributes.  See C.7.2(19).

There are no implementation-defined aspects of Task_Attributes.

93.  Values of all Metrics.  See D(2).

The metrics information for GNAT depends on the performance of the underlying operating system.  The sources of the run-time for tasking implementation, together with the output from -gnatG can be used to determine the exact sequence of operating systems calls made to implement various tasking constructs.  Together with appropriate information on the performance of the underlying operating system, on the exact target in use, this information can be used to determine the required metrics.

94.  The declarations of Any_Priority and Priority.  See D.1(11).

See declarations in file system.ads.

95.  Implementation-defined execution resources.  See D.1(15).

There are no implementation-defined execution resources.

96.  Whether, on a multiprocessor, a task that is waiting for access to a protected object keeps its processor busy.  See D.2.1(3).

On a multi-processor, a task that is waiting for access to a protected object does not keep its processor busy.

97.  The affect of implementation defined execution resources on task dispatching.  See D.2.1(9).

Tasks map to threads in the threads package used by GNAT.  Where possible and appropriate, these threads correspond to native threads of the underlying operating system.

98.  Implementation-defined policy_identifiers allowed in a pragma Task_Dispatching_Policy.  See D.2.2(3).

There are no implementation-defined policy-identifiers allowed in this pragma.

99.  Implementation-defined aspects of priority inversion.  See D.2.2(16).

Execution of a task cannot be preempted by the implementation processing of delay expirations for lower priority tasks.

100.  Implementation defined task dispatching.  See D.2.2(18).

The policy is the same as that of the underlying threads implementation.

101.  Implementation-defined policy_identifiers allowed in a pragma Locking_Policy.  See D.3(4).

The two implementation defined policies permitted in GNAT are Inheritance_Locking and  Conccurent_Readers_Locking.  On targets that support the Inheritance_Locking policy, locking is implemented by inheritance, i.e. the task owning the lock operates at a priority equal to the highest priority of any task currently requesting the lock.  On targets that support the Conccurent_Readers_Locking policy, locking is implemented with a read/write lock allowing multiple propected object functions to enter concurrently.

102.  Default ceiling priorities.  See D.3(10).

The ceiling priority of protected objects of the type System.Interrupt_Priority'Last as described in the Ada Reference Manual D.3(10),

103.  The ceiling of any protected object used internally by the implementation.  See D.3(16).

The ceiling priority of internal protected objects is System.Priority'Last.

104.  Implementation-defined queuing policies.  See D.4(1).

There are no implementation-defined queuing policies.

105.  On a multiprocessor, any conditions that cause the completion of an aborted construct to be delayed later than what is specified for a single processor.  See D.6(3).

The semantics for abort on a multi-processor is the same as on a single processor, there are no further delays.

106.  Any operations that implicitly require heap storage allocation.  See D.7(8).

The only operation that implicitly requires heap storage allocation is task creation.

107.  Implementation-defined aspects of pragma Restrictions.  See D.7(20).

There are no such implementation-defined aspects.

108.  Implementation-defined aspects of package Real_Time.  See D.8(17).

There are no implementation defined aspects of package Real_Time.

109.  Implementation-defined aspects of delay_statements.  See D.9(8).

Any difference greater than one microsecond will cause the task to be delayed (see D.9(7)).

110.  The upper bound on the duration of interrupt blocking caused by the implementation.  See D.12(5).

The upper bound is determined by the underlying operating system.  In no cases is it more than 10 milliseconds.

111.  The means for creating and executing distributed programs.  See E(5).

The GLADE package provides a utility GNATDIST for creating and executing distributed programs.  See the GLADE reference manual for further details.

112.  Any events that can result in a partition becoming inaccessible.  See E.1(7).

See the GLADE reference manual for full details on such events.

113.  The scheduling policies, treatment of priorities, and management of shared resources between partitions in certain cases.  See E.1(11).

See the GLADE reference manual for full details on these aspects of multi-partition execution.

114.  Events that cause the version of a compilation unit to change.  See E.3(5).

Editing the source file of a compilation unit, or the source files of any units on which it is dependent in a significant way cause the version to change.  No other actions cause the version number to change.  All changes are significant except those which affect only layout, capitalization or comments.

115.  Whether the execution of the remote subprogram is immediately aborted as a result of cancellation.  See E.4(13).

See the GLADE reference manual for details on the effect of abort in a distributed application.

116.  Implementation-defined aspects of the PCS.  See E.5(25).

See the GLADE reference manual for a full description of all implementation defined aspects of the PCS.

117.  Implementation-defined interfaces in the PCS.  See E.5(26).

See the GLADE reference manual for a full description of all implementation defined interfaces.

118.  The values of named numbers in the package Decimal.  See F.2(7).

Max_Scale
     +18

Min_Scale
     -18

Min_Delta
     1.0E-18

Max_Delta
     1.0E+18

Max_Decimal_Digits
     18

119.  The value of Max_Picture_Length in the package Text_IO.Editing.  See F.3.3(16).

64

120.  The value of Max_Picture_Length in the package Wide_Text_IO.Editing.  See F.3.4(5).

64

121.  The accuracy actually achieved by the complex elementary functions and by other complex arithmetic operations.  See G.1(1).

Standard library functions are used for the complex arithmetic operations.  Only fast math mode is currently supported.

122.  The sign of a zero result (or a component thereof) from any operator or function in Numerics.Generic_Complex_Types, when Real'Signed_Zeros is True.  See G.1.1(53).

The signs of zero values are as recommended by the relevant implementation advice.

123.  The sign of a zero result (or a component thereof) from any operator or function in Numerics.Generic_Complex_Elementary_Functions, when Real'Signed_Zeros is True.  See G.1.2(45).

The signs of zero values are as recommended by the relevant implementation advice.

124.  Whether the strict mode or the relaxed mode is the default.  See G.2(2).

The strict mode is the default.  There is no separate relaxed mode.  GNAT provides a highly efficient implementation of strict mode.

125.  The result interval in certain cases of fixed-to-float conversion.  See G.2.1(10).

For cases where the result interval is implementation dependent, the accuracy is that provided by performing all operations in 64-bit IEEE floating-point format.

126.  The result of a floating point arithmetic operation in overflow situations, when the Machine_Overflows attribute of the result type is False.  See G.2.1(13).

Infinite and NaN values are produced as dictated by the IEEE floating-point standard.

Note that on machines that are not fully compliant with the IEEE floating-point standard, such as Alpha, the -mieee compiler flag must be used for achieving IEEE conforming behavior (although at the cost of a significant performance penalty), so infinite and NaN values are properly generated.

127.  The result interval for division (or exponentiation by a negative exponent), when the floating point hardware implements division as multiplication by a reciprocal.  See G.2.1(16).

Not relevant, division is IEEE exact.

128.  The definition of close result set, which determines the accuracy of certain fixed point multiplications and divisions.  See G.2.3(5).

Operations in the close result set are performed using IEEE long format floating-point arithmetic.  The input operands are converted to floating-point, the operation is done in floating-point, and the result is converted to the target type.

129.  Conditions on a universal_real operand of a fixed point multiplication or division for which the result shall be in the perfect result set.  See G.2.3(22).

The result is only defined to be in the perfect result set if the result can be computed by a single scaling operation involving a scale factor representable in 64-bits.

130.  The result of a fixed point arithmetic operation in overflow situations, when the Machine_Overflows attribute of the result type is False.  See G.2.3(27).

Not relevant, Machine_Overflows is True for fixed-point types.

131.  The result of an elementary function reference in overflow situations, when the Machine_Overflows attribute of the result type is False.  See G.2.4(4).

IEEE infinite and Nan values are produced as appropriate.

132.  The value of the angle threshold, within which certain elementary functions, complex arithmetic operations, and complex elementary functions yield results conforming to a maximum relative error bound.  See G.2.4(10).

Information on this subject is not yet available.

133.  The accuracy of certain elementary functions for parameters beyond the angle threshold.  See G.2.4(10).

Information on this subject is not yet available.

134.  The result of a complex arithmetic operation or complex elementary function reference in overflow situations, when the Machine_Overflows attribute of the corresponding real type is False.  See G.2.6(5).

IEEE infinite and Nan values are produced as appropriate.

135.  The accuracy of certain complex arithmetic operations and certain complex elementary functions for parameters (or components thereof) beyond the angle threshold.  See G.2.6(8).

Information on those subjects is not yet available.

136.  Information regarding bounded errors and erroneous execution.  See H.2(1).

Information on this subject is not yet available.

137.  Implementation-defined aspects of pragma Inspection_Point.  See H.3.2(8).

Pragma Inspection_Point ensures that the variable is live and can be examined by the debugger at the inspection point.

138.  Implementation-defined aspects of pragma Restrictions.  See H.4(25).

There are no implementation-defined aspects of pragma Restrictions.  The use of pragma Restrictions [No_Exceptions] has no effect on the generated code.  Checks must suppressed by use of pragma Suppress.

139.  Any restrictions on pragma Restrictions.  See H.4(27).

There are no restrictions on pragma Restrictions.

Intrinsic Subprograms
*********************

GNAT allows a user application program to write the declaration:

        pragma Import (Intrinsic, name);

providing that the name corresponds to one of the implemented intrinsic subprograms in GNAT, and that the parameter profile of the referenced subprogram meets the requirements.  This chapter describes the set of implemented intrinsic subprograms, and the requirements on parameter profiles. Note that no body is supplied; as with other uses of pragma Import, the body is supplied elsewhere (in this case by the compiler itself).  Note that any use of this feature is potentially non-portable, since the Ada standard does not require Ada compilers to implement this feature.

Intrinsic Operators
===================

All the predefined numeric operators in package Standard in pragma Import (Intrinsic,..) declarations.  In the binary operator case, the operands must have the same size.  The operand or operands must also be appropriate for the operator.  For example, for addition, the operands must both be floating-point or both be fixed-point, and the right operand for "**" must have a root type of Standard.Integer'Base. You can use an intrinsic operator declaration as in the following example:

        type Int1 is new Integer;
        type Int2 is new Integer;

        function "+" (X1 : Int1; X2 : Int2) return Int1;
        function "+" (X1 : Int1; X2 : Int2) return Int2;
        pragma Import (Intrinsic, "+");

This declaration would permit ``mixed mode'' arithmetic on items of the differing types Int1 and Int2. It is also possible to specify such operators for private types, if the full views are appropriate arithmetic types.

Enclosing_Entity
================

This intrinsic subprogram is used in the implementation of the library routine GNAT.Source_Info.  The only useful use of the intrinsic import in this case is the one in this unit, so an application program should simply call the function GNAT.Source_Info.Enclosing_Entity to obtain the name of the current subprogram, package, task, entry, or protected subprogram.

Exception_Information
=====================

This intrinsic subprogram is used in the implementation of the library routine GNAT.Current_Exception.  The only useful use of the intrinsic import in this case is the one in this unit, so an application program should simply call the function GNAT.Current_Exception.Exception_Information to obtain the exception information associated with the current exception.

Exception_Message
=================

This intrinsic subprogram is used in the implementation of the library routine GNAT.Current_Exception.  The only useful use of the intrinsic import in this case is the one in this unit, so an application program should simply call the function GNAT.Current_Exception.Exception_Message to obtain the message associated with the current exception.

Exception_Name
==============

This intrinsic subprogram is used in the implementation of the library routine GNAT.Current_Exception.  The only useful use of the intrinsic import in this case is the one in this unit, so an application program should simply call the function GNAT.Current_Exception.Exception_Name to obtain the name of the current exception.

File
====

This intrinsic subprogram is used in the implementation of the library routine GNAT.Source_Info.  The only useful use of the intrinsic import in this case is the one in this unit, so an application program should simply call the function GNAT.Source_Info.File to obtain the name of the current file.

Line
====

This intrinsic subprogram is used in the implementation of the library routine GNAT.Source_Info.  The only useful use of the intrinsic import in this case is the one in this unit, so an application program should simply call the function GNAT.Source_Info.Line to obtain the number of the current source line.

Rotate_Left
===========

In standard Ada, the Rotate_Left function is available only for the predefined modular types in package Interfaces.  However, in GNAT it is possible to define a Rotate_Left function for a user defined modular type or any signed integer type as in this example:

        function Shift_Left
          (Value  : My_Modular_Type;
           Amount : Natural)
           return   My_Modular_Type;

The requirements are that the profile be exactly as in the example above.  The only modifications allowed are in the formal parameter names, and in the type of Value and the return type, which must be the same, and must be either a signed integer type, or a modular integer type with a binary modulus, and the size must be 8.  16, 32 or 64 bits.

Rotate_Right
============

A Rotate_Right function can be defined for any user defined binary modular integer type, or signed integer type, as described above for Rotate_Left.

Shift_Left
==========

A Shift_Left function can be defined for any user defined binary modular integer type, or signed integer type, as described above for Rotate_Left.

Shift_Right
===========

A Shift_Right function can be defined for any user defined binary modular integer type, or signed integer type, as described above for Rotate_Left.

Shift_Right_Arithmetic
======================

A Shift_Right_Arithmetic function can be defined for any user defined binary modular integer type, or signed integer type, as described above for Rotate_Left.

Source_Location
===============

This intrinsic subprogram is used in the implementation of the library routine GNAT.Source_Info.  The only useful use of the intrinsic import in this case is the one in this unit, so an application program should simply call the function GNAT.Source_Info.Source_Location to obtain the current source file location.

Representation Clauses and Pragmas
**********************************

This section describes the representation clauses accepted by GNAT, and their effect on the representation of corresponding data objects.

GNAT fully implements Annex C (Systems Programming).  This means that all the implementation advice sections in chapter 13 are fully implemented. However, these sections only require a minimal level of support for representation clauses.  GNAT provides much more extensive capabilities, and this section describes the additional capabilities provided.

Alignment Clauses
=================

GNAT requires that all alignment clauses specify a power of 2, and all default alignments are always a power of 2.  The default alignment values are as follows:

  * Primitive Types. For primitive types, the alignment is the minimum of the actual size of objects of the type divided by Storage_Unit, and the maximum alignment supported by the target. (This maximum alignment is given by the GNAT-specific attribute Standard'Maximum_Alignment; see Maximum_Alignment.) For example, for type Long_Float, the object size is 8 bytes, and the default alignment will be 8 on any target that supports alignments this large, but on some targets, the maximum alignment may be smaller than 8, in which case objects of type Long_Float will be maximally aligned.

  * Arrays. For arrays, the alignment is equal to the alignment of the component type for the normal case where no packing or component size is given.  If the array is packed, and the packing is effective (see separate section on packed arrays), then the alignment will be one for long packed arrays, or arrays whose length is not known at compile time.  For short packed arrays, which are handled internally as modular types, the alignment will be as described for primitive types, e.g. a packed array of length 31 bits will have an object size of four bytes, and an alignment of 4.

  * Records. For the normal non-packed case, the alignment of a record is equal to the maximum alignment of any of its components.  For tagged records, this includes the implicit access type used for the tag.  If a pragma Pack is used and all components are packable (see separate section on pragma Pack), then the resulting alignment is 1, unless the layout of the record makes it profitable to increase it.

     A special case is when:

       * the size of the record is given explicitly, or a full record representation clause is given, and

       * the size of the record is 2, 4, or 8 bytes.

     In this case, an alignment is chosen to match the size of the record. For example, if we have:

             type Small is record
                A, B : Character;
             end record;
             for Small'Size use 16;

     then the default alignment of the record type Small is 2, not 1. This leads to more efficient code when the record is treated as a unit, and also allows the type to specified as Atomic on architectures requiring strict alignment.

An alignment clause may specify a larger alignment than the default value up to some maximum value dependent on the target (obtainable by using the attribute reference Standard'Maximum_Alignment). It may also specify a smaller alignment than the default value for enumeration, integer and fixed point types, as well as for record types, for example

       type V is record
          A : Integer;
       end record;

       for V'alignment use 1;

The default alignment for the type V is 4, as a result of the Integer field in the record, but it is permissible, as shown, to override the default alignment of the record with a smaller value.

Size Clauses
============

The default size for a type T is obtainable through the language-defined attribute T'Size and also through the equivalent GNAT-defined attribute T'Value_Size. For objects of type T, GNAT will generally increase the type size so that the object size (obtainable through the GNAT-defined attribute T'Object_Size) is a multiple of T'Alignment * Storage_Unit. For example

        type Smallint is range 1 .. 6;

        type Rec is record
           Y1 : integer;
           Y2 : boolean;
        end record;

In this example, Smallint'Size = Smallint'Value_Size = 3, as specified by the RM rules, but objects of this type will have a size of 8 (Smallint'Object_Size = 8), since objects by default occupy an integral number of storage units.  On some targets, notably older versions of the Digital Alpha, the size of stand alone objects of this type may be 32, reflecting the inability of the hardware to do byte load/stores.

Similarly, the size of type Rec is 40 bits (Rec'Size = Rec'Value_Size = 40), but the alignment is 4, so objects of this type will have their size increased to 64 bits so that it is a multiple of the alignment (in bits).  This decision is in accordance with the specific Implementation Advice in RM 13.3(43):

     A Size clause should be supported for an object if the specified Size is at least as large as its subtype's Size, and corresponds to a size in storage elements that is a multiple of the object's Alignment (if the Alignment is nonzero).

An explicit size clause may be used to override the default size by increasing it.  For example, if we have:

        type My_Boolean is new Boolean;
        for My_Boolean'Size use 32;

then values of this type will always be 32 bits long.  In the case of discrete types, the size can be increased up to 64 bits, with the effect that the entire specified field is used to hold the value, sign- or zero-extended as appropriate.  If more than 64 bits is specified, then padding space is allocated after the value, and a warning is issued that there are unused bits.

Similarly the size of records and arrays may be increased, and the effect is to add padding bits after the value.  This also causes a warning message to be generated.

The largest Size value permitted in GNAT is 2**31-1.  Since this is a Size in bits, this corresponds to an object of size 256 megabytes (minus one).  This limitation is true on all targets.  The reason for this limitation is that it improves the quality of the code in many cases if it is known that a Size value can be accommodated in an object of type Integer.

Storage_Size Clauses
====================

For tasks, the Storage_Size clause specifies the amount of space to be allocated for the task stack.  This cannot be extended, and if the stack is exhausted, then Storage_Error will be raised (if stack checking is enabled).  Use a Storage_Size attribute definition clause, or a Storage_Size pragma in the task definition to set the appropriate required size.  A useful technique is to include in every task definition a pragma of the form:

        pragma Storage_Size (Default_Stack_Size);

Then Default_Stack_Size can be defined in a global package, and modified as required. Any tasks requiring stack sizes different from the default can have an appropriate alternative reference in the pragma.

You can also use the -d binder switch to modify the default stack size.

For access types, the Storage_Size clause specifies the maximum space available for allocation of objects of the type.  If this space is exceeded then Storage_Error will be raised by an allocation attempt. In the case where the access type is declared local to a subprogram, the use of a Storage_Size clause triggers automatic use of a special predefined storage pool (System.Pool_Size) that ensures that all space for the pool is automatically reclaimed on exit from the scope in which the type is declared.

A special case recognized by the compiler is the specification of a Storage_Size of zero for an access type.  This means that no items can be allocated from the pool, and this is recognized at compile time, and all the overhead normally associated with maintaining a fixed size storage pool is eliminated.  Consider the following example:

        procedure p is
           type R is array (Natural) of Character;
           type P is access all R;
           for P'Storage_Size use 0;
           --  Above access type intended only for interfacing purposes

           y : P;

           procedure g (m : P);
           pragma Import (C, g);

           --  ...

        begin
           --  ...
           y := new R;
        end;

As indicated in this example, these dummy storage pools are often useful in connection with interfacing where no object will ever be allocated.  If you compile the above example, you get the warning:

        p.adb:16:09: warning: allocation from empty storage pool
        p.adb:16:09: warning: Storage_Error will be raised at run time

Of course in practice, there will not be any explicit allocators in the case of such an access declaration.

Size of Variant Record Objects
==============================

In the case of variant record objects, there is a question whether Size gives information about a particular variant, or the maximum size required for any variant.  Consider the following program

     with Text_IO; use Text_IO;
     procedure q is
        type R1 (A : Boolean := False) is record
          case A is
            when True  => X : Character;
            when False => null;
          end case;
        end record;

        V1 : R1 (False);
        V2 : R1;

     begin
        Put_Line (Integer'Image (V1'Size));
        Put_Line (Integer'Image (V2'Size));
     end q;

Here we are dealing with a variant record, where the True variant requires 16 bits, and the False variant requires 8 bits. In the above example, both V1 and V2 contain the False variant, which is only 8 bits long.  However, the result of running the program is:

     8
     16

The reason for the difference here is that the discriminant value of V1 is fixed, and will always be False.  It is not possible to assign a True variant value to V1, therefore 8 bits is sufficient.  On the other hand, in the case of V2, the initial discriminant value is False (from the default), but it is possible to assign a True variant value to V2, therefore 16 bits must be allocated for V2 in the general case, even fewer bits may be needed at any particular point during the program execution.

As can be seen from the output of this program, the 'Size attribute applied to such an object in GNAT gives the actual allocated size of the variable, which is the largest size of any of the variants. The Ada Reference Manual is not completely clear on what choice should be made here, but the GNAT behavior seems most consistent with the language in the RM.

In some cases, it may be desirable to obtain the size of the current variant, rather than the size of the largest variant.  This can be achieved in GNAT by making use of the fact that in the case of a subprogram parameter, GNAT does indeed return the size of the current variant (because a subprogram has no way of knowing how much space is actually allocated for the actual).

Consider the following modified version of the above program:

     with Text_IO; use Text_IO;
     procedure q is
        type R1 (A : Boolean := False) is record
          case A is
            when True  => X : Character;
            when False => null;
          end case;
        end record;

        V2 : R1;

        function Size (V : R1) return Integer is
        begin
           return V'Size;
        end Size;

     begin
        Put_Line (Integer'Image (V2'Size));
        Put_Line (Integer'IMage (Size (V2)));
        V2 := (True, 'x');
        Put_Line (Integer'Image (V2'Size));
        Put_Line (Integer'IMage (Size (V2)));
     end q;

The output from this program is

     16
     8
     16
     16

Here we see that while the 'Size attribute always returns the maximum size, regardless of the current variant value, the Size function does indeed return the size of the current variant value.

Biased Representation
=====================

In the case of scalars with a range starting at other than zero, it is possible in some cases to specify a size smaller than the default minimum value, and in such cases, GNAT uses an unsigned biased representation, in which zero is used to represent the lower bound, and successive values represent successive values of the type.

For example, suppose we have the declaration:

        type Small is range -7 .. -4;
        for Small'Size use 2;

Although the default size of type Small is 4, the Size clause is accepted by GNAT and results in the following representation scheme:

       -7 is represented as 2#00#
       -6 is represented as 2#01#
       -5 is represented as 2#10#
       -4 is represented as 2#11#

Biased representation is only used if the specified Size clause cannot be accepted in any other manner.  These reduced sizes that force biased representation can be used for all discrete types except for enumeration types for which a representation clause is given.

Value_Size and Object_Size Clauses
==================================

In Ada 95 and Ada 2005, T'Size for a type T is the minimum number of bits required to hold values of type T. Although this interpretation was allowed in Ada 83, it was not required, and this requirement in practice can cause some significant difficulties. For example, in most Ada 83 compilers, Natural'Size was 32. However, in Ada 95 and Ada 2005, Natural'Size is typically 31.  This means that code may change in behavior when moving from Ada 83 to Ada 95 or Ada 2005.  For example, consider:

        type Rec is record;
           A : Natural;
           B : Natural;
        end record;

        for Rec use record
           at 0  range 0 .. Natural'Size - 1;
           at 0  range Natural'Size .. 2 * Natural'Size - 1;
        end record;

In the above code, since the typical size of Natural objects is 32 bits and Natural'Size is 31, the above code can cause unexpected inefficient packing in Ada 95 and Ada 2005, and in general there are cases where the fact that the object size can exceed the size of the type causes surprises.

To help get around this problem GNAT provides two implementation defined attributes, Value_Size and Object_Size.  When applied to a type, these attributes yield the size of the type (corresponding to the RM defined size attribute), and the size of objects of the type respectively.

The Object_Size is used for determining the default size of objects and components.  This size value can be referred to using the Object_Size attribute.  The phrase ``is used'' here means that it is the basis of the determination of the size.  The backend is free to pad this up if necessary for efficiency, e.g. an 8-bit stand-alone character might be stored in 32 bits on a machine with no efficient byte access instructions such as the Alpha.

The default rules for the value of Object_Size for discrete types are as follows:

  * The Object_Size for base subtypes reflect the natural hardware size in bits (run the compiler with -gnatS to find those values for numeric types). Enumeration types and fixed-point base subtypes have 8, 16, 32 or 64 bits for this size, depending on the range of values to be stored.

  * The Object_Size of a subtype is the same as the Object_Size of the type from which it is obtained.

  * The Object_Size of a derived base type is copied from the parent base type, and the Object_Size of a derived first subtype is copied from the parent first subtype.

The Value_Size attribute is the (minimum) number of bits required to store a value of the type. This value is used to determine how tightly to pack records or arrays with components of this type, and also affects the semantics of unchecked conversion (unchecked conversions where the Value_Size values differ generate a warning, and are potentially target dependent).

The default rules for the value of Value_Size are as follows:

  * The Value_Size for a base subtype is the minimum number of bits required to store all values of the type (including the sign bit only if negative values are possible).

  * If a subtype statically matches the first subtype of a given type, then it has by default the same Value_Size as the first subtype.  This is a consequence of RM 13.1(14) (``if two subtypes statically match, then their subtype-specific aspects are the same''.)

  * All other subtypes have a Value_Size corresponding to the minimum number of bits required to store all values of the subtype.  For dynamic bounds, it is assumed that the value can range down or up to the corresponding bound of the ancestor

The RM defined attribute Size corresponds to the Value_Size attribute.

The Size attribute may be defined for a first-named subtype.  This sets the Value_Size of the first-named subtype to the given value, and the Object_Size of this first-named subtype to the given value padded up to an appropriate boundary.  It is a consequence of the default rules above that this Object_Size will apply to all further subtypes.  On the other hand, Value_Size is affected only for the first subtype, any dynamic subtypes obtained from it directly, and any statically matching subtypes.  The Value_Size of any other static subtypes is not affected.

Value_Size and Object_Size may be explicitly set for any subtype using an attribute definition clause.  Note that the use of these attributes can cause the RM 13.1(14) rule to be violated.  If two access types reference aliased objects whose subtypes have differing Object_Size values as a result of explicit attribute definition clauses, then it is erroneous to convert from one access subtype to the other.

At the implementation level, Esize stores the Object_Size and the RM_Size field stores the Value_Size (and hence the value of the Size attribute, which, as noted above, is equivalent to Value_Size).

To get a feel for the difference, consider the following examples (note that in each case the base is Short_Short_Integer with a size of 8):

                                            Object_Size     Value_Size

     type x1 is range 0 .. 5;                    8               3

     type x2 is range 0 .. 5;
     for x2'size use 12;                        16              12

     subtype x3 is x2 range 0 .. 3;             16               2

     subtype x4 is x2'base range 0 .. 10;        8               4

     subtype x5 is x2 range 0 .. dynamic;       16               3*

     subtype x6 is x2'base range 0 .. dynamic;   8               3*

Note: the entries marked ``3*'' are not actually specified by the Ada Reference Manual, but it seems in the spirit of the RM rules to allocate the minimum number of bits (here 3, given the range for x2) known to be large enough to hold the given range of values.

So far, so good, but GNAT has to obey the RM rules, so the question is under what conditions must the RM Size be used. The following is a list of the occasions on which the RM Size must be used:

  * Component size for packed arrays or records

  * Value of the attribute Size for a type

  * Warning about sizes not matching for unchecked conversion

For record types, the Object_Size is always a multiple of the alignment of the type (this is true for all types). In some cases the Value_Size can be smaller. Consider:

        type R is record
          X : Integer;
          Y : Character;
        end record;

On a typical 32-bit architecture, the X component will be four bytes, and require four-byte alignment, and the Y component will be one byte. In this case R'Value_Size will be 40 (bits) since this is the minimum size required to store a value of this type, and for example, it is permissible to have a component of type R in an outer array whose component size is specified to be 48 bits. However, R'Object_Size will be 64 (bits), since it must be rounded up so that this value is a multiple of the alignment (4 bytes = 32 bits).

For all other types, the Object_Size and Value_Size are the same (and equivalent to the RM attribute Size). Only Size may be specified for such types.

Component_Size Clauses
======================

Normally, the value specified in a component size clause must be consistent with the subtype of the array component with regard to size and alignment. In other words, the value specified must be at least equal to the size of this subtype, and must be a multiple of the alignment value.

In addition, component size clauses are allowed which cause the array to be packed, by specifying a smaller value.  A first case is for component size values in the range 1 through 63.  The value specified must not be smaller than the Size of the subtype.  GNAT will accurately honor all packing requests in this range.  For example, if we have:

     type r is array (1 .. 8) of Natural;
     for r'Component_Size use 31;

then the resulting array has a length of 31 bytes (248 bits = 8 * 31). Of course access to the components of such an array is considerably less efficient than if the natural component size of 32 is used. A second case is when the subtype of the component is a record type padded because of its default alignment.  For example, if we have:

     type r is record
       i : Integer;
       j : Integer;
       b : Boolean;
     end record;

     type a is array (1 .. 8) of r;
     for a'Component_Size use 72;

then the resulting array has a length of 72 bytes, instead of 96 bytes if the alignment of the record (4) was obeyed.

Note that there is no point in giving both a component size clause and a pragma Pack for the same array type. if such duplicate clauses are given, the pragma Pack will be ignored.

Bit_Order Clauses
=================

For record subtypes, GNAT permits the specification of the Bit_Order attribute.  The specification may either correspond to the default bit order for the target, in which case the specification has no effect and places no additional restrictions, or it may be for the non-standard setting (that is the opposite of the default).

In the case where the non-standard value is specified, the effect is to renumber bits within each byte, but the ordering of bytes is not affected.  There are certain restrictions placed on component clauses as follows:

  * Components fitting within a single storage unit.

     These are unrestricted, and the effect is merely to renumber bits.  For example if we are on a little-endian machine with Low_Order_First being the default, then the following two declarations have exactly the same effect:

             type R1 is record
                A : Boolean;
                B : Integer range 1 .. 120;
             end record;

             for R1 use record
                A at 0 range 0 .. 0;
                B at 0 range 1 .. 7;
             end record;

             type R2 is record
                A : Boolean;
                B : Integer range 1 .. 120;
             end record;

             for R2'Bit_Order use High_Order_First;

             for R2 use record
                A at 0 range 7 .. 7;
                B at 0 range 0 .. 6;
             end record;

     The useful application here is to write the second declaration with the Bit_Order attribute definition clause, and know that it will be treated the same, regardless of whether the target is little-endian or big-endian.

  * Components occupying an integral number of bytes.

     These are components that exactly fit in two or more bytes.  Such component declarations are allowed, but have no effect, since it is important to realize that the Bit_Order specification does not affect the ordering of bytes. In particular, the following attempt at getting an endian-independent integer does not work:

             type R2 is record
                A : Integer;
             end record;

             for R2'Bit_Order use High_Order_First;

             for R2 use record
                A at 0 range 0 .. 31;
             end record;

     This declaration will result in a little-endian integer on a little-endian machine, and a big-endian integer on a big-endian machine. If byte flipping is required for interoperability between big- and little-endian machines, this must be explicitly programmed.  This capability is not provided by Bit_Order.

  * Components that are positioned across byte boundaries

     but do not occupy an integral number of bytes.  Given that bytes are not reordered, such fields would occupy a non-contiguous sequence of bits in memory, requiring non-trivial code to reassemble.  They are for this reason not permitted, and any component clause specifying such a layout will be flagged as illegal by GNAT.

Since the misconception that Bit_Order automatically deals with all endian-related incompatibilities is a common one, the specification of a component field that is an integral number of bytes will always generate a warning.  This warning may be suppressed using pragma Warnings (Off) if desired.  The following section contains additional details regarding the issue of byte ordering.

Effect of Bit_Order on Byte Ordering
====================================

In this section we will review the effect of the Bit_Order attribute definition clause on byte ordering.  Briefly, it has no effect at all, but a detailed example will be helpful.  Before giving this example, let us review the precise definition of the effect of defining Bit_Order.  The effect of a non-standard bit order is described in section 15.5.3 of the Ada Reference Manual:

     2   A bit ordering is a method of interpreting the meaning of the storage place attributes.

To understand the precise definition of storage place attributes in this context, we visit section 13.5.1 of the manual:

     13   A record_representation_clause (without the mod_clause) specifies the layout.  The storage place attributes (see 13.5.2) are taken from the values of the position, first_bit, and last_bit expressions after normalizing those values so that first_bit is less than Storage_Unit.

The critical point here is that storage places are taken from the values after normalization, not before.  So the Bit_Order interpretation applies to normalized values.  The interpretation is described in the later part of the 15.5.3 paragraph:

     2   A bit ordering is a method of interpreting the meaning of the storage place attributes.  High_Order_First (known in the vernacular as ``big endian'') means that the first bit of a storage element (bit 0) is the most significant bit (interpreting the sequence of bits that represent a component as an unsigned integer value).  Low_Order_First (known in the vernacular as ``little endian'') means the opposite: the first bit is the least significant.

Note that the numbering is with respect to the bits of a storage unit.  In other words, the specification affects only the numbering of bits within a single storage unit.

We can make the effect clearer by giving an example.

Suppose that we have an external device which presents two bytes, the first byte presented, which is the first (low addressed byte) of the two byte record is called Master, and the second byte is called Slave.

The left most (most significant bit is called Control for each byte, and the remaining 7 bits are called V1, V2, ... V7, where V7 is the rightmost (least significant) bit.

On a big-endian machine, we can write the following representation clause

        type Data is record
           Master_Control : Bit;
           Master_V1      : Bit;
           Master_V2      : Bit;
           Master_V3      : Bit;
           Master_V4      : Bit;
           Master_V5      : Bit;
           Master_V6      : Bit;
           Master_V7      : Bit;
           Slave_Control  : Bit;
           Slave_V1       : Bit;
           Slave_V2       : Bit;
           Slave_V3       : Bit;
           Slave_V4       : Bit;
           Slave_V5       : Bit;
           Slave_V6       : Bit;
           Slave_V7       : Bit;
        end record;

        for Data use record
           Master_Control at 0 range 0 .. 0;
           Master_V1      at 0 range 1 .. 1;
           Master_V2      at 0 range 2 .. 2;
           Master_V3      at 0 range 3 .. 3;
           Master_V4      at 0 range 4 .. 4;
           Master_V5      at 0 range 5 .. 5;
           Master_V6      at 0 range 6 .. 6;
           Master_V7      at 0 range 7 .. 7;
           Slave_Control  at 1 range 0 .. 0;
           Slave_V1       at 1 range 1 .. 1;
           Slave_V2       at 1 range 2 .. 2;
           Slave_V3       at 1 range 3 .. 3;
           Slave_V4       at 1 range 4 .. 4;
           Slave_V5       at 1 range 5 .. 5;
           Slave_V6       at 1 range 6 .. 6;
           Slave_V7       at 1 range 7 .. 7;
        end record;

Now if we move this to a little endian machine, then the bit ordering within the byte is backwards, so we have to rewrite the record rep clause as:

        for Data use record
           Master_Control at 0 range 7 .. 7;
           Master_V1      at 0 range 6 .. 6;
           Master_V2      at 0 range 5 .. 5;
           Master_V3      at 0 range 4 .. 4;
           Master_V4      at 0 range 3 .. 3;
           Master_V5      at 0 range 2 .. 2;
           Master_V6      at 0 range 1 .. 1;
           Master_V7      at 0 range 0 .. 0;
           Slave_Control  at 1 range 7 .. 7;
           Slave_V1       at 1 range 6 .. 6;
           Slave_V2       at 1 range 5 .. 5;
           Slave_V3       at 1 range 4 .. 4;
           Slave_V4       at 1 range 3 .. 3;
           Slave_V5       at 1 range 2 .. 2;
           Slave_V6       at 1 range 1 .. 1;
           Slave_V7       at 1 range 0 .. 0;
        end record;

It is a nuisance to have to rewrite the clause, especially if the code has to be maintained on both machines.  However, this is a case that we can handle with the Bit_Order attribute if it is implemented. Note that the implementation is not required on byte addressed machines, but it is indeed implemented in GNAT. This means that we can simply use the first record clause, together with the declaration

        for Data'Bit_Order use High_Order_First;

and the effect is what is desired, namely the layout is exactly the same, independent of whether the code is compiled on a big-endian or little-endian machine.

The important point to understand is that byte ordering is not affected. A Bit_Order attribute definition never affects which byte a field ends up in, only where it ends up in that byte. To make this clear, let us rewrite the record rep clause of the previous example as:

        for Data'Bit_Order use High_Order_First;
        for Data use record
           Master_Control at 0 range  0 .. 0;
           Master_V1      at 0 range  1 .. 1;
           Master_V2      at 0 range  2 .. 2;
           Master_V3      at 0 range  3 .. 3;
           Master_V4      at 0 range  4 .. 4;
           Master_V5      at 0 range  5 .. 5;
           Master_V6      at 0 range  6 .. 6;
           Master_V7      at 0 range  7 .. 7;
           Slave_Control  at 0 range  8 .. 8;
           Slave_V1       at 0 range  9 .. 9;
           Slave_V2       at 0 range 10 .. 10;
           Slave_V3       at 0 range 11 .. 11;
           Slave_V4       at 0 range 12 .. 12;
           Slave_V5       at 0 range 13 .. 13;
           Slave_V6       at 0 range 14 .. 14;
           Slave_V7       at 0 range 15 .. 15;
        end record;

This is exactly equivalent to saying (a repeat of the first example):

        for Data'Bit_Order use High_Order_First;
        for Data use record
           Master_Control at 0 range 0 .. 0;
           Master_V1      at 0 range 1 .. 1;
           Master_V2      at 0 range 2 .. 2;
           Master_V3      at 0 range 3 .. 3;
           Master_V4      at 0 range 4 .. 4;
           Master_V5      at 0 range 5 .. 5;
           Master_V6      at 0 range 6 .. 6;
           Master_V7      at 0 range 7 .. 7;
           Slave_Control  at 1 range 0 .. 0;
           Slave_V1       at 1 range 1 .. 1;
           Slave_V2       at 1 range 2 .. 2;
           Slave_V3       at 1 range 3 .. 3;
           Slave_V4       at 1 range 4 .. 4;
           Slave_V5       at 1 range 5 .. 5;
           Slave_V6       at 1 range 6 .. 6;
           Slave_V7       at 1 range 7 .. 7;
        end record;

Why are they equivalent? Well take a specific field, the Slave_V2 field.  The storage place attributes are obtained by normalizing the values given so that the First_Bit value is less than 8.  After normalizing the values (0,10,10) we get (1,2,2) which is exactly what we specified in the other case.

Now one might expect that the Bit_Order attribute might affect bit numbering within the entire record component (two bytes in this case, thus affecting which byte fields end up in), but that is not the way this feature is defined, it only affects numbering of bits, not which byte they end up in.

Consequently it never makes sense to specify a starting bit number greater than 7 (for a byte addressable field) if an attribute definition for Bit_Order has been given, and indeed it may be actively confusing to specify such a value, so the compiler generates a warning for such usage.

If you do need to control byte ordering then appropriate conditional values must be used.  If in our example, the slave byte came first on some machines we might write:

        Master_Byte_First constant Boolean := ...;

        Master_Byte : constant Natural :=
                        1 - Boolean'Pos (Master_Byte_First);
        Slave_Byte  : constant Natural :=
                        Boolean'Pos (Master_Byte_First);

        for Data'Bit_Order use High_Order_First;
        for Data use record
           Master_Control at Master_Byte range 0 .. 0;
           Master_V1      at Master_Byte range 1 .. 1;
           Master_V2      at Master_Byte range 2 .. 2;
           Master_V3      at Master_Byte range 3 .. 3;
           Master_V4      at Master_Byte range 4 .. 4;
           Master_V5      at Master_Byte range 5 .. 5;
           Master_V6      at Master_Byte range 6 .. 6;
           Master_V7      at Master_Byte range 7 .. 7;
           Slave_Control  at Slave_Byte  range 0 .. 0;
           Slave_V1       at Slave_Byte  range 1 .. 1;
           Slave_V2       at Slave_Byte  range 2 .. 2;
           Slave_V3       at Slave_Byte  range 3 .. 3;
           Slave_V4       at Slave_Byte  range 4 .. 4;
           Slave_V5       at Slave_Byte  range 5 .. 5;
           Slave_V6       at Slave_Byte  range 6 .. 6;
           Slave_V7       at Slave_Byte  range 7 .. 7;
        end record;

Now to switch between machines, all that is necessary is to set the boolean constant Master_Byte_First in an appropriate manner.

Pragma Pack for Arrays
======================

Pragma Pack applied to an array has no effect unless the component type is packable.  For a component type to be packable, it must be one of the following cases:

  * Any scalar type

  * Any type whose size is specified with a size clause

  * Any packed array type with a static size

  * Any record type padded because of its default alignment

For all these cases, if the component subtype size is in the range 1 through 63, then the effect of the pragma Pack is exactly as though a component size were specified giving the component subtype size. For example if we have:

        type r is range 0 .. 17;

        type ar is array (1 .. 8) of r;
        pragma Pack (ar);

Then the component size of ar will be set to 5 (i.e. to r'size, and the size of the array ar will be exactly 40 bits.

Note that in some cases this rather fierce approach to packing can produce unexpected effects.  For example, in Ada 95 and Ada 2005, subtype Natural typically has a size of 31, meaning that if you pack an array of Natural, you get 31-bit close packing, which saves a few bits, but results in far less efficient access.  Since many other Ada compilers will ignore such a packing request, GNAT will generate a warning on some uses of pragma Pack that it guesses might not be what is intended.  You can easily remove this warning by using an explicit Component_Size setting instead, which never generates a warning, since the intention of the programmer is clear in this case.

GNAT treats packed arrays in one of two ways.  If the size of the array is known at compile time and is less than 64 bits, then internally the array is represented as a single modular type, of exactly the appropriate number of bits.  If the length is greater than 63 bits, or is not known at compile time, then the packed array is represented as an array of bytes, and the length is always a multiple of 8 bits.

Note that to represent a packed array as a modular type, the alignment must be suitable for the modular type involved. For example, on typical machines a 32-bit packed array will be represented by a 32-bit modular integer with an alignment of four bytes. If you explicitly override the default alignment with an alignment clause that is too small, the modular representation cannot be used. For example, consider the following set of declarations:

        type R is range 1 .. 3;
        type S is array (1 .. 31) of R;
        for S'Component_Size use 2;
        for S'Size use 62;
        for S'Alignment use 1;

If the alignment clause were not present, then a 62-bit modular representation would be chosen (typically with an alignment of 4 or 8 bytes depending on the target). But the default alignment is overridden with the explicit alignment clause. This means that the modular representation cannot be used, and instead the array of bytes representation must be used, meaning that the length must be a multiple of 8. Thus the above set of declarations will result in a diagnostic rejecting the size clause and noting that the minimum size allowed is 64.

One special case that is worth noting occurs when the base type of the component size is 8/16/32 and the subtype is one bit less. Notably this occurs with subtype Natural. Consider:

        type Arr is array (1 .. 32) of Natural;
        pragma Pack (Arr);

In all commonly used Ada 83 compilers, this pragma Pack would be ignored, since typically Natural'Size is 32 in Ada 83, and in any case most Ada 83 compilers did not attempt 31 bit packing.

In Ada 95 and Ada 2005, Natural'Size is required to be 31. Furthermore, GNAT really does pack 31-bit subtype to 31 bits. This may result in a substantial unintended performance penalty when porting legacy Ada 83 code. To help prevent this, GNAT generates a warning in such cases. If you really want 31 bit packing in a case like this, you can set the component size explicitly:

        type Arr is array (1 .. 32) of Natural;
        for Arr'Component_Size use 31;

Here 31-bit packing is achieved as required, and no warning is generated, since in this case the programmer intention is clear.

Pragma Pack for Records
=======================

Pragma Pack applied to a record will pack the components to reduce wasted space from alignment gaps and by reducing the amount of space taken by components.  We distinguish between packable components and non-packable components. Components of the following types are considered packable:

  * All primitive types are packable.

  * Small packed arrays, whose size does not exceed 64 bits, and where the size is statically known at compile time, are represented internally as modular integers, and so they are also packable.

All packable components occupy the exact number of bits corresponding to their Size value, and are packed with no padding bits, i.e. they can start on an arbitrary bit boundary.

All other types are non-packable, they occupy an integral number of storage units, and are placed at a boundary corresponding to their alignment requirements.

For example, consider the record

        type Rb1 is array (1 .. 13) of Boolean;
        pragma Pack (rb1);

        type Rb2 is array (1 .. 65) of Boolean;
        pragma Pack (rb2);

        type x2 is record
           l1 : Boolean;
           l2 : Duration;
           l3 : Float;
           l4 : Boolean;
           l5 : Rb1;
           l6 : Rb2;
        end record;
        pragma Pack (x2);

The representation for the record x2 is as follows:

     for x2'Size use 224;
     for x2 use record
        l1 at  0 range  0 .. 0;
        l2 at  0 range  1 .. 64;
        l3 at 12 range  0 .. 31;
        l4 at 16 range  0 .. 0;
        l5 at 16 range  1 .. 13;
        l6 at 18 range  0 .. 71;
     end record;

Studying this example, we see that the packable fields l1 and l2 are of length equal to their sizes, and placed at specific bit boundaries (and not byte boundaries) to eliminate padding.  But l3 is of a non-packable float type, so it is on the next appropriate alignment boundary.

The next two fields are fully packable, so l4 and l5 are minimally packed with no gaps.  However, type Rb2 is a packed array that is longer than 64 bits, so it is itself non-packable.  Thus the l6 field is aligned to the next byte boundary, and takes an integral number of bytes, i.e. 72 bits.

Record Representation Clauses
=============================

Record representation clauses may be given for all record types, including types obtained by record extension.  Component clauses are allowed for any static component.  The restrictions on component clauses depend on the type of the component.

For all components of an elementary type, the only restriction on component clauses is that the size must be at least the 'Size value of the type (actually the Value_Size).  There are no restrictions due to alignment, and such components may freely cross storage boundaries.

Packed arrays with a size up to and including 64 bits are represented internally using a modular type with the appropriate number of bits, and thus the same lack of restriction applies.  For example, if you declare:

        type R is array (1 .. 49) of Boolean;
        pragma Pack (R);
        for R'Size use 49;

then a component clause for a component of type R may start on any specified bit boundary, and may specify a value of 49 bits or greater.

For packed bit arrays that are longer than 64 bits, there are two cases. If the component size is a power of 2 (1,2,4,8,16,32 bits), including the important case of single bits or boolean values, then there are no limitations on placement of such components, and they may start and end at arbitrary bit boundaries.

If the component size is not a power of 2 (e.g. 3 or 5), then an array of this type longer than 64 bits must always be placed on on a storage unit (byte) boundary and occupy an integral number of storage units (bytes). Any component clause that does not meet this requirement will be rejected.

Any aliased component, or component of an aliased type, must have its normal alignment and size. A component clause that does not meet this requirement will be rejected.

The tag field of a tagged type always occupies an address sized field at the start of the record.  No component clause may attempt to overlay this tag. When a tagged type appears as a component, the tag field must have proper alignment

In the case of a record extension T1, of a type T, no component clause applied to the type T1 can specify a storage location that would overlap the first T'Size bytes of the record.

For all other component types, including non-bit-packed arrays, the component can be placed at an arbitrary bit boundary, so for example, the following is permitted:

        type R is array (1 .. 10) of Boolean;
        for R'Size use 80;

        type Q is record
           G, H : Boolean;
           L, M : R;
        end record;

        for Q use record
           G at 0 range  0 ..   0;
           H at 0 range  1 ..   1;
           L at 0 range  2 ..  81;
           R at 0 range 82 .. 161;
        end record;

Note: the above rules apply to recent releases of GNAT 5. In GNAT 3, there are more severe restrictions on larger components. For non-primitive types, including packed arrays with a size greater than 64 bits, component clauses must respect the alignment requirement of the type, in particular, always starting on a byte boundary, and the length must be a multiple of the storage unit.

Enumeration Clauses
===================

The only restriction on enumeration clauses is that the range of values must be representable.  For the signed case, if one or more of the representation values are negative, all values must be in the range:

        System.Min_Int .. System.Max_Int

For the unsigned case, where all values are nonnegative, the values must be in the range:

        0 .. System.Max_Binary_Modulus;

A confirming representation clause is one in which the values range from 0 in sequence, i.e. a clause that confirms the default representation for an enumeration type. Such a confirming representation is permitted by these rules, and is specially recognized by the compiler so that no extra overhead results from the use of such a clause.

If an array has an index type which is an enumeration type to which an enumeration clause has been applied, then the array is stored in a compact manner.  Consider the declarations:

        type r is (A, B, C);
        for r use (A => 1, B => 5, C => 10);
        type t is array (r) of Character;

The array type t corresponds to a vector with exactly three elements and has a default size equal to 3*Character'Size.  This ensures efficient use of space, but means that accesses to elements of the array will incur the overhead of converting representation values to the corresponding positional values, (i.e. the value delivered by the Pos attribute).

Address Clauses
===============

The reference manual allows a general restriction on representation clauses, as found in RM 13.1(22):

     An implementation need not support representation items containing nonstatic expressions, except that an implementation should support a representation item for a given entity if each nonstatic expression in the representation item is a name that statically denotes a constant declared before the entity.

In practice this is applicable only to address clauses, since this is the only case in which a non-static expression is permitted by the syntax.  As the AARM notes in sections 13.1 (22.a-22.h):

       22.a   Reason: This is to avoid the following sort of thing:

       22.b        X : Integer := F(...);
                   Y : Address := G(...);
                   for X'Address use Y;

       22.c   In the above, we have to evaluate the
              initialization expression for X before we
              know where to put the result.  This seems
              like an unreasonable implementation burden.

       22.d   The above code should instead be written
              like this:

       22.e        Y : constant Address := G(...);
                   X : Integer := F(...);
                   for X'Address use Y;

       22.f   This allows the expression ``Y'' to be safely
              evaluated before X is created.

       22.g   The constant could be a formal parameter of mode in.

       22.h   An implementation can support other nonstatic
              expressions if it wants to.  Expressions of type
              Address are hardly ever static, but their value
              might be known at compile time anyway in many
              cases.

GNAT does indeed permit many additional cases of non-static expressions.  In particular, if the type involved is elementary there are no restrictions (since in this case, holding a temporary copy of the initialization value, if one is present, is inexpensive).  In addition, if there is no implicit or explicit initialization, then there are no restrictions.  GNAT will reject only the case where all three of these conditions hold:

  * The type of the item is non-elementary (e.g. a record or array).

  * There is explicit or implicit initialization required for the object. Note that access values are always implicitly initialized.

  * The address value is non-static.  Here GNAT is more permissive than the RM, and allows the address value to be the address of a previously declared stand-alone variable, as long as it does not itself have an address clause.

                     Anchor  : Some_Initialized_Type;
                     Overlay : Some_Initialized_Type;
                     for Overlay'Address use Anchor'Address;

     However, the prefix of the address clause cannot be an array component, or a component of a discriminated record.

As noted above in section 22.h, address values are typically non-static.  In particular the To_Address function, even if applied to a literal value, is a non-static function call.  To avoid this minor annoyance, GNAT provides the implementation defined attribute 'To_Address.  The following two expressions have identical values:

        To_Address (16#1234_0000#)
        System'To_Address (16#1234_0000#);

except that the second form is considered to be a static expression, and thus when used as an address clause value is always permitted.

Additionally, GNAT treats as static an address clause that is an unchecked_conversion of a static integer value.  This simplifies the porting of legacy code, and provides a portable equivalent to the GNAT attribute To_Address.

Another issue with address clauses is the interaction with alignment requirements.  When an address clause is given for an object, the address value must be consistent with the alignment of the object (which is usually the same as the alignment of the type of the object).  If an address clause is given that specifies an inappropriately aligned address value, then the program execution is erroneous.

Since this source of erroneous behavior can have unfortunate effects, GNAT checks (at compile time if possible, generating a warning, or at execution time with a run-time check) that the alignment is appropriate.  If the run-time check fails, then Program_Error is raised.  This run-time check is suppressed if range checks are suppressed, or if the special GNAT check Alignment_Check is suppressed, or if pragma Restrictions (No_Elaboration_Code) is in effect.

Finally, GNAT does not permit overlaying of objects of controlled types or composite types containing a controlled component. In most cases, the compiler can detect an attempt at such overlays and will generate a warning at compile time and a Program_Error exception at run time.

An address clause cannot be given for an exported object.  More understandably the real restriction is that objects with an address clause cannot be exported.  This is because such variables are not defined by the Ada program, so there is no external object to export.

It is permissible to give an address clause and a pragma Import for the same object.  In this case, the variable is not really defined by the Ada program, so there is no external symbol to be linked.  The link name and the external name are ignored in this case.  The reason that we allow this combination is that it provides a useful idiom to avoid unwanted initializations on objects with address clauses.

When an address clause is given for an object that has implicit or explicit initialization, then by default initialization takes place.  This means that the effect of the object declaration is to overwrite the memory at the specified address.  This is almost always not what the programmer wants, so GNAT will output a warning:

       with System;
       package G is
          type R is record
             M : Integer := 0;
          end record;

          Ext : R;
          for Ext'Address use System'To_Address (16#1234_1234#);
              |
       >>> warning: implicit initialization of "Ext" may
           modify overlaid storage
       >>> warning: use pragma Import for "Ext" to suppress
           initialization (RM B(24))

       end G;

As indicated by the warning message, the solution is to use a (dummy) pragma Import to suppress this initialization.  The pragma tell the compiler that the object is declared and initialized elsewhere.  The following package compiles without warnings (and the initialization is suppressed):

        with System;
        package G is
           type R is record
              M : Integer := 0;
           end record;

           Ext : R;
           for Ext'Address use System'To_Address (16#1234_1234#);
           pragma Import (Ada, Ext);
        end G;

A final issue with address clauses involves their use for overlaying variables, as in the following example:

       A : Integer;
       B : Integer;
       for B'Address use A'Address;

or alternatively, using the form recommended by the RM:

       A    : Integer;
       Addr : constant Address := A'Address;
       B    : Integer;
       for B'Address use Addr;

In both of these cases, A and B become aliased to one another via the address clause. This use of address clauses to overlay variables, achieving an effect similar to unchecked conversion was erroneous in Ada 83, but in Ada 95 and Ada 2005 the effect is implementation defined. Furthermore, the Ada RM specifically recommends that in a situation like this, B should be subject to the following implementation advice (RM 13.3(19)):

     19  If the Address of an object is specified, or it is imported or exported, then the implementation should not perform optimizations based on assumptions of no aliases.

GNAT follows this recommendation, and goes further by also applying this recommendation to the overlaid variable (A in the above example) in this case. This means that the overlay works "as expected", in that a modification to one of the variables will affect the value of the other.

Effect of Convention on Representation
======================================

Normally the specification of a foreign language convention for a type or an object has no effect on the chosen representation.  In particular, the representation chosen for data in GNAT generally meets the standard system conventions, and for example records are laid out in a manner that is consistent with C.  This means that specifying convention C (for example) has no effect.

There are four exceptions to this general rule:

  * Convention Fortran and array subtypes If pragma Convention Fortran is specified for an array subtype, then in accordance with the implementation advice in section 3.6.2(11) of the Ada Reference Manual, the array will be stored in a Fortran-compatible column-major manner, instead of the normal default row-major order.

  * Convention C and enumeration types GNAT normally stores enumeration types in 8, 16, or 32 bits as required to accommodate all values of the type.  For example, for the enumeration type declared by:

             type Color is (Red, Green, Blue);

     8 bits is sufficient to store all values of the type, so by default, objects of type Color will be represented using 8 bits.  However, normal C convention is to use 32 bits for all enum values in C, since enum values are essentially of type int.  If pragma Convention C is specified for an Ada enumeration type, then the size is modified as necessary (usually to 32 bits) to be consistent with the C convention for enum values.

     Note that this treatment applies only to types. If Convention C is given for an enumeration object, where the enumeration type is not Convention C, then Object_Size bits are allocated. For example, for a normal enumeration type, with less than 256 elements, only 8 bits will be allocated for the object. Since this may be a surprise in terms of what C expects, GNAT will issue a warning in this situation. The warning can be suppressed by giving an explicit size clause specifying the desired size.

  * Convention C/Fortran and Boolean types In C, the usual convention for boolean values, that is values used for conditions, is that zero represents false, and nonzero values represent true.  In Ada, the normal convention is that two specific values, typically 0/1, are used to represent false/true respectively.

     Fortran has a similar convention for LOGICAL values (any nonzero value represents true).

     To accommodate the Fortran and C conventions, if a pragma Convention specifies C or Fortran convention for a derived Boolean, as in the following example:

             type C_Switch is new Boolean;
             pragma Convention (C, C_Switch);

     then the GNAT generated code will treat any nonzero value as true.  For truth values generated by GNAT, the conventional value 1 will be used for True, but when one of these values is read, any nonzero value is treated as True.

  * Access types on OpenVMS For 64-bit OpenVMS systems, access types (other than those for unconstrained arrays) are 64-bits long. An exception to this rule is for the case of C-convention access types where there is no explicit size clause present (or inherited for derived types). In this case, GNAT chooses to make these pointers 32-bits, which provides an easier path for migration of 32-bit legacy code. size clause specifying 64-bits must be used to obtain a 64-bit pointer.

Determining the Representations chosen by GNAT
==============================================

Although the descriptions in this section are intended to be complete, it is often easier to simply experiment to see what GNAT accepts and what the effect is on the layout of types and objects.

As required by the Ada RM, if a representation clause is not accepted, then it must be rejected as illegal by the compiler.  However, when a representation clause or pragma is accepted, there can still be questions of what the compiler actually does.  For example, if a partial record representation clause specifies the location of some components and not others, then where are the non-specified components placed? Or if pragma Pack is used on a record, then exactly where are the resulting fields placed? The section on pragma Pack in this chapter can be used to answer the second question, but it is often easier to just see what the compiler does.

For this purpose, GNAT provides the option -gnatR.  If you compile with this option, then the compiler will output information on the actual representations chosen, in a format similar to source representation clauses.  For example, if we compile the package:

     package q is
        type r (x : boolean) is tagged record
           case x is
              when True => S : String (1 .. 100);
              when False => null;
           end case;
        end record;

        type r2 is new r (false) with record
           y2 : integer;
        end record;

        for r2 use record
           y2 at 16 range 0 .. 31;
        end record;

        type x is record
           y : character;
        end record;

        type x1 is array (1 .. 10) of x;
        for x1'component_size use 11;

        type ia is access integer;

        type Rb1 is array (1 .. 13) of Boolean;
        pragma Pack (rb1);

        type Rb2 is array (1 .. 65) of Boolean;
        pragma Pack (rb2);

        type x2 is record
           l1 : Boolean;
           l2 : Duration;
           l3 : Float;
           l4 : Boolean;
           l5 : Rb1;
           l6 : Rb2;
        end record;
        pragma Pack (x2);
     end q;

using the switch -gnatR we obtain the following output:

     Representation information for unit q
     -------------------------------------

     for r'Size use ??;
     for r'Alignment use 4;
     for r use record
        x    at 4 range  0 .. 7;
        _tag at 0 range  0 .. 31;
        s    at 5 range  0 .. 799;
     end record;

     for r2'Size use 160;
     for r2'Alignment use 4;
     for r2 use record
        x       at  4 range  0 .. 7;
        _tag    at  0 range  0 .. 31;
        _parent at  0 range  0 .. 63;
        y2      at 16 range  0 .. 31;
     end record;

     for x'Size use 8;
     for x'Alignment use 1;
     for x use record
        y at 0 range  0 .. 7;
     end record;

     for x1'Size use 112;
     for x1'Alignment use 1;
     for x1'Component_Size use 11;

     for rb1'Size use 13;
     for rb1'Alignment use 2;
     for rb1'Component_Size use 1;

     for rb2'Size use 72;
     for rb2'Alignment use 1;
     for rb2'Component_Size use 1;

     for x2'Size use 224;
     for x2'Alignment use 4;
     for x2 use record
        l1 at  0 range  0 .. 0;
        l2 at  0 range  1 .. 64;
        l3 at 12 range  0 .. 31;
        l4 at 16 range  0 .. 0;
        l5 at 16 range  1 .. 13;
        l6 at 18 range  0 .. 71;
     end record;

The Size values are actually the Object_Size, i.e. the default size that will be allocated for objects of the type. The ?? size for type r indicates that we have a variant record, and the actual size of objects will depend on the discriminant value.

The Alignment values show the actual alignment chosen by the compiler for each record or array type.

The record representation clause for type r shows where all fields are placed, including the compiler generated tag field (whose location cannot be controlled by the programmer).

The record representation clause for the type extension r2 shows all the fields present, including the parent field, which is a copy of the fields of the parent type of r2, i.e. r1.

The component size and size clauses for types rb1 and rb2 show the exact effect of pragma Pack on these arrays, and the record representation clause for type x2 shows how pragma Pack affects this record type.

In some cases, it may be useful to cut and paste the representation clauses generated by the compiler into the original source to fix and guarantee the actual representation to be used.

Standard Library Routines
*************************

The Ada Reference Manual contains in Annex A a full description of an extensive set of standard library routines that can be used in any Ada program, and which must be provided by all Ada compilers.  They are analogous to the standard C library used by C programs.

GNAT implements all of the facilities described in annex A, and for most purposes the description in the Ada Reference Manual, or appropriate Ada text book, will be sufficient for making use of these facilities.

In the case of the input-output facilities, see The Implementation of Standard I/O, gives details on exactly how GNAT interfaces to the file system.  For the remaining packages, the Ada Reference Manual should be sufficient.  The following is a list of the packages included, together with a brief description of the functionality that is provided.

For completeness, references are included to other predefined library routines defined in other sections of the Ada Reference Manual (these are cross-indexed from Annex A).

Ada (A.2)
     This is a parent package for all the standard library packages.  It is usually included implicitly in your program, and itself contains no useful data or routines.

Ada.Calendar (9.6)
     Calendar provides time of day access, and routines for manipulating times and durations.

Ada.Characters (A.3.1)
     This is a dummy parent package that contains no useful entities

Ada.Characters.Handling (A.3.2)
     This package provides some basic character handling capabilities, including classification functions for classes of characters (e.g. test for letters, or digits).

Ada.Characters.Latin_1 (A.3.3)
     This package includes a complete set of definitions of the characters that appear in type CHARACTER.  It is useful for writing programs that will run in international environments.  For example, if you want an upper case E with an acute accent in a string, it is often better to use the definition of UC_E_Acute in this package.  Then your program will print in an understandable manner even if your environment does not support these extended characters.

Ada.Command_Line (A.15)
     This package provides access to the command line parameters and the name of the current program (analogous to the use of argc and argv in C), and also allows the exit status for the program to be set in a system-independent manner.

Ada.Decimal (F.2)
     This package provides constants describing the range of decimal numbers implemented, and also a decimal divide routine (analogous to the COBOL verb DIVIDE ... GIVING ... REMAINDER ...)

Ada.Direct_IO (A.8.4)
     This package provides input-output using a model of a set of records of fixed-length, containing an arbitrary definite Ada type, indexed by an integer record number.

Ada.Dynamic_Priorities (D.5)
     This package allows the priorities of a task to be adjusted dynamically as the task is running.

Ada.Exceptions (11.4.1)
     This package provides additional information on exceptions, and also contains facilities for treating exceptions as data objects, and raising exceptions with associated messages.

Ada.Finalization (7.6)
     This package contains the declarations and subprograms to support the use of controlled types, providing for automatic initialization and finalization (analogous to the constructors and destructors of C++)

Ada.Interrupts (C.3.2)
     This package provides facilities for interfacing to interrupts, which includes the set of signals or conditions that can be raised and recognized as interrupts.

Ada.Interrupts.Names (C.3.2)
     This package provides the set of interrupt names (actually signal or condition names) that can be handled by GNAT.

Ada.IO_Exceptions (A.13)
     This package defines the set of exceptions that can be raised by use of the standard IO packages.

Ada.Numerics
     This package contains some standard constants and exceptions used throughout the numerics packages.  Note that the constants pi and e are defined here, and it is better to use these definitions than rolling your own.

Ada.Numerics.Complex_Elementary_Functions
     Provides the implementation of standard elementary functions (such as log and trigonometric functions) operating on complex numbers using the standard Float and the Complex and Imaginary types created by the package Numerics.Complex_Types.

Ada.Numerics.Complex_Types
     This is a predefined instantiation of Numerics.Generic_Complex_Types using Standard.Float to build the type Complex and Imaginary.

Ada.Numerics.Discrete_Random
     This generic package provides a random number generator suitable for generating uniformly distributed values of a specified discrete subtype.

Ada.Numerics.Float_Random
     This package provides a random number generator suitable for generating uniformly distributed floating point values in the unit interval.

Ada.Numerics.Generic_Complex_Elementary_Functions
     This is a generic version of the package that provides the implementation of standard elementary functions (such as log and trigonometric functions) for an arbitrary complex type.

     The following predefined instantiations of this package are provided:

     Short_Float
          Ada.Numerics.Short_Complex_Elementary_Functions

     Float
          Ada.Numerics.Complex_Elementary_Functions

     Long_Float
          Ada.Numerics.Long_Complex_Elementary_Functions

Ada.Numerics.Generic_Complex_Types
     This is a generic package that allows the creation of complex types, with associated complex arithmetic operations.

     The following predefined instantiations of this package exist

     Short_Float
          Ada.Numerics.Short_Complex_Complex_Types

     Float
          Ada.Numerics.Complex_Complex_Types

     Long_Float
          Ada.Numerics.Long_Complex_Complex_Types

Ada.Numerics.Generic_Elementary_Functions
     This is a generic package that provides the implementation of standard elementary functions (such as log an trigonometric functions) for an arbitrary float type.

     The following predefined instantiations of this package exist

     Short_Float
          Ada.Numerics.Short_Elementary_Functions

     Float
          Ada.Numerics.Elementary_Functions

     Long_Float
          Ada.Numerics.Long_Elementary_Functions

Ada.Real_Time (D.8)
     This package provides facilities similar to those of Calendar, but operating with a finer clock suitable for real time control. Note that annex D requires that there be no backward clock jumps, and GNAT generally guarantees this behavior, but of course if the external clock on which the GNAT runtime depends is deliberately reset by some external event, then such a backward jump may occur.

Ada.Sequential_IO (A.8.1)
     This package provides input-output facilities for sequential files, which can contain a sequence of values of a single type, which can be any Ada type, including indefinite (unconstrained) types.

Ada.Storage_IO (A.9)
     This package provides a facility for mapping arbitrary Ada types to and from a storage buffer.  It is primarily intended for the creation of new IO packages.

Ada.Streams (13.13.1)
     This is a generic package that provides the basic support for the concept of streams as used by the stream attributes (Input, Output, Read and Write).

Ada.Streams.Stream_IO (A.12.1)
     This package is a specialization of the type Streams defined in package Streams together with a set of operations providing Stream_IO capability.  The Stream_IO model permits both random and sequential access to a file which can contain an arbitrary set of values of one or more Ada types.

Ada.Strings (A.4.1)
     This package provides some basic constants used by the string handling packages.

Ada.Strings.Bounded (A.4.4)
     This package provides facilities for handling variable length strings.  The bounded model requires a maximum length.  It is thus somewhat more limited than the unbounded model, but avoids the use of dynamic allocation or finalization.

Ada.Strings.Fixed (A.4.3)
     This package provides facilities for handling fixed length strings.

Ada.Strings.Maps (A.4.2)
     This package provides facilities for handling character mappings and arbitrarily defined subsets of characters.  For instance it is useful in defining specialized translation tables.

Ada.Strings.Maps.Constants (A.4.6)
     This package provides a standard set of predefined mappings and predefined character sets.  For example, the standard upper to lower case conversion table is found in this package.  Note that upper to lower case conversion is non-trivial if you want to take the entire set of characters, including extended characters like E with an acute accent, into account.  You should use the mappings in this package (rather than adding 32 yourself) to do case mappings.

Ada.Strings.Unbounded (A.4.5)
     This package provides facilities for handling variable length strings.  The unbounded model allows arbitrary length strings, but requires the use of dynamic allocation and finalization.

Ada.Strings.Wide_Bounded (A.4.7)
Ada.Strings.Wide_Fixed (A.4.7)
Ada.Strings.Wide_Maps (A.4.7)
Ada.Strings.Wide_Maps.Constants (A.4.7)
Ada.Strings.Wide_Unbounded (A.4.7)
     These packages provide analogous capabilities to the corresponding packages without Wide_ in the name, but operate with the types Wide_String and Wide_Character instead of String and Character.

Ada.Strings.Wide_Wide_Bounded (A.4.7)
Ada.Strings.Wide_Wide_Fixed (A.4.7)
Ada.Strings.Wide_Wide_Maps (A.4.7)
Ada.Strings.Wide_Wide_Maps.Constants (A.4.7)
Ada.Strings.Wide_Wide_Unbounded (A.4.7)
     These packages provide analogous capabilities to the corresponding packages without Wide_ in the name, but operate with the types Wide_Wide_String and Wide_Wide_Character instead of String and Character.

Ada.Synchronous_Task_Control (D.10)
     This package provides some standard facilities for controlling task communication in a synchronous manner.

Ada.Tags
     This package contains definitions for manipulation of the tags of tagged values.

Ada.Task_Attributes
     This package provides the capability of associating arbitrary task-specific data with separate tasks.

Ada.Text_IO
     This package provides basic text input-output capabilities for character, string and numeric data.  The subpackages of this package are listed next.

Ada.Text_IO.Decimal_IO
     Provides input-output facilities for decimal fixed-point types

Ada.Text_IO.Enumeration_IO
     Provides input-output facilities for enumeration types.

Ada.Text_IO.Fixed_IO
     Provides input-output facilities for ordinary fixed-point types.

Ada.Text_IO.Float_IO
     Provides input-output facilities for float types.  The following predefined instantiations of this generic package are available:

     Short_Float
          Short_Float_Text_IO

     Float
          Float_Text_IO

     Long_Float
          Long_Float_Text_IO

Ada.Text_IO.Integer_IO
     Provides input-output facilities for integer types.  The following predefined instantiations of this generic package are available:

     Short_Short_Integer
          Ada.Short_Short_Integer_Text_IO

     Short_Integer
          Ada.Short_Integer_Text_IO

     Integer
          Ada.Integer_Text_IO

     Long_Integer
          Ada.Long_Integer_Text_IO

     Long_Long_Integer
          Ada.Long_Long_Integer_Text_IO

Ada.Text_IO.Modular_IO
     Provides input-output facilities for modular (unsigned) types

Ada.Text_IO.Complex_IO (G.1.3)
     This package provides basic text input-output capabilities for complex data.

Ada.Text_IO.Editing (F.3.3)
     This package contains routines for edited output, analogous to the use of pictures in COBOL.  The picture formats used by this package are a close copy of the facility in COBOL.

Ada.Text_IO.Text_Streams (A.12.2)
     This package provides a facility that allows Text_IO files to be treated as streams, so that the stream attributes can be used for writing arbitrary data, including binary data, to Text_IO files.

Ada.Unchecked_Conversion (13.9)
     This generic package allows arbitrary conversion from one type to another of the same size, providing for breaking the type safety in special circumstances.

     If the types have the same Size (more accurately the same Value_Size), then the effect is simply to transfer the bits from the source to the target type without any modification.  This usage is well defined, and for simple types whose representation is typically the same across all implementations, gives a portable method of performing such conversions.

     If the types do not have the same size, then the result is implementation defined, and thus may be non-portable.  The following describes how GNAT handles such unchecked conversion cases.

     If the types are of different sizes, and are both discrete types, then the effect is of a normal type conversion without any constraint checking. In particular if the result type has a larger size, the result will be zero or sign extended.  If the result type has a smaller size, the result will be truncated by ignoring high order bits.

     If the types are of different sizes, and are not both discrete types, then the conversion works as though pointers were created to the source and target, and the pointer value is converted.  The effect is that bits are copied from successive low order storage units and bits of the source up to the length of the target type.

     A warning is issued if the lengths differ, since the effect in this case is implementation dependent, and the above behavior may not match that of some other compiler.

     A pointer to one type may be converted to a pointer to another type using unchecked conversion.  The only case in which the effect is undefined is when one or both pointers are pointers to unconstrained array types.  In this case, the bounds information may get incorrectly transferred, and in particular, GNAT uses double size pointers for such types, and it is meaningless to convert between such pointer types.  GNAT will issue a warning if the alignment of the target designated type is more strict than the alignment of the source designated type (since the result may be unaligned in this case).

     A pointer other than a pointer to an unconstrained array type may be converted to and from System.Address.  Such usage is common in Ada 83 programs, but note that Ada.Address_To_Access_Conversions is the preferred method of performing such conversions in Ada 95 and Ada 2005. Neither unchecked conversion nor Ada.Address_To_Access_Conversions should be used in conjunction with pointers to unconstrained objects, since the bounds information cannot be handled correctly in this case.

Ada.Unchecked_Deallocation (13.11.2)
     This generic package allows explicit freeing of storage previously allocated by use of an allocator.

Ada.Wide_Text_IO (A.11)
     This package is similar to Ada.Text_IO, except that the external file supports wide character representations, and the internal types are Wide_Character and Wide_String instead of Character and String.  It contains generic subpackages listed next.

Ada.Wide_Text_IO.Decimal_IO
     Provides input-output facilities for decimal fixed-point types

Ada.Wide_Text_IO.Enumeration_IO
     Provides input-output facilities for enumeration types.

Ada.Wide_Text_IO.Fixed_IO
     Provides input-output facilities for ordinary fixed-point types.

Ada.Wide_Text_IO.Float_IO
     Provides input-output facilities for float types.  The following predefined instantiations of this generic package are available:

     Short_Float
          Short_Float_Wide_Text_IO

     Float
          Float_Wide_Text_IO

     Long_Float
          Long_Float_Wide_Text_IO

Ada.Wide_Text_IO.Integer_IO
     Provides input-output facilities for integer types.  The following predefined instantiations of this generic package are available:

     Short_Short_Integer
          Ada.Short_Short_Integer_Wide_Text_IO

     Short_Integer
          Ada.Short_Integer_Wide_Text_IO

     Integer
          Ada.Integer_Wide_Text_IO

     Long_Integer
          Ada.Long_Integer_Wide_Text_IO

     Long_Long_Integer
          Ada.Long_Long_Integer_Wide_Text_IO

Ada.Wide_Text_IO.Modular_IO
     Provides input-output facilities for modular (unsigned) types

Ada.Wide_Text_IO.Complex_IO (G.1.3)
     This package is similar to Ada.Text_IO.Complex_IO, except that the external file supports wide character representations.

Ada.Wide_Text_IO.Editing (F.3.4)
     This package is similar to Ada.Text_IO.Editing, except that the types are Wide_Character and Wide_String instead of Character and String.

Ada.Wide_Text_IO.Streams (A.12.3)
     This package is similar to Ada.Text_IO.Streams, except that the types are Wide_Character and Wide_String instead of Character and String.

Ada.Wide_Wide_Text_IO (A.11)
     This package is similar to Ada.Text_IO, except that the external file supports wide character representations, and the internal types are Wide_Character and Wide_String instead of Character and String.  It contains generic subpackages listed next.

Ada.Wide_Wide_Text_IO.Decimal_IO
     Provides input-output facilities for decimal fixed-point types

Ada.Wide_Wide_Text_IO.Enumeration_IO
     Provides input-output facilities for enumeration types.

Ada.Wide_Wide_Text_IO.Fixed_IO
     Provides input-output facilities for ordinary fixed-point types.

Ada.Wide_Wide_Text_IO.Float_IO
     Provides input-output facilities for float types.  The following predefined instantiations of this generic package are available:

     Short_Float
          Short_Float_Wide_Wide_Text_IO

     Float
          Float_Wide_Wide_Text_IO

     Long_Float
          Long_Float_Wide_Wide_Text_IO

Ada.Wide_Wide_Text_IO.Integer_IO
     Provides input-output facilities for integer types.  The following predefined instantiations of this generic package are available:

     Short_Short_Integer
          Ada.Short_Short_Integer_Wide_Wide_Text_IO

     Short_Integer
          Ada.Short_Integer_Wide_Wide_Text_IO

     Integer
          Ada.Integer_Wide_Wide_Text_IO

     Long_Integer
          Ada.Long_Integer_Wide_Wide_Text_IO

     Long_Long_Integer
          Ada.Long_Long_Integer_Wide_Wide_Text_IO

Ada.Wide_Wide_Text_IO.Modular_IO
     Provides input-output facilities for modular (unsigned) types

Ada.Wide_Wide_Text_IO.Complex_IO (G.1.3)
     This package is similar to Ada.Text_IO.Complex_IO, except that the external file supports wide character representations.

Ada.Wide_Wide_Text_IO.Editing (F.3.4)
     This package is similar to Ada.Text_IO.Editing, except that the types are Wide_Character and Wide_String instead of Character and String.

Ada.Wide_Wide_Text_IO.Streams (A.12.3)
     This package is similar to Ada.Text_IO.Streams, except that the types are Wide_Character and Wide_String instead of Character and String.

The Implementation of Standard I/O
**********************************

GNAT implements all the required input-output facilities described in A.6 through A.14.  These sections of the Ada Reference Manual describe the required behavior of these packages from the Ada point of view, and if you are writing a portable Ada program that does not need to know the exact manner in which Ada maps to the outside world when it comes to reading or writing external files, then you do not need to read this chapter.  As long as your files are all regular files (not pipes or devices), and as long as you write and read the files only from Ada, the description in the Ada Reference Manual is sufficient.

However, if you want to do input-output to pipes or other devices, such as the keyboard or screen, or if the files you are dealing with are either generated by some other language, or to be read by some other language, then you need to know more about the details of how the GNAT implementation of these input-output facilities behaves.

In this chapter we give a detailed description of exactly how GNAT interfaces to the file system.  As always, the sources of the system are available to you for answering questions at an even more detailed level, but for most purposes the information in this chapter will suffice.

Another reason that you may need to know more about how input-output is implemented arises when you have a program written in mixed languages where, for example, files are shared between the C and Ada sections of the same program.  GNAT provides some additional facilities, in the form of additional child library packages, that facilitate this sharing, and these additional facilities are also described in this chapter.

Standard I/O Packages
=====================

The Standard I/O packages described in Annex A for

  * Ada.Text_IO

  * Ada.Text_IO.Complex_IO

  * Ada.Text_IO.Text_Streams

  * Ada.Wide_Text_IO

  * Ada.Wide_Text_IO.Complex_IO

  * Ada.Wide_Text_IO.Text_Streams

  * Ada.Wide_Wide_Text_IO

  * Ada.Wide_Wide_Text_IO.Complex_IO

  * Ada.Wide_Wide_Text_IO.Text_Streams

  * Ada.Stream_IO

  * Ada.Sequential_IO

  * Ada.Direct_IO

are implemented using the C library streams facility; where

  * All files are opened using fopen.

  * All input/output operations use fread/fwrite.

There is no internal buffering of any kind at the Ada library level. The only buffering is that provided at the system level in the implementation of the library routines that support streams. This facilitates shared use of these streams by mixed language programs. Note though that system level buffering is explicitly enabled at elaboration of the standard I/O packages and that can have an impact on mixed language programs, in particular those using I/O before calling the Ada elaboration routine (e.g. adainit). It is recommended to call the Ada elaboration routine before performing any I/O or when impractical, flush the common I/O streams and in particular Standard_Output before elaborating the Ada code.

FORM Strings
============

The format of a FORM string in GNAT is:

     "keyword=value,keyword=value,...,keyword=value"

where letters may be in upper or lower case, and there are no spaces between values.  The order of the entries is not important.  Currently the following keywords defined.

     TEXT_TRANSLATION=[YES|NO]
     SHARED=[YES|NO]
     WCEM=[n|h|u|s|e|8|b]
     ENCODING=[UTF8|8BITS]

The use of these parameters is described later in this section.

Direct_IO
=========

Direct_IO can only be instantiated for definite types.  This is a restriction of the Ada language, which means that the records are fixed length (the length being determined by TYPE'Size, rounded up to the next storage unit boundary if necessary).

The records of a Direct_IO file are simply written to the file in index sequence, with the first record starting at offset zero, and subsequent records following.  There is no control information of any kind.  For example, if 32-bit integers are being written, each record takes 4-bytes, so the record at index K starts at offset (K-1)*4.

There is no limit on the size of Direct_IO files, they are expanded as necessary to accommodate whatever records are written to the file.

Sequential_IO
=============

Sequential_IO may be instantiated with either a definite (constrained) or indefinite (unconstrained) type.

For the definite type case, the elements written to the file are simply the memory images of the data values with no control information of any kind.  The resulting file should be read using the same type, no validity checking is performed on input.

For the indefinite type case, the elements written consist of two parts.  First is the size of the data item, written as the memory image of a Interfaces.C.size_t value, followed by the memory image of the data value.  The resulting file can only be read using the same (unconstrained) type.  Normal assignment checks are performed on these read operations, and if these checks fail, Data_Error is raised.  In particular, in the array case, the lengths must match, and in the variant record case, if the variable for a particular read operation is constrained, the discriminants must match.

Note that it is not possible to use Sequential_IO to write variable length array items, and then read the data back into different length arrays.  For example, the following will raise Data_Error:

      package IO is new Sequential_IO (String);
      F : IO.File_Type;
      S : String (1..4);
      ...
      IO.Create (F)
      IO.Write (F, "hello!")
      IO.Reset (F, Mode=>In_File);
      IO.Read (F, S);
      Put_Line (S);

On some Ada implementations, this will print hell, but the program is clearly incorrect, since there is only one element in the file, and that element is the string hello!.

In Ada 95 and Ada 2005, this kind of behavior can be legitimately achieved using Stream_IO, and this is the preferred mechanism.  In particular, the above program fragment rewritten to use Stream_IO will work correctly.

Text_IO
=======

Text_IO files consist of a stream of characters containing the following special control characters:

     LF (line feed, 16#0A#) Line Mark
     FF (form feed, 16#0C#) Page Mark

A canonical Text_IO file is defined as one in which the following conditions are met:

  * The character LF is used only as a line mark, i.e. to mark the end of the line.

  * The character FF is used only as a page mark, i.e. to mark the end of a page and consequently can appear only immediately following a LF (line mark) character.

  * The file ends with either LF (line mark) or LF-FF (line mark, page mark).  In the former case, the page mark is implicitly assumed to be present.

A file written using Text_IO will be in canonical form provided that no explicit LF or FF characters are written using Put or Put_Line.  There will be no FF character at the end of the file unless an explicit New_Page operation was performed before closing the file.

A canonical Text_IO file that is a regular file (i.e., not a device or a pipe) can be read using any of the routines in Text_IO.  The semantics in this case will be exactly as defined in the Ada Reference Manual, and all the routines in Text_IO are fully implemented.

A text file that does not meet the requirements for a canonical Text_IO file has one of the following:

  * The file contains FF characters not immediately following a LF character.

  * The file contains LF or FF characters written by Put or Put_Line, which are not logically considered to be line marks or page marks.

  * The file ends in a character other than LF or FF, i.e. there is no explicit line mark or page mark at the end of the file.

Text_IO can be used to read such non-standard text files but subprograms to do with line or page numbers do not have defined meanings.  In particular, a FF character that does not follow a LF character may or may not be treated as a page mark from the point of view of page and line numbering.  Every LF character is considered to end a line, and there is an implied LF character at the end of the file.

Stream Pointer Positioning
--------------------------

Ada.Text_IO has a definition of current position for a file that is being read.  No internal buffering occurs in Text_IO, and usually the physical position in the stream used to implement the file corresponds to this logical position defined by Text_IO.  There are two exceptions:

  * After a call to End_Of_Page that returns True, the stream is positioned past the LF (line mark) that precedes the page mark.  Text_IO maintains an internal flag so that subsequent read operations properly handle the logical position which is unchanged by the End_Of_Page call.

  * After a call to End_Of_File that returns True, if the Text_IO file was positioned before the line mark at the end of file before the call, then the logical position is unchanged, but the stream is physically positioned right at the end of file (past the line mark, and past a possible page mark following the line mark.  Again Text_IO maintains internal flags so that subsequent read operations properly handle the logical position.

These discrepancies have no effect on the observable behavior of Text_IO, but if a single Ada stream is shared between a C program and Ada program, or shared (using shared=yes in the form string) between two Ada files, then the difference may be observable in some situations.

Reading and Writing Non-Regular Files
-------------------------------------

A non-regular file is a device (such as a keyboard), or a pipe.  Text_IO can be used for reading and writing.  Writing is not affected and the sequence of characters output is identical to the normal file case, but for reading, the behavior of Text_IO is modified to avoid undesirable look-ahead as follows:

An input file that is not a regular file is considered to have no page marks.  Any Ascii.FF characters (the character normally used for a page mark) appearing in the file are considered to be data characters.  In particular:

  * Get_Line and Skip_Line do not test for a page mark following a line mark.  If a page mark appears, it will be treated as a data character.

  * This avoids the need to wait for an extra character to be typed or entered from the pipe to complete one of these operations.

  * End_Of_Page always returns False

  * End_Of_File will return False if there is a page mark at the end of the file.

Output to non-regular files is the same as for regular files.  Page marks may be written to non-regular files using New_Page, but as noted above they will not be treated as page marks on input if the output is piped to another Ada program.

Another important discrepancy when reading non-regular files is that the end of file indication is not ``sticky''.  If an end of file is entered, e.g. by pressing the EOT key, then end of file is signaled once (i.e. the test End_Of_File will yield True, or a read will raise End_Error), but then reading can resume to read data past that end of file indication, until another end of file indication is entered.

Get_Immediate
-------------

Get_Immediate returns the next character (including control characters) from the input file.  In particular, Get_Immediate will return LF or FF characters used as line marks or page marks.  Such operations leave the file positioned past the control character, and it is thus not treated as having its normal function.  This means that page, line and column counts after this kind of Get_Immediate call are set as though the mark did not occur.  In the case where a Get_Immediate leaves the file positioned between the line mark and page mark (which is not normally possible), it is undefined whether the FF character will be treated as a page mark.

Treating Text_IO Files as Streams
---------------------------------

The package Text_IO.Streams allows a Text_IO file to be treated as a stream.  Data written to a Text_IO file in this stream mode is binary data.  If this binary data contains bytes 16#0A# (LF) or 16#0C# (FF), the resulting file may have non-standard format.  Similarly if read operations are used to read from a Text_IO file treated as a stream, then LF and FF characters may be skipped and the effect is similar to that described above for Get_Immediate.

Text_IO Extensions
------------------

A package GNAT.IO_Aux in the GNAT library provides some useful extensions to the standard Text_IO package:

  * function File_Exists (Name : String) return Boolean; Determines if a file of the given name exists.

  * function Get_Line return String; Reads a string from the standard input file.  The value returned is exactly the length of the line that was read.

  * function Get_Line (File : Ada.Text_IO.File_Type) return String; Similar, except that the parameter File specifies the file from which the string is to be read.

Text_IO Facilities for Unbounded Strings
----------------------------------------

The package Ada.Strings.Unbounded.Text_IO in library files a-suteio.ads/adb contains some GNAT-specific subprograms useful for Text_IO operations on unbounded strings:

  * function Get_Line (File : File_Type) return Unbounded_String; Reads a line from the specified file and returns the result as an unbounded string.

  * procedure Put (File : File_Type; U : Unbounded_String); Writes the value of the given unbounded string to the specified file Similar to the effect of Put (To_String (U)) except that an extra copy is avoided.

  * procedure Put_Line (File : File_Type; U : Unbounded_String); Writes the value of the given unbounded string to the specified file, followed by a New_Line. Similar to the effect of Put_Line (To_String (U)) except that an extra copy is avoided.

In the above procedures, File is of type Ada.Text_IO.File_Type and is optional.  If the parameter is omitted, then the standard input or output file is referenced as appropriate.

The package Ada.Strings.Wide_Unbounded.Wide_Text_IO in library files a-swuwti.ads and a-swuwti.adb provides similar extended Wide_Text_IO functionality for unbounded wide strings.

The package Ada.Strings.Wide_Wide_Unbounded.Wide_Wide_Text_IO in library files a-szuzti.ads and a-szuzti.adb provides similar extended Wide_Wide_Text_IO functionality for unbounded wide wide strings.

Wide_Text_IO
============

Wide_Text_IO is similar in most respects to Text_IO, except that both input and output files may contain special sequences that represent wide character values.  The encoding scheme for a given file may be specified using a FORM parameter:

     WCEM=X

as part of the FORM string (WCEM = wide character encoding method), where X is one of the following characters

h
     Hex ESC encoding

u
     Upper half encoding

s
     Shift-JIS encoding

e
     EUC Encoding

8
     UTF-8 encoding

b
     Brackets encoding

The encoding methods match those that can be used in a source program, but there is no requirement that the encoding method used for the source program be the same as the encoding method used for files, and different files may use different encoding methods.

The default encoding method for the standard files, and for opened files for which no WCEM parameter is given in the FORM string matches the wide character encoding specified for the main program (the default being brackets encoding if no coding method was specified with -gnatW).

Hex Coding
     In this encoding, a wide character is represented by a five character sequence:

          ESC a b c d

     where A, B, C, D are the four hexadecimal characters (using upper case letters) of the wide character code.  For example, ESC A345 is used to represent the wide character with code 16#A345#.  This scheme is compatible with use of the full Wide_Character set.

Upper Half Coding
     The wide character with encoding 16#abcd#, where the upper bit is on (i.e. a is in the range 8-F) is represented as two bytes 16#ab# and 16#cd#.  The second byte may never be a format control character, but is not required to be in the upper half.  This method can be also used for shift-JIS or EUC where the internal coding matches the external coding.

Shift JIS Coding
     A wide character is represented by a two character sequence 16#ab# and 16#cd#, with the restrictions described for upper half encoding as described above.  The internal character code is the corresponding JIS character according to the standard algorithm for Shift-JIS conversion.  Only characters defined in the JIS code set table can be used with this encoding method.

EUC Coding
     A wide character is represented by a two character sequence 16#ab# and 16#cd#, with both characters being in the upper half.  The internal character code is the corresponding JIS character according to the EUC encoding algorithm.  Only characters defined in the JIS code set table can be used with this encoding method.

UTF-8 Coding
     A wide character is represented using UCS Transformation Format 8 (UTF-8) as defined in Annex R of ISO 10646-1/Am.2.  Depending on the character value, the representation is a one, two, or three byte sequence:

          16#0000#-16#007f#: 2#0xxxxxxx#
          16#0080#-16#07ff#: 2#110xxxxx# 2#10xxxxxx#
          16#0800#-16#ffff#: 2#1110xxxx# 2#10xxxxxx# 2#10xxxxxx#

     where the XXX bits correspond to the left-padded bits of the 16-bit character value.  Note that all lower half ASCII characters are represented as ASCII bytes and all upper half characters and other wide characters are represented as sequences of upper-half (The full UTF-8 scheme allows for encoding 31-bit characters as 6-byte sequences, but in this implementation, all UTF-8 sequences of four or more bytes length will raise a Constraint_Error, as will all invalid UTF-8 sequences.)

Brackets Coding
     In this encoding, a wide character is represented by the following eight character sequence:

          [ " a b c d " ]

     where a, b, c, d are the four hexadecimal characters (using uppercase letters) of the wide character code.  For example, ["A345"] is used to represent the wide character with code 16#A345#. This scheme is compatible with use of the full Wide_Character set. On input, brackets coding can also be used for upper half characters, e.g. ["C1"] for lower case a.  However, on output, brackets notation is only used for wide characters with a code greater than 16#FF#.

     Note that brackets coding is not normally used in the context of Wide_Text_IO or Wide_Wide_Text_IO, since it is really just designed as a portable way of encoding source files. In the context of Wide_Text_IO or Wide_Wide_Text_IO, it can only be used if the file does not contain any instance of the left bracket character other than to encode wide character values using the brackets encoding method. In practice it is expected that some standard wide character encoding method such as UTF-8 will be used for text input output.

     If brackets notation is used, then any occurrence of a left bracket in the input file which is not the start of a valid wide character sequence will cause Constraint_Error to be raised. It is possible to encode a left bracket as ["5B"] and Wide_Text_IO and Wide_Wide_Text_IO input will interpret this as a left bracket.

     However, when a left bracket is output, it will be output as a left bracket and not as ["5B"]. We make this decision because for normal use of Wide_Text_IO for outputting messages, it is unpleasant to clobber left brackets. For example, if we write:

             Put_Line ("Start of output [first run]");

     we really do not want to have the left bracket in this message clobbered so that the output reads:

             Start of output ["5B"]first run]

     In practice brackets encoding is reasonably useful for normal Put_Line use since we won't get confused between left brackets and wide character sequences in the output. But for input, or when files are written out and read back in, it really makes better sense to use one of the standard encoding methods such as UTF-8.

For the coding schemes other than UTF-8, Hex, or Brackets encoding, not all wide character values can be represented.  An attempt to output a character that cannot be represented using the encoding scheme for the file causes Constraint_Error to be raised.  An invalid wide character sequence on input also causes Constraint_Error to be raised.

Stream Pointer Positioning
--------------------------

Ada.Wide_Text_IO is similar to Ada.Text_IO in its handling of stream pointer positioning (see Text_IO).  There is one additional case:

If Ada.Wide_Text_IO.Look_Ahead reads a character outside the normal lower ASCII set (i.e. a character in the range:

     Wide_Character'Val (16#0080#) .. Wide_Character'Val (16#FFFF#)

then although the logical position of the file pointer is unchanged by the Look_Ahead call, the stream is physically positioned past the wide character sequence.  Again this is to avoid the need for buffering or backup, and all Wide_Text_IO routines check the internal indication that this situation has occurred so that this is not visible to a normal program using Wide_Text_IO.  However, this discrepancy can be observed if the wide text file shares a stream with another file.

Reading and Writing Non-Regular Files
-------------------------------------

As in the case of Text_IO, when a non-regular file is read, it is assumed that the file contains no page marks (any form characters are treated as data characters), and End_Of_Page always returns False.  Similarly, the end of file indication is not sticky, so it is possible to read beyond an end of file.

Wide_Wide_Text_IO
=================

Wide_Wide_Text_IO is similar in most respects to Text_IO, except that both input and output files may contain special sequences that represent wide wide character values.  The encoding scheme for a given file may be specified using a FORM parameter:

     WCEM=X

as part of the FORM string (WCEM = wide character encoding method), where X is one of the following characters

h
     Hex ESC encoding

u
     Upper half encoding

s
     Shift-JIS encoding

e
     EUC Encoding

8
     UTF-8 encoding

b
     Brackets encoding

The encoding methods match those that can be used in a source program, but there is no requirement that the encoding method used for the source program be the same as the encoding method used for files, and different files may use different encoding methods.

The default encoding method for the standard files, and for opened files for which no WCEM parameter is given in the FORM string matches the wide character encoding specified for the main program (the default being brackets encoding if no coding method was specified with -gnatW).

UTF-8 Coding
     A wide character is represented using UCS Transformation Format 8 (UTF-8) as defined in Annex R of ISO 10646-1/Am.2.  Depending on the character value, the representation is a one, two, three, or four byte sequence:

          16#000000#-16#00007f#: 2#0xxxxxxx#
          16#000080#-16#0007ff#: 2#110xxxxx# 2#10xxxxxx#
          16#000800#-16#00ffff#: 2#1110xxxx# 2#10xxxxxx# 2#10xxxxxx#
          16#010000#-16#10ffff#: 2#11110xxx# 2#10xxxxxx# 2#10xxxxxx# 2#10xxxxxx#

     where the XXX bits correspond to the left-padded bits of the 21-bit character value.  Note that all lower half ASCII characters are represented as ASCII bytes and all upper half characters and other wide characters are represented as sequences of upper-half characters.

Brackets Coding
     In this encoding, a wide wide character is represented by the following eight character sequence if is in wide character range

          [ " a b c d " ]

     and by the following ten character sequence if not

          [ " a b c d e f " ]

     where a, b, c, d, e, and f are the four or six hexadecimal characters (using uppercase letters) of the wide wide character code.  For example, ["01A345"] is used to represent the wide wide character with code 16#01A345#.

     This scheme is compatible with use of the full Wide_Wide_Character set. On input, brackets coding can also be used for upper half characters, e.g. ["C1"] for lower case a.  However, on output, brackets notation is only used for wide characters with a code greater than 16#FF#.

If is also possible to use the other Wide_Character encoding methods, such as Shift-JIS, but the other schemes cannot support the full range of wide wide characters. An attempt to output a character that cannot be represented using the encoding scheme for the file causes Constraint_Error to be raised.  An invalid wide character sequence on input also causes Constraint_Error to be raised.

Stream Pointer Positioning
--------------------------

Ada.Wide_Wide_Text_IO is similar to Ada.Text_IO in its handling of stream pointer positioning (see Text_IO).  There is one additional case:

If Ada.Wide_Wide_Text_IO.Look_Ahead reads a character outside the normal lower ASCII set (i.e. a character in the range:

     Wide_Wide_Character'Val (16#0080#) .. Wide_Wide_Character'Val (16#10FFFF#)

then although the logical position of the file pointer is unchanged by the Look_Ahead call, the stream is physically positioned past the wide character sequence.  Again this is to avoid the need for buffering or backup, and all Wide_Wide_Text_IO routines check the internal indication that this situation has occurred so that this is not visible to a normal program using Wide_Wide_Text_IO.  However, this discrepancy can be observed if the wide text file shares a stream with another file.

Reading and Writing Non-Regular Files
-------------------------------------

As in the case of Text_IO, when a non-regular file is read, it is assumed that the file contains no page marks (any form characters are treated as data characters), and End_Of_Page always returns False.  Similarly, the end of file indication is not sticky, so it is possible to read beyond an end of file.

Stream_IO
=========

A stream file is a sequence of bytes, where individual elements are written to the file as described in the Ada Reference Manual.  The type Stream_Element is simply a byte.  There are two ways to read or write a stream file.

  * The operations Read and Write directly read or write a sequence of stream elements with no control information.

  * The stream attributes applied to a stream file transfer data in the manner described for stream attributes.

Text Translation
================

Text_Translation=XXX may be used as the Form parameter passed to Text_IO.Create and Text_IO.Open: Text_Translation=YES is the default, which means to translate LF to/from CR/LF on Windows systems. Text_Translation=NO disables this translation; i.e. it uses binary mode. For output files, Text_Translation=NO may be used to create Unix-style files on Windows. Text_Translation=XXX has no effect on Unix systems.

Shared Files
============

Section A.14 of the Ada Reference Manual allows implementations to provide a wide variety of behavior if an attempt is made to access the same external file with two or more internal files.

To provide a full range of functionality, while at the same time minimizing the problems of portability caused by this implementation dependence, GNAT handles file sharing as follows:

  * In the absence of a shared=XXX form parameter, an attempt to open two or more files with the same full name is considered an error and is not supported.  The exception Use_Error will be raised.  Note that a file that is not explicitly closed by the program remains open until the program terminates.

  * If the form parameter shared=no appears in the form string, the file can be opened or created with its own separate stream identifier, regardless of whether other files sharing the same external file are opened.  The exact effect depends on how the C stream routines handle multiple accesses to the same external files using separate streams.

  * If the form parameter shared=yes appears in the form string for each of two or more files opened using the same full name, the same stream is shared between these files, and the semantics are as described in Ada Reference Manual, Section A.14.

When a program that opens multiple files with the same name is ported from another Ada compiler to GNAT, the effect will be that Use_Error is raised.

The documentation of the original compiler and the documentation of the program should then be examined to determine if file sharing was expected, and shared=XXX parameters added to Open and Create calls as required.

When a program is ported from GNAT to some other Ada compiler, no special attention is required unless the shared=XXX form parameter is used in the program.  In this case, you must examine the documentation of the new compiler to see if it supports the required file sharing semantics, and form strings modified appropriately.  Of course it may be the case that the program cannot be ported if the target compiler does not support the required functionality.  The best approach in writing portable code is to avoid file sharing (and hence the use of the shared=XXX parameter in the form string) completely.

One common use of file sharing in Ada 83 is the use of instantiations of Sequential_IO on the same file with different types, to achieve heterogeneous input-output.  Although this approach will work in GNAT if shared=yes is specified, it is preferable in Ada to use Stream_IO for this purpose (using the stream attributes)

Filenames encoding
==================

An encoding form parameter can be used to specify the filename encoding encoding=XXX.

  * If the form parameter encoding=utf8 appears in the form string, the filename must be encoded in UTF-8.

  * If the form parameter encoding=8bits appears in the form string, the filename must be a standard 8bits string.

In the absence of a encoding=XXX form parameter, the encoding is controlled by the GNAT_CODE_PAGE environment variable. And if not set utf8 is assumed.

CP_ACP
     The current system Windows ANSI code page.

CP_UTF8
     UTF-8 encoding

This encoding form parameter is only supported on the Windows platform. On the other Operating Systems the run-time is supporting UTF-8 natively.

Open Modes
==========

Open and Create calls result in a call to fopen using the mode shown in the following table:

Open and Create Call Modes

                                    OPEN            CREATE
     Append_File                    "r+"             "w+"
     In_File                        "r"              "w+"
     Out_File (Direct_IO)           "r+"             "w"
     Out_File (all other cases)     "w"              "w"
     Inout_File                     "r+"             "w+"

If text file translation is required, then either b or t is added to the mode, depending on the setting of Text.  Text file translation refers to the mapping of CR/LF sequences in an external file to LF characters internally.  This mapping only occurs in DOS and DOS-like systems, and is not relevant to other systems.

A special case occurs with Stream_IO.  As shown in the above table, the file is initially opened in r or w mode for the In_File and Out_File cases.  If a Set_Mode operation subsequently requires switching from reading to writing or vice-versa, then the file is reopened in r+ mode to permit the required operation.

Operations on C Streams
=======================

The package Interfaces.C_Streams provides an Ada program with direct access to the C library functions for operations on C streams:

     package Interfaces.C_Streams is
       -- Note: the reason we do not use the types that are in
       -- Interfaces.C is that we want to avoid dragging in the
       -- code in this unit if possible.
       subtype chars is System.Address;
       -- Pointer to null-terminated array of characters
       subtype FILEs is System.Address;
       -- Corresponds to the C type FILE*
       subtype voids is System.Address;
       -- Corresponds to the C type void*
       subtype int is Integer;
       subtype long is Long_Integer;
       -- Note: the above types are subtypes deliberately, and it
       -- is part of this spec that the above correspondences are
       -- guaranteed.  This means that it is legitimate to, for
       -- example, use Integer instead of int.  We provide these
       -- synonyms for clarity, but in some cases it may be
       -- convenient to use the underlying types (for example to
       -- avoid an unnecessary dependency of a spec on the spec
       -- of this unit).
       type size_t is mod 2 ** Standard'Address_Size;
       NULL_Stream : constant FILEs;
       -- Value returned (NULL in C) to indicate an
       -- fdopen/fopen/tmpfile error
       ----------------------------------
       -- Constants Defined in stdio.h --
       ----------------------------------
       EOF : constant int;
       -- Used by a number of routines to indicate error or
       -- end of file
       IOFBF : constant int;
       IOLBF : constant int;
       IONBF : constant int;
       -- Used to indicate buffering mode for setvbuf call
       SEEK_CUR : constant int;
       SEEK_END : constant int;
       SEEK_SET : constant int;
       -- Used to indicate origin for fseek call
       function stdin return FILEs;
       function stdout return FILEs;
       function stderr return FILEs;
       -- Streams associated with standard files
       --------------------------
       -- Standard C functions --
       --------------------------
       -- The functions selected below are ones that are
       -- available in UNIX (but not necessarily in ANSI C).
       -- These are very thin interfaces
       -- which copy exactly the C headers.  For more
       -- documentation on these functions, see the Microsoft C
       -- "Run-Time Library Reference" (Microsoft Press, 1990,
       -- ISBN 1-55615-225-6), which includes useful information
       -- on system compatibility.
       procedure clearerr (stream : FILEs);
       function fclose (stream : FILEs) return int;
       function fdopen (handle : int; mode : chars) return FILEs;
       function feof (stream : FILEs) return int;
       function ferror (stream : FILEs) return int;
       function fflush (stream : FILEs) return int;
       function fgetc (stream : FILEs) return int;
       function fgets (strng : chars; n : int; stream : FILEs)
           return chars;
       function fileno (stream : FILEs) return int;
       function fopen (filename : chars; Mode : chars)
           return FILEs;
       -- Note: to maintain target independence, use
       -- text_translation_required, a boolean variable defined in
       -- a-sysdep.c to deal with the target dependent text
       -- translation requirement.  If this variable is set,
       -- then  b/t should be appended to the standard mode
       -- argument to set the text translation mode off or on
       -- as required.
       function fputc (C : int; stream : FILEs) return int;
       function fputs (Strng : chars; Stream : FILEs) return int;
       function fread
          (buffer : voids;
           size : size_t;
           count : size_t;
           stream : FILEs)
           return size_t;
       function freopen
          (filename : chars;
           mode : chars;
           stream : FILEs)
           return FILEs;
       function fseek
          (stream : FILEs;
           offset : long;
           origin : int)
           return int;
       function ftell (stream : FILEs) return long;
       function fwrite
          (buffer : voids;
           size : size_t;
           count : size_t;
           stream : FILEs)
           return size_t;
       function isatty (handle : int) return int;
       procedure mktemp (template : chars);
       -- The return value (which is just a pointer to template)
       -- is discarded
       procedure rewind (stream : FILEs);
       function rmtmp return int;
       function setvbuf
          (stream : FILEs;
           buffer : chars;
           mode : int;
           size : size_t)
           return int;

       function tmpfile return FILEs;
       function ungetc (c : int; stream : FILEs) return int;
       function unlink (filename : chars) return int;
       ---------------------
       -- Extra functions --
       ---------------------
       -- These functions supply slightly thicker bindings than
       -- those above.  They are derived from functions in the
       -- C Run-Time Library, but may do a bit more work than
       -- just directly calling one of the Library functions.
       function is_regular_file (handle : int) return int;
       -- Tests if given handle is for a regular file (result 1)
       -- or for a non-regular file (pipe or device, result 0).
       ---------------------------------
       -- Control of Text/Binary Mode --
       ---------------------------------
       -- If text_translation_required is true, then the following
       -- functions may be used to dynamically switch a file from
       -- binary to text mode or vice versa.  These functions have
       -- no effect if text_translation_required is false (i.e. in
       -- normal UNIX mode).  Use fileno to get a stream handle.
       procedure set_binary_mode (handle : int);
       procedure set_text_mode (handle : int);
       ----------------------------
       -- Full Path Name support --
       ----------------------------
       procedure full_name (nam : chars; buffer : chars);
       -- Given a NUL terminated string representing a file
       -- name, returns in buffer a NUL terminated string
       -- representing the full path name for the file name.
       -- On systems where it is relevant the   drive is also
       -- part of the full path name.  It is the responsibility
       -- of the caller to pass an actual parameter for buffer
       -- that is big enough for any full path name.  Use
       -- max_path_len given below as the size of buffer.
       max_path_len : integer;
       -- Maximum length of an allowable full path name on the
       -- system, including a terminating NUL character.
     end Interfaces.C_Streams;

Interfacing to C Streams
========================

The packages in this section permit interfacing Ada files to C Stream operations.

      with Interfaces.C_Streams;
      package Ada.Sequential_IO.C_Streams is
         function C_Stream (F : File_Type)
            return Interfaces.C_Streams.FILEs;
         procedure Open
           (File : in out File_Type;
            Mode : in File_Mode;
            C_Stream : in Interfaces.C_Streams.FILEs;
            Form : in String := "");
      end Ada.Sequential_IO.C_Streams;

       with Interfaces.C_Streams;
       package Ada.Direct_IO.C_Streams is
          function C_Stream (F : File_Type)
             return Interfaces.C_Streams.FILEs;
          procedure Open
            (File : in out File_Type;
             Mode : in File_Mode;
             C_Stream : in Interfaces.C_Streams.FILEs;
             Form : in String := "");
       end Ada.Direct_IO.C_Streams;

       with Interfaces.C_Streams;
       package Ada.Text_IO.C_Streams is
          function C_Stream (F : File_Type)
             return Interfaces.C_Streams.FILEs;
          procedure Open
            (File : in out File_Type;
             Mode : in File_Mode;
             C_Stream : in Interfaces.C_Streams.FILEs;
             Form : in String := "");
       end Ada.Text_IO.C_Streams;

       with Interfaces.C_Streams;
       package Ada.Wide_Text_IO.C_Streams is
          function C_Stream (F : File_Type)
             return Interfaces.C_Streams.FILEs;
          procedure Open
            (File : in out File_Type;
             Mode : in File_Mode;
             C_Stream : in Interfaces.C_Streams.FILEs;
             Form : in String := "");
      end Ada.Wide_Text_IO.C_Streams;

       with Interfaces.C_Streams;
       package Ada.Wide_Wide_Text_IO.C_Streams is
          function C_Stream (F : File_Type)
             return Interfaces.C_Streams.FILEs;
          procedure Open
            (File : in out File_Type;
             Mode : in File_Mode;
             C_Stream : in Interfaces.C_Streams.FILEs;
             Form : in String := "");
      end Ada.Wide_Wide_Text_IO.C_Streams;

      with Interfaces.C_Streams;
      package Ada.Stream_IO.C_Streams is
         function C_Stream (F : File_Type)
            return Interfaces.C_Streams.FILEs;
         procedure Open
           (File : in out File_Type;
            Mode : in File_Mode;
            C_Stream : in Interfaces.C_Streams.FILEs;
            Form : in String := "");
      end Ada.Stream_IO.C_Streams;

In each of these six packages, the C_Stream function obtains the FILE pointer from a currently opened Ada file.  It is then possible to use the Interfaces.C_Streams package to operate on this stream, or the stream can be passed to a C program which can operate on it directly.  Of course the program is responsible for ensuring that only appropriate sequences of operations are executed.

One particular use of relevance to an Ada program is that the setvbuf function can be used to control the buffering of the stream used by an Ada file.  In the absence of such a call the standard default buffering is used.

The Open procedures in these packages open a file giving an existing C Stream instead of a file name.  Typically this stream is imported from a C program, allowing an Ada file to operate on an existing C file.

The GNAT Library
****************

The GNAT library contains a number of general and special purpose packages. It represents functionality that the GNAT developers have found useful, and which is made available to GNAT users.  The packages described here are fully supported, and upwards compatibility will be maintained in future releases, so you can use these facilities with the confidence that the same functionality will be available in future releases.

The chapter here simply gives a brief summary of the facilities available. The full documentation is found in the spec file for the package.  The full sources of these library packages, including both spec and body, are provided with all GNAT releases.  For example, to find out the full specifications of the SPITBOL pattern matching capability, including a full tutorial and extensive examples, look in the g-spipat.ads file in the library.

For each entry here, the package name (as it would appear in a with clause) is given, followed by the name of the corresponding spec file in parentheses.  The packages are children in four hierarchies, Ada, Interfaces, System, and GNAT, the latter being a GNAT-specific hierarchy.

Note that an application program should only use packages in one of these four hierarchies if the package is defined in the Ada Reference Manual, or is listed in this section of the GNAT Programmers Reference Manual. All other units should be considered internal implementation units and should not be directly with'ed by application code.  The use of a with statement that references one of these internal implementation units makes an application potentially dependent on changes in versions of GNAT, and will generate a warning message.

Ada.Characters.Latin_9 (a-chlat9.ads)
=====================================

This child of Ada.Characters provides a set of definitions corresponding to those in the RM-defined package Ada.Characters.Latin_1 but with the few modifications required for Latin-9 The provision of such a package is specifically authorized by the Ada Reference Manual (RM A.3.3(27)).

Ada.Characters.Wide_Latin_1 (a-cwila1.ads)
==========================================

This child of Ada.Characters provides a set of definitions corresponding to those in the RM-defined package Ada.Characters.Latin_1 but with the types of the constants being Wide_Character instead of Character.  The provision of such a package is specifically authorized by the Ada Reference Manual (RM A.3.3(27)).

Ada.Characters.Wide_Latin_9 (a-cwila1.ads)
==========================================

This child of Ada.Characters provides a set of definitions corresponding to those in the GNAT defined package Ada.Characters.Latin_9 but with the types of the constants being Wide_Character instead of Character.  The provision of such a package is specifically authorized by the Ada Reference Manual (RM A.3.3(27)).

Ada.Characters.Wide_Wide_Latin_1 (a-chzla1.ads)
===============================================

This child of Ada.Characters provides a set of definitions corresponding to those in the RM-defined package Ada.Characters.Latin_1 but with the types of the constants being Wide_Wide_Character instead of Character.  The provision of such a package is specifically authorized by the Ada Reference Manual (RM A.3.3(27)).

Ada.Characters.Wide_Wide_Latin_9 (a-chzla9.ads)
===============================================

This child of Ada.Characters provides a set of definitions corresponding to those in the GNAT defined package Ada.Characters.Latin_9 but with the types of the constants being Wide_Wide_Character instead of Character.  The provision of such a package is specifically authorized by the Ada Reference Manual (RM A.3.3(27)).

Ada.Containers.Formal_Doubly_Linked_Lists (a-cfdlli.ads)
========================================================

This child of Ada.Containers defines a modified version of the Ada 2005 container for doubly linked lists, meant to facilitate formal verification of code using such containers.

Ada.Containers.Formal_Hashed_Maps (a-cfhama.ads)
================================================

This child of Ada.Containers defines a modified version of the Ada 2005 container for hashed maps, meant to facilitate formal verification of code using such containers.

Ada.Containers.Formal_Hashed_Sets (a-cfhase.ads)
================================================

This child of Ada.Containers defines a modified version of the Ada 2005 container for hashed sets, meant to facilitate formal verification of code using such containers.

Ada.Containers.Formal_Ordered_Maps (a-cforma.ads)
=================================================

This child of Ada.Containers defines a modified version of the Ada 2005 container for ordered maps, meant to facilitate formal verification of code using such containers.

Ada.Containers.Formal_Ordered_Sets (a-cforse.ads)
=================================================

This child of Ada.Containers defines a modified version of the Ada 2005 container for ordered sets, meant to facilitate formal verification of code using such containers.

Ada.Containers.Formal_Vectors (a-cofove.ads)
============================================

This child of Ada.Containers defines a modified version of the Ada 2005 container for vectors, meant to facilitate formal verification of code using such containers.

Ada.Command_Line.Environment (a-colien.ads)
===========================================

This child of Ada.Command_Line provides a mechanism for obtaining environment values on systems where this concept makes sense.

Ada.Command_Line.Remove (a-colire.ads)
======================================

This child of Ada.Command_Line provides a mechanism for logically removing arguments from the argument list.  Once removed, an argument is not visible to further calls on the subprograms in Ada.Command_Line will not see the removed argument.

Ada.Command_Line.Response_File (a-clrefi.ads)
=============================================

This child of Ada.Command_Line provides a mechanism facilities for getting command line arguments from a text file, called a "response file". Using a response file allow passing a set of arguments to an executable longer than the maximum allowed by the system on the command line.

Ada.Direct_IO.C_Streams (a-diocst.ads)
======================================

This package provides subprograms that allow interfacing between C streams and Direct_IO.  The stream identifier can be extracted from a file opened on the Ada side, and an Ada file can be constructed from a stream opened on the C side.

Ada.Exceptions.Is_Null_Occurrence (a-einuoc.ads)
================================================

This child subprogram provides a way of testing for the null exception occurrence (Null_Occurrence) without raising an exception.

Ada.Exceptions.Last_Chance_Handler (a-elchha.ads)
=================================================

This child subprogram is used for handling otherwise unhandled exceptions (hence the name last chance), and perform clean ups before terminating the program. Note that this subprogram never returns.

Ada.Exceptions.Traceback (a-exctra.ads)
=======================================

This child package provides the subprogram (Tracebacks) to give a traceback array of addresses based on an exception occurrence.

Ada.Sequential_IO.C_Streams (a-siocst.ads)
==========================================

This package provides subprograms that allow interfacing between C streams and Sequential_IO.  The stream identifier can be extracted from a file opened on the Ada side, and an Ada file can be constructed from a stream opened on the C side.

Ada.Streams.Stream_IO.C_Streams (a-ssicst.ads)
==============================================

This package provides subprograms that allow interfacing between C streams and Stream_IO.  The stream identifier can be extracted from a file opened on the Ada side, and an Ada file can be constructed from a stream opened on the C side.

Ada.Strings.Unbounded.Text_IO (a-suteio.ads)
============================================

This package provides subprograms for Text_IO for unbounded strings, avoiding the necessity for an intermediate operation with ordinary strings.

Ada.Strings.Wide_Unbounded.Wide_Text_IO (a-swuwti.ads)
======================================================

This package provides subprograms for Text_IO for unbounded wide strings, avoiding the necessity for an intermediate operation with ordinary wide strings.

Ada.Strings.Wide_Wide_Unbounded.Wide_Wide_Text_IO (a-szuzti.ads)
================================================================

This package provides subprograms for Text_IO for unbounded wide wide strings, avoiding the necessity for an intermediate operation with ordinary wide wide strings.

Ada.Text_IO.C_Streams (a-tiocst.ads)
====================================

This package provides subprograms that allow interfacing between C streams and Text_IO.  The stream identifier can be extracted from a file opened on the Ada side, and an Ada file can be constructed from a stream opened on the C side.

Ada.Text_IO.Reset_Standard_Files (a-tirsfi.ads)
===============================================

This procedure is used to reset the status of the standard files used by Ada.Text_IO.  This is useful in a situation (such as a restart in an embedded application) where the status of the files may change during execution (for example a standard input file may be redefined to be interactive).

Ada.Wide_Characters.Unicode (a-wichun.ads)
==========================================

This package provides subprograms that allow categorization of Wide_Character values according to Unicode categories.

Ada.Wide_Text_IO.C_Streams (a-wtcstr.ads)
=========================================

This package provides subprograms that allow interfacing between C streams and Wide_Text_IO.  The stream identifier can be extracted from a file opened on the Ada side, and an Ada file can be constructed from a stream opened on the C side.

Ada.Wide_Text_IO.Reset_Standard_Files (a-wrstfi.ads)
====================================================

This procedure is used to reset the status of the standard files used by Ada.Wide_Text_IO.  This is useful in a situation (such as a restart in an embedded application) where the status of the files may change during execution (for example a standard input file may be redefined to be interactive).

Ada.Wide_Wide_Characters.Unicode (a-zchuni.ads)
===============================================

This package provides subprograms that allow categorization of Wide_Wide_Character values according to Unicode categories.

Ada.Wide_Wide_Text_IO.C_Streams (a-ztcstr.ads)
==============================================

This package provides subprograms that allow interfacing between C streams and Wide_Wide_Text_IO.  The stream identifier can be extracted from a file opened on the Ada side, and an Ada file can be constructed from a stream opened on the C side.

Ada.Wide_Wide_Text_IO.Reset_Standard_Files (a-zrstfi.ads)
=========================================================

This procedure is used to reset the status of the standard files used by Ada.Wide_Wide_Text_IO. This is useful in a situation (such as a restart in an embedded application) where the status of the files may change during execution (for example a standard input file may be redefined to be interactive).

GNAT.Altivec (g-altive.ads)
===========================

This is the root package of the GNAT AltiVec binding. It provides definitions of constants and types common to all the versions of the binding.

GNAT.Altivec.Conversions (g-altcon.ads)
=======================================

This package provides the Vector/View conversion routines.

GNAT.Altivec.Vector_Operations (g-alveop.ads)
=============================================

This package exposes the Ada interface to the AltiVec operations on vector objects. A soft emulation is included by default in the GNAT library. The hard binding is provided as a separate package. This unit is common to both bindings.

GNAT.Altivec.Vector_Types (g-alvety.ads)
========================================

This package exposes the various vector types part of the Ada binding to AltiVec facilities.

GNAT.Altivec.Vector_Views (g-alvevi.ads)
========================================

This package provides public 'View' data types from/to which private vector representations can be converted via GNAT.Altivec.Conversions. This allows convenient access to individual vector elements and provides a simple way to initialize vector objects.

GNAT.Array_Split (g-arrspl.ads)
===============================

Useful array-manipulation routines: given a set of separators, split an array wherever the separators appear, and provide direct access to the resulting slices.

GNAT.AWK (g-awk.ads)
====================

Provides AWK-like parsing functions, with an easy interface for parsing one or more files containing formatted data.  The file is viewed as a database where each record is a line and a field is a data element in this line.

GNAT.Bounded_Buffers (g-boubuf.ads)
===================================

Provides a concurrent generic bounded buffer abstraction.  Instances are useful directly or as parts of the implementations of other abstractions, such as mailboxes.

GNAT.Bounded_Mailboxes (g-boumai.ads)
=====================================

Provides a thread-safe asynchronous intertask mailbox communication facility.

GNAT.Bubble_Sort (g-bubsor.ads)
===============================

Provides a general implementation of bubble sort usable for sorting arbitrary data items.  Exchange and comparison procedures are provided by passing access-to-procedure values.

GNAT.Bubble_Sort_A (g-busora.ads)
=================================

Provides a general implementation of bubble sort usable for sorting arbitrary data items.  Move and comparison procedures are provided by passing access-to-procedure values. This is an older version, retained for compatibility. Usually GNAT.Bubble_Sort will be preferable.

GNAT.Bubble_Sort_G (g-busorg.ads)
=================================

Similar to Bubble_Sort_A except that the move and sorting procedures are provided as generic parameters, this improves efficiency, especially if the procedures can be inlined, at the expense of duplicating code for multiple instantiations.

GNAT.Byte_Order_Mark (g-byorma.ads)
===================================

Provides a routine which given a string, reads the start of the string to see whether it is one of the standard byte order marks (BOM's) which signal the encoding of the string. The routine includes detection of special XML sequences for various UCS input formats.

GNAT.Byte_Swapping (g-bytswa.ads)
=================================

General routines for swapping the bytes in 2-, 4-, and 8-byte quantities. Machine-specific implementations are available in some cases.

GNAT.Calendar (g-calend.ads)
============================

Extends the facilities provided by Ada.Calendar to include handling of days of the week, an extended Split and Time_Of capability. Also provides conversion of Ada.Calendar.Time values to and from the C timeval format.

GNAT.Calendar.Time_IO (g-catiio.ads)
====================================

GNAT.CRC32 (g-crc32.ads)
========================

This package implements the CRC-32 algorithm.  For a full description of this algorithm see ``Computation of Cyclic Redundancy Checks via Table Look-Up'', Communications of the ACM, Vol. 31 No. 8, pp. 1008-1013, Aug. 1988.  Sarwate, D.V.

GNAT.Case_Util (g-casuti.ads)
=============================

A set of simple routines for handling upper and lower casing of strings without the overhead of the full casing tables in Ada.Characters.Handling.

GNAT.CGI (g-cgi.ads)
====================

This is a package for interfacing a GNAT program with a Web server via the Common Gateway Interface (CGI).  Basically this package parses the CGI parameters, which are a set of key/value pairs sent by the Web server.  It builds a table whose index is the key and provides some services to deal with this table.

GNAT.CGI.Cookie (g-cgicoo.ads)
==============================

This is a package to interface a GNAT program with a Web server via the Common Gateway Interface (CGI).  It exports services to deal with Web cookies (piece of information kept in the Web client software).

GNAT.CGI.Debug (g-cgideb.ads)
=============================

This is a package to help debugging CGI (Common Gateway Interface) programs written in Ada.

GNAT.Command_Line (g-comlin.ads)
================================

Provides a high level interface to Ada.Command_Line facilities, including the ability to scan for named switches with optional parameters and expand file names using wild card notations.

GNAT.Compiler_Version (g-comver.ads)
====================================

Provides a routine for obtaining the version of the compiler used to compile the program. More accurately this is the version of the binder used to bind the program (this will normally be the same as the version of the compiler if a consistent tool set is used to compile all units of a partition).

GNAT.Ctrl_C (g-ctrl_c.ads)
==========================

Provides a simple interface to handle Ctrl-C keyboard events.

GNAT.Current_Exception (g-curexc.ads)
=====================================

Provides access to information on the current exception that has been raised without the need for using the Ada 95 / Ada 2005 exception choice parameter specification syntax. This is particularly useful in simulating typical facilities for obtaining information about exceptions provided by Ada 83 compilers.

GNAT.Debug_Pools (g-debpoo.ads)
===============================

Provide a debugging storage pools that helps tracking memory corruption problems.  See The GNAT Debug Pool Facility (GNAT User's Guide).

GNAT.Debug_Utilities (g-debuti.ads)
===================================

Provides a few useful utilities for debugging purposes, including conversion to and from string images of address values. Supports both C and Ada formats for hexadecimal literals.

GNAT.Decode_String (g-decstr.ads)
=================================

A generic package providing routines for decoding wide character and wide wide character strings encoded as sequences of 8-bit characters using a specified encoding method. Includes validation routines, and also routines for stepping to next or previous encoded character in an encoded string. Useful in conjunction with Unicode character coding. Note there is a preinstantiation for UTF-8. See next entry.

GNAT.Decode_UTF8_String (g-deutst.ads)
======================================

A preinstantiation of GNAT.Decode_Strings for UTF-8 encoding.

GNAT.Directory_Operations (g-dirope.ads)
========================================

Provides a set of routines for manipulating directories, including changing the current directory, making new directories, and scanning the files in a directory.

GNAT.Directory_Operations.Iteration (g-diopit.ads)
==================================================

A child unit of GNAT.Directory_Operations providing additional operations for iterating through directories.

GNAT.Dynamic_HTables (g-dynhta.ads)
===================================

A generic implementation of hash tables that can be used to hash arbitrary data.  Provided in two forms, a simple form with built in hash functions, and a more complex form in which the hash function is supplied.

This package provides a facility similar to that of GNAT.HTable, except that this package declares a type that can be used to define dynamic instances of the hash table, while an instantiation of GNAT.HTable creates a single instance of the hash table.

GNAT.Dynamic_Tables (g-dyntab.ads)
==================================

A generic package providing a single dimension array abstraction where the length of the array can be dynamically modified.

This package provides a facility similar to that of GNAT.Table, except that this package declares a type that can be used to define dynamic instances of the table, while an instantiation of GNAT.Table creates a single instance of the table type.

GNAT.Encode_String (g-encstr.ads)
=================================

A generic package providing routines for encoding wide character and wide wide character strings as sequences of 8-bit characters using a specified encoding method. Useful in conjunction with Unicode character coding. Note there is a preinstantiation for UTF-8. See next entry.

GNAT.Encode_UTF8_String (g-enutst.ads)
======================================

A preinstantiation of GNAT.Encode_Strings for UTF-8 encoding.

GNAT.Exception_Actions (g-excact.ads)
=====================================

Provides callbacks when an exception is raised. Callbacks can be registered for specific exceptions, or when any exception is raised. This can be used for instance to force a core dump to ease debugging.

GNAT.Exception_Traces (g-exctra.ads)
====================================

Provides an interface allowing to control automatic output upon exception occurrences.

GNAT.Exceptions (g-expect.ads)
==============================

Normally it is not possible to raise an exception with a message from a subprogram in a pure package, since the necessary types and subprograms are in Ada.Exceptions which is not a pure unit. GNAT.Exceptions provides a facility for getting around this limitation for a few predefined exceptions, and for example allow raising Constraint_Error with a message from a pure subprogram.

GNAT.Expect (g-expect.ads)
==========================

Provides a set of subprograms similar to what is available with the standard Tcl Expect tool. It allows you to easily spawn and communicate with an external process. You can send commands or inputs to the process, and compare the output with some expected regular expression. Currently GNAT.Expect is implemented on all native GNAT ports except for OpenVMS. It is not implemented for cross ports, and in particular is not implemented for VxWorks or LynxOS.

GNAT.Float_Control (g-flocon.ads)
=================================

Provides an interface for resetting the floating-point processor into the mode required for correct semantic operation in Ada.  Some third party library calls may cause this mode to be modified, and the Reset procedure in this package can be used to reestablish the required mode.

GNAT.Heap_Sort (g-heasor.ads)
=============================

Provides a general implementation of heap sort usable for sorting arbitrary data items. Exchange and comparison procedures are provided by passing access-to-procedure values.  The algorithm used is a modified heap sort that performs approximately N*log(N) comparisons in the worst case.

GNAT.Heap_Sort_A (g-hesora.ads)
===============================

Provides a general implementation of heap sort usable for sorting arbitrary data items. Move and comparison procedures are provided by passing access-to-procedure values.  The algorithm used is a modified heap sort that performs approximately N*log(N) comparisons in the worst case. This differs from GNAT.Heap_Sort in having a less convenient interface, but may be slightly more efficient.

GNAT.Heap_Sort_G (g-hesorg.ads)
===============================

Similar to Heap_Sort_A except that the move and sorting procedures are provided as generic parameters, this improves efficiency, especially if the procedures can be inlined, at the expense of duplicating code for multiple instantiations.

GNAT.HTable (g-htable.ads)
==========================

A generic implementation of hash tables that can be used to hash arbitrary data.  Provides two approaches, one a simple static approach, and the other allowing arbitrary dynamic hash tables.

GNAT.IO (g-io.ads)
==================

A simple preelaborable input-output package that provides a subset of simple Text_IO functions for reading characters and strings from Standard_Input, and writing characters, strings and integers to either Standard_Output or Standard_Error.

GNAT.IO_Aux (g-io_aux.ads)
==========================

Provides some auxiliary functions for use with Text_IO, including a test for whether a file exists, and functions for reading a line of text.

GNAT.Lock_Files (g-locfil.ads)
==============================

Provides a general interface for using files as locks.  Can be used for providing program level synchronization.

GNAT.MBBS_Discrete_Random (g-mbdira.ads)
========================================

The original implementation of Ada.Numerics.Discrete_Random.  Uses a modified version of the Blum-Blum-Shub generator.

GNAT.MBBS_Float_Random (g-mbflra.ads)
=====================================

The original implementation of Ada.Numerics.Float_Random.  Uses a modified version of the Blum-Blum-Shub generator.

GNAT.MD5 (g-md5.ads)
====================

Implements the MD5 Message-Digest Algorithm as described in RFC 1321.

GNAT.Memory_Dump (g-memdum.ads)
===============================

Provides a convenient routine for dumping raw memory to either the standard output or standard error files. Uses GNAT.IO for actual output.

GNAT.Most_Recent_Exception (g-moreex.ads)
=========================================

Provides access to the most recently raised exception.  Can be used for various logging purposes, including duplicating functionality of some Ada 83 implementation dependent extensions.

GNAT.OS_Lib (g-os_lib.ads)
==========================

Provides a range of target independent operating system interface functions, including time/date management, file operations, subprocess management, including a portable spawn procedure, and access to environment variables and error return codes.

GNAT.Perfect_Hash_Generators (g-pehage.ads)
===========================================

Provides a generator of static minimal perfect hash functions. No collisions occur and each item can be retrieved from the table in one probe (perfect property). The hash table size corresponds to the exact size of the key set and no larger (minimal property). The key set has to be know in advance (static property). The hash functions are also order preserving. If w2 is inserted after w1 in the generator, their hashcode are in the same order. These hashing functions are very convenient for use with realtime applications.

GNAT.Random_Numbers (g-rannum.ads)
==================================

Provides random number capabilities which extend those available in the standard Ada library and are more convenient to use.

GNAT.Regexp (g-regexp.ads)
==========================

A simple implementation of regular expressions, using a subset of regular expression syntax copied from familiar Unix style utilities.  This is the simples of the three pattern matching packages provided, and is particularly suitable for ``file globbing'' applications.

GNAT.Registry (g-regist.ads)
============================

This is a high level binding to the Windows registry.  It is possible to do simple things like reading a key value, creating a new key.  For full registry API, but at a lower level of abstraction, refer to the Win32.Winreg package provided with the Win32Ada binding

GNAT.Regpat (g-regpat.ads)
==========================

A complete implementation of Unix-style regular expression matching, copied from the original V7 style regular expression library written in C by Henry Spencer (and binary compatible with this C library).

GNAT.Secondary_Stack_Info (g-sestin.ads)
========================================

Provide the capability to query the high water mark of the current task's secondary stack.

GNAT.Semaphores (g-semaph.ads)
==============================

Provides classic counting and binary semaphores using protected types.

GNAT.Serial_Communications (g-sercom.ads)
=========================================

Provides a simple interface to send and receive data over a serial port. This is only supported on GNU/Linux and Windows.

GNAT.SHA1 (g-sha1.ads)
======================

Implements the SHA-1 Secure Hash Algorithm as described in FIPS PUB 180-3 and RFC 3174.

GNAT.SHA224 (g-sha224.ads)
==========================

Implements the SHA-224 Secure Hash Algorithm as described in FIPS PUB 180-3.

GNAT.SHA256 (g-sha256.ads)
==========================

Implements the SHA-256 Secure Hash Algorithm as described in FIPS PUB 180-3.

GNAT.SHA384 (g-sha384.ads)
==========================

Implements the SHA-384 Secure Hash Algorithm as described in FIPS PUB 180-3.

GNAT.SHA512 (g-sha512.ads)
==========================

Implements the SHA-512 Secure Hash Algorithm as described in FIPS PUB 180-3.

GNAT.Signals (g-signal.ads)
===========================

Provides the ability to manipulate the blocked status of signals on supported targets.

GNAT.Sockets (g-socket.ads)
===========================

A high level and portable interface to develop sockets based applications. This package is based on the sockets thin binding found in GNAT.Sockets.Thin. Currently GNAT.Sockets is implemented on all native GNAT ports except for OpenVMS.  It is not implemented for the LynxOS cross port.

GNAT.Source_Info (g-souinf.ads)
===============================

Provides subprograms that give access to source code information known at compile time, such as the current file name and line number.

GNAT.Spelling_Checker (g-speche.ads)
====================================

Provides a function for determining whether one string is a plausible near misspelling of another string.

GNAT.Spelling_Checker_Generic (g-spchge.ads)
============================================

Provides a generic function that can be instantiated with a string type for determining whether one string is a plausible near misspelling of another string.

GNAT.Spitbol.Patterns (g-spipat.ads)
====================================

A complete implementation of SNOBOL4 style pattern matching.  This is the most elaborate of the pattern matching packages provided.  It fully duplicates the SNOBOL4 dynamic pattern construction and matching capabilities, using the efficient algorithm developed by Robert Dewar for the SPITBOL system.

GNAT.Spitbol (g-spitbo.ads)
===========================

The top level package of the collection of SPITBOL-style functionality, this package provides basic SNOBOL4 string manipulation functions, such as Pad, Reverse, Trim, Substr capability, as well as a generic table function useful for constructing arbitrary mappings from strings in the style of the SNOBOL4 TABLE function.

GNAT.Spitbol.Table_Boolean (g-sptabo.ads)
=========================================

A library level of instantiation of GNAT.Spitbol.Patterns.Table for type Standard.Boolean, giving an implementation of sets of string values.

GNAT.Spitbol.Table_Integer (g-sptain.ads)
=========================================

A library level of instantiation of GNAT.Spitbol.Patterns.Table for type Standard.Integer, giving an implementation of maps from string to integer values.

GNAT.Spitbol.Table_VString (g-sptavs.ads)
=========================================

A library level of instantiation of GNAT.Spitbol.Patterns.Table for a variable length string type, giving an implementation of general maps from strings to strings.

GNAT.SSE (g-sse.ads)
====================

Root of a set of units aimed at offering Ada bindings to a subset of the Intel(r) Streaming SIMD Extensions with GNAT on the x86 family of targets.  It exposes vector component types together with a general introduction to the binding contents and use.

GNAT.SSE.Vector_Types (g-ssvety.ads)
====================================

SSE vector types for use with SSE related intrinsics.

GNAT.Strings (g-string.ads)
===========================

Common String access types and related subprograms. Basically it defines a string access and an array of string access types.

GNAT.String_Split (g-strspl.ads)
================================

Useful string manipulation routines: given a set of separators, split a string wherever the separators appear, and provide direct access to the resulting slices. This package is instantiated from GNAT.Array_Split.

GNAT.Table (g-table.ads)
========================

A generic package providing a single dimension array abstraction where the length of the array can be dynamically modified.

This package provides a facility similar to that of GNAT.Dynamic_Tables, except that this package declares a single instance of the table type, while an instantiation of GNAT.Dynamic_Tables creates a type that can be used to define dynamic instances of the table.

GNAT.Task_Lock (g-tasloc.ads)
=============================

A very simple facility for locking and unlocking sections of code using a single global task lock.  Appropriate for use in situations where contention between tasks is very rarely expected.

GNAT.Time_Stamp (g-timsta.ads)
==============================

Provides a simple function that returns a string YYYY-MM-DD HH:MM:SS.SS that represents the current date and time in ISO 8601 format. This is a very simple routine with minimal code and there are no dependencies on any other unit.

GNAT.Threads (g-thread.ads)
===========================

Provides facilities for dealing with foreign threads which need to be known by the GNAT run-time system. Consult the documentation of this package for further details if your program has threads that are created by a non-Ada environment which then accesses Ada code.

GNAT.Traceback (g-traceb.ads)
=============================

Provides a facility for obtaining non-symbolic traceback information, useful in various debugging situations.

GNAT.Traceback.Symbolic (g-trasym.ads)
======================================

GNAT.UTF_32 (g-table.ads)
=========================

This is a package intended to be used in conjunction with the Wide_Character type in Ada 95 and the Wide_Wide_Character type in Ada 2005 (available in GNAT in Ada 2005 mode). This package contains Unicode categorization routines, as well as lexical categorization routines corresponding to the Ada 2005 lexical rules for identifiers and strings, and also a lower case to upper case fold routine corresponding to the Ada 2005 rules for identifier equivalence.

GNAT.Wide_Spelling_Checker (g-u3spch.ads)
=========================================

Provides a function for determining whether one wide wide string is a plausible near misspelling of another wide wide string, where the strings are represented using the UTF_32_String type defined in System.Wch_Cnv.

GNAT.Wide_Spelling_Checker (g-wispch.ads)
=========================================

Provides a function for determining whether one wide string is a plausible near misspelling of another wide string.

GNAT.Wide_String_Split (g-wistsp.ads)
=====================================

Useful wide string manipulation routines: given a set of separators, split a wide string wherever the separators appear, and provide direct access to the resulting slices. This package is instantiated from GNAT.Array_Split.

GNAT.Wide_Wide_Spelling_Checker (g-zspche.ads)
==============================================

Provides a function for determining whether one wide wide string is a plausible near misspelling of another wide wide string.

GNAT.Wide_Wide_String_Split (g-zistsp.ads)
==========================================

Useful wide wide string manipulation routines: given a set of separators, split a wide wide string wherever the separators appear, and provide direct access to the resulting slices. This package is instantiated from GNAT.Array_Split.

Interfaces.C.Extensions (i-cexten.ads)
======================================

This package contains additional C-related definitions, intended for use with either manually or automatically generated bindings to C libraries.

Interfaces.C.Streams (i-cstrea.ads)
===================================

This package is a binding for the most commonly used operations on C streams.

Interfaces.CPP (i-cpp.ads)
==========================

This package provides facilities for use in interfacing to C++.  It is primarily intended to be used in connection with automated tools for the generation of C++ interfaces.

Interfaces.Packed_Decimal (i-pacdec.ads)
========================================

This package provides a set of routines for conversions to and from a packed decimal format compatible with that used on IBM mainframes.

Interfaces.VxWorks (i-vxwork.ads)
=================================

This package provides a limited binding to the VxWorks API. In particular, it interfaces with the VxWorks hardware interrupt facilities.

Interfaces.VxWorks.IO (i-vxwoio.ads)
====================================

This package provides a binding to the ioctl (IO/Control) function of VxWorks, defining a set of option values and function codes. A particular use of this package is to enable the use of Get_Immediate under VxWorks.

System.Address_Image (s-addima.ads)
===================================

This function provides a useful debugging function that gives an (implementation dependent) string which identifies an address.

System.Assertions (s-assert.ads)
================================

This package provides the declaration of the exception raised by an run-time assertion failure, as well as the routine that is used internally to raise this assertion.

System.Memory (s-memory.ads)
============================

This package provides the interface to the low level routines used by the generated code for allocation and freeing storage for the default storage pool (analogous to the C routines malloc and free. It also provides a reallocation interface analogous to the C routine realloc. The body of this unit may be modified to provide alternative allocation mechanisms for the default pool, and in addition, direct calls to this unit may be made for low level allocation uses (for example see the body of GNAT.Tables).

System.Partition_Interface (s-parint.ads)
=========================================

This package provides facilities for partition interfacing.  It is used primarily in a distribution context when using Annex E with GLADE.

System.Pool_Global (s-pooglo.ads)
=================================

This package provides a storage pool that is equivalent to the default storage pool used for access types for which no pool is specifically declared. It uses malloc/free to allocate/free and does not attempt to do any automatic reclamation.

System.Pool_Local (s-pooloc.ads)
================================

This package provides a storage pool that is intended for use with locally defined access types. It uses malloc/free for allocate/free, and maintains a list of allocated blocks, so that all storage allocated for the pool can be freed automatically when the pool is finalized.

System.Restrictions (s-restri.ads)
==================================

This package provides facilities for accessing at run time the status of restrictions specified at compile time for the partition. Information is available both with regard to actual restrictions specified, and with regard to compiler determined information on which restrictions are violated by one or more packages in the partition.

System.Rident (s-rident.ads)
============================

This package provides definitions of the restrictions identifiers supported by GNAT, and also the format of the restrictions provided in package System.Restrictions. It is not normally necessary to with this generic package since the necessary instantiation is included in package System.Restrictions.

System.Strings.Stream_Ops (s-ststop.ads)
========================================

This package provides a set of stream subprograms for standard string types. It is intended primarily to support implicit use of such subprograms when stream attributes are applied to string types, but the subprograms in this package can be used directly by application programs.

System.Task_Info (s-tasinf.ads)
===============================

This package provides target dependent functionality that is used to support the Task_Info pragma

System.Wch_Cnv (s-wchcnv.ads)
=============================

This package provides routines for converting between wide and wide wide characters and a representation as a value of type Standard.String, using a specified wide character encoding method.  It uses definitions in package System.Wch_Con.

System.Wch_Con (s-wchcon.ads)
=============================

This package provides definitions and descriptions of the various methods used for encoding wide characters in ordinary strings.  These definitions are used by the package System.Wch_Cnv.

Interfacing to Other Languages
******************************

The facilities in annex B of the Ada Reference Manual are fully implemented in GNAT, and in addition, a full interface to C++ is provided.

Interfacing to C
================

Interfacing to C with GNAT can use one of two approaches:

  * The types in the package Interfaces.C may be used.

  * Standard Ada types may be used directly.  This may be less portable to other compilers, but will work on all GNAT compilers, which guarantee correspondence between the C and Ada types.

Pragma Convention C may be applied to Ada types, but mostly has no effect, since this is the default.  The following table shows the correspondence between Ada scalar types and the corresponding C types.

Integer
     int

Short_Integer
     short

Short_Short_Integer
     signed char

Long_Integer
     long

Long_Long_Integer
     long long

Short_Float
     float

Float
     float

Long_Float
     double

Long_Long_Float
     This is the longest floating-point type supported by the hardware.

Additionally, there are the following general correspondences between Ada and C types:

  * Ada enumeration types map to C enumeration types directly if pragma Convention C is specified, which causes them to have int length.  Without pragma Convention C, Ada enumeration types map to 8, 16, or 32 bits (i.e. C types signed char, short, int, respectively) depending on the number of values passed. This is the only case in which pragma Convention C affects the representation of an Ada type.

  * Ada access types map to C pointers, except for the case of pointers to unconstrained types in Ada, which have no direct C equivalent.

  * Ada arrays map directly to C arrays.

  * Ada records map directly to C structures.

  * Packed Ada records map to C structures where all members are bit fields of the length corresponding to the TYPE'Size value in Ada.

Interfacing to C++
==================

The interface to C++ makes use of the following pragmas, which are primarily intended to be constructed automatically using a binding generator tool, although it is possible to construct them by hand.  No suitable binding generator tool is supplied with GNAT though.

Using these pragmas it is possible to achieve complete inter-operability between Ada tagged types and C++ class definitions. See Implementation Defined Pragmas, for more details.

pragma CPP_Class ([Entity =>] LOCAL_NAME)
     The argument denotes an entity in the current declarative region that is declared as a tagged or untagged record type. It indicates that the type corresponds to an externally declared C++ class type, and is to be laid out the same way that C++ would lay out the type.

     Note: Pragma CPP_Class is currently obsolete. It is supported for backward compatibility but its functionality is available using pragma Import with Convention = CPP.

pragma CPP_Constructor ([Entity =>] LOCAL_NAME)
     This pragma identifies an imported function (imported in the usual way with pragma Import) as corresponding to a C++ constructor.

Interfacing to COBOL
====================

Interfacing to COBOL is achieved as described in section B.4 of the Ada Reference Manual.

Interfacing to Fortran
======================

Interfacing to Fortran is achieved as described in section B.5 of the Ada Reference Manual.  The pragma Convention Fortran, applied to a multi-dimensional array causes the array to be stored in column-major order as required for convenient interface to Fortran.

Interfacing to non-GNAT Ada code
================================

It is possible to specify the convention Ada in a pragma Import or pragma Export.  However this refers to the calling conventions used by GNAT, which may or may not be similar enough to those used by some other Ada 83 / Ada 95 / Ada 2005 compiler to allow interoperation.

If arguments types are kept simple, and if the foreign compiler generally follows system calling conventions, then it may be possible to integrate files compiled by other Ada compilers, provided that the elaboration issues are adequately addressed (for example by eliminating the need for any load time elaboration).

In particular, GNAT running on VMS is designed to be highly compatible with the DEC Ada 83 compiler, so this is one case in which it is possible to import foreign units of this type, provided that the data items passed are restricted to simple scalar values or simple record types without variants, or simple array types with fixed bounds.

Specialized Needs Annexes
*************************

Ada 95 and Ada 2005 define a number of Specialized Needs Annexes, which are not required in all implementations.  However, as described in this chapter, GNAT implements all of these annexes:

Systems Programming (Annex C)
     The Systems Programming Annex is fully implemented.

Real-Time Systems (Annex D)
     The Real-Time Systems Annex is fully implemented.

Distributed Systems (Annex E)
     Stub generation is fully implemented in the GNAT compiler.  In addition, a complete compatible PCS is available as part of the GLADE system, a separate product.  When the two products are used in conjunction, this annex is fully implemented.

Information Systems (Annex F)
     The Information Systems annex is fully implemented.

Numerics (Annex G)
     The Numerics Annex is fully implemented.

Safety and Security / High-Integrity Systems (Annex H)
     The Safety and Security Annex (termed the High-Integrity Systems Annex in Ada 2005) is fully implemented.

Implementation of Specific Ada Features
***************************************

This chapter describes the GNAT implementation of several Ada language facilities.

Machine Code Insertions
=======================

Package Machine_Code provides machine code support as described in the Ada Reference Manual in two separate forms:

  * Machine code statements, consisting of qualified expressions that fit the requirements of RM section 13.8.

  * An intrinsic callable procedure, providing an alternative mechanism of including machine instructions in a subprogram.

The two features are similar, and both are closely related to the mechanism provided by the asm instruction in the GNU C compiler.  Full understanding and use of the facilities in this package requires understanding the asm instruction, see Assembler Instructions with C Expression Operands (Using the GNU Compiler Collection (GCC)).

Calls to the function Asm and the procedure Asm have identical semantic restrictions and effects as described below.  Both are provided so that the procedure call can be used as a statement, and the function call can be used to form a code_statement.

The first example given in the GCC documentation is the C asm instruction:

        asm ("fsinx %1 %0" : "=f" (result) : "f" (angle));

The equivalent can be written for GNAT as:

     Asm ("fsinx %1 %0",
          My_Float'Asm_Output ("=f", result),
          My_Float'Asm_Input  ("f",  angle));

The first argument to Asm is the assembler template, and is identical to what is used in GNU C.  This string must be a static expression.  The second argument is the output operand list.  It is either a single Asm_Output attribute reference, or a list of such references enclosed in parentheses (technically an array aggregate of such references).

The Asm_Output attribute denotes a function that takes two parameters.  The first is a string, the second is the name of a variable of the type designated by the attribute prefix.  The first (string) argument is required to be a static expression and designates the constraint for the parameter (e.g. what kind of register is required).  The second argument is the variable to be updated with the result.  The possible values for constraint are the same as those used in the RTL, and are dependent on the configuration file used to build the GCC back end.  If there are no output operands, then this argument may either be omitted, or explicitly given as No_Output_Operands.

The second argument of MY_FLOAT'Asm_Output functions as though it were an out parameter, which is a little curious, but all names have the form of expressions, so there is no syntactic irregularity, even though normally functions would not be permitted out parameters.  The third argument is the list of input operands.  It is either a single Asm_Input attribute reference, or a list of such references enclosed in parentheses (technically an array aggregate of such references).

The Asm_Input attribute denotes a function that takes two parameters.  The first is a string, the second is an expression of the type designated by the prefix.  The first (string) argument is required to be a static expression, and is the constraint for the parameter, (e.g. what kind of register is required).  The second argument is the value to be used as the input argument.  The possible values for the constant are the same as those used in the RTL, and are dependent on the configuration file used to built the GCC back end.

If there are no input operands, this argument may either be omitted, or explicitly given as No_Input_Operands.  The fourth argument, not present in the above example, is a list of register names, called the clobber argument.  This argument, if given, must be a static string expression, and is a space or comma separated list of names of registers that must be considered destroyed as a result of the Asm call.  If this argument is the null string (the default value), then the code generator assumes that no additional registers are destroyed.

The fifth argument, not present in the above example, called the volatile argument, is by default False.  It can be set to the literal value True to indicate to the code generator that all optimizations with respect to the instruction specified should be suppressed, and that in particular, for an instruction that has outputs, the instruction will still be generated, even if none of the outputs are used.  See Assembler Instructions with C Expression Operands (Using the GNU Compiler Collection (GCC)), for the full description. Generally it is strongly advisable to use Volatile for any ASM statement that is missing either input or output operands, or when two or more ASM statements appear in sequence, to avoid unwanted optimizations. A warning is generated if this advice is not followed.

The Asm subprograms may be used in two ways.  First the procedure forms can be used anywhere a procedure call would be valid, and correspond to what the RM calls ``intrinsic'' routines.  Such calls can be used to intersperse machine instructions with other Ada statements. Second, the function forms, which return a dummy value of the limited private type Asm_Insn, can be used in code statements, and indeed this is the only context where such calls are allowed.  Code statements appear as aggregates of the form:

     Asm_Insn'(Asm (...));
     Asm_Insn'(Asm_Volatile (...));

In accordance with RM rules, such code statements are allowed only within subprograms whose entire body consists of such statements.  It is not permissible to intermix such statements with other Ada statements.

Typically the form using intrinsic procedure calls is more convenient and more flexible.  The code statement form is provided to meet the RM suggestion that such a facility should be made available.  The following is the exact syntax of the call to Asm. As usual, if named notation is used, the arguments may be given in arbitrary order, following the normal rules for use of positional and named arguments)

     ASM_CALL ::= Asm (
                      [Template =>] static_string_EXPRESSION
                    [,[Outputs  =>] OUTPUT_OPERAND_LIST      ]
                    [,[Inputs   =>] INPUT_OPERAND_LIST       ]
                    [,[Clobber  =>] static_string_EXPRESSION ]
                    [,[Volatile =>] static_boolean_EXPRESSION] )

     OUTPUT_OPERAND_LIST ::=
       [PREFIX.]No_Output_Operands
     | OUTPUT_OPERAND_ATTRIBUTE
     | (OUTPUT_OPERAND_ATTRIBUTE {,OUTPUT_OPERAND_ATTRIBUTE})

     OUTPUT_OPERAND_ATTRIBUTE ::=
       SUBTYPE_MARK'Asm_Output (static_string_EXPRESSION, NAME)

     INPUT_OPERAND_LIST ::=
       [PREFIX.]No_Input_Operands
     | INPUT_OPERAND_ATTRIBUTE
     | (INPUT_OPERAND_ATTRIBUTE {,INPUT_OPERAND_ATTRIBUTE})

     INPUT_OPERAND_ATTRIBUTE ::=
       SUBTYPE_MARK'Asm_Input (static_string_EXPRESSION, EXPRESSION)

The identifiers No_Input_Operands and No_Output_Operands are declared in the package Machine_Code and must be referenced according to normal visibility rules. In particular if there is no use clause for this package, then appropriate package name qualification is required.

GNAT Implementation of Tasking
==============================

This chapter outlines the basic GNAT approach to tasking (in particular, a multi-layered library for portability) and discusses issues related to compliance with the Real-Time Systems Annex.

Mapping Ada Tasks onto the Underlying Kernel Threads
----------------------------------------------------

GNAT's run-time support comprises two layers:

  * GNARL (GNAT Run-time Layer)

  * GNULL (GNAT Low-level Library)

In GNAT, Ada's tasking services rely on a platform and OS independent layer known as GNARL.  This code is responsible for implementing the correct semantics of Ada's task creation, rendezvous, protected operations etc.

GNARL decomposes Ada's tasking semantics into simpler lower level operations such as create a thread, set the priority of a thread, yield, create a lock, lock/unlock, etc.  The spec for these low-level operations constitutes GNULLI, the GNULL Interface.  This interface is directly inspired from the POSIX real-time API.

If the underlying executive or OS implements the POSIX standard faithfully, the GNULL Interface maps as is to the services offered by the underlying kernel.  Otherwise, some target dependent glue code maps the services offered by the underlying kernel to the semantics expected by GNARL.

Whatever the underlying OS (VxWorks, UNIX, Windows, etc.) the key point is that each Ada task is mapped on a thread in the underlying kernel.  For example, in the case of VxWorks, one Ada task = one VxWorks task.

In addition Ada task priorities map onto the underlying thread priorities. Mapping Ada tasks onto the underlying kernel threads has several advantages:

  * The underlying scheduler is used to schedule the Ada tasks.  This makes Ada tasks as efficient as kernel threads from a scheduling standpoint.

  * Interaction with code written in C containing threads is eased since at the lowest level Ada tasks and C threads map onto the same underlying kernel concept.

  * When an Ada task is blocked during I/O the remaining Ada tasks are able to proceed.

  * On multiprocessor systems Ada tasks can execute in parallel.

Some threads libraries offer a mechanism to fork a new process, with the child process duplicating the threads from the parent. GNAT does not support this functionality when the parent contains more than one task.

Ensuring Compliance with the Real-Time Annex
--------------------------------------------

Although mapping Ada tasks onto the underlying threads has significant advantages, it does create some complications when it comes to respecting the scheduling semantics specified in the real-time annex (Annex D).

For instance the Annex D requirement for the FIFO_Within_Priorities scheduling policy states:

     When the active priority of a ready task that is not running changes, or the setting of its base priority takes effect, the task is removed from the ready queue for its old active priority and is added at the tail of the ready queue for its new active priority, except in the case where the active priority is lowered due to the loss of inherited priority, in which case the task is added at the head of the ready queue for its new active priority.

While most kernels do put tasks at the end of the priority queue when a task changes its priority, (which respects the main FIFO_Within_Priorities requirement), almost none keep a thread at the beginning of its priority queue when its priority drops from the loss of inherited priority.

As a result most vendors have provided incomplete Annex D implementations.

The GNAT run-time, has a nice cooperative solution to this problem which ensures that accurate FIFO_Within_Priorities semantics are respected.

The principle is as follows.  When an Ada task T is about to start running, it checks whether some other Ada task R with the same priority as T has been suspended due to the loss of priority inheritance.  If this is the case, T yields and is placed at the end of its priority queue.  When R arrives at the front of the queue it executes.

Note that this simple scheme preserves the relative order of the tasks that were ready to execute in the priority queue where R has been placed at the end.

GNAT Implementation of Shared Passive Packages
==============================================

GNAT fully implements the pragma Shared_Passive for the purpose of designating shared passive packages. This allows the use of passive partitions in the context described in the Ada Reference Manual; i.e., for communication between separate partitions of a distributed application using the features in Annex E.

However, the implementation approach used by GNAT provides for more extensive usage as follows:

Communication between separate programs

     This allows separate programs to access the data in passive partitions, using protected objects for synchronization where needed. The only requirement is that the two programs have a common shared file system. It is even possible for programs running on different machines with different architectures (e.g. different endianness) to communicate via the data in a passive partition.

Persistence between program runs

     The data in a passive package can persist from one run of a program to another, so that a later program sees the final values stored by a previous run of the same program.

The implementation approach used is to store the data in files. A separate stream file is created for each object in the package, and an access to an object causes the corresponding file to be read or written.

The environment variable SHARED_MEMORY_DIRECTORY should be set to the directory to be used for these files. The files in this directory have names that correspond to their fully qualified names. For example, if we have the package

     package X is
       pragma Shared_Passive (X);
       Y : Integer;
       Z : Float;
     end X;

and the environment variable is set to /stemp/, then the files created will have the names:

     /stemp/x.y
     /stemp/x.z

These files are created when a value is initially written to the object, and the files are retained until manually deleted. This provides the persistence semantics. If no file exists, it means that no partition has assigned a value to the variable; in this case the initial value declared in the package will be used. This model ensures that there are no issues in synchronizing the elaboration process, since elaboration of passive packages elaborates the initial values, but does not create the files.

The files are written using normal Stream_IO access. If you want to be able to communicate between programs or partitions running on different architectures, then you should use the XDR versions of the stream attribute routines, since these are architecture independent.

If active synchronization is required for access to the variables in the shared passive package, then as described in the Ada Reference Manual, the package may contain protected objects used for this purpose. In this case a lock file (whose name is ___lock (three underscores) is created in the shared memory directory. This is used to provide the required locking semantics for proper protected object synchronization.

As of January 2003, GNAT supports shared passive packages on all platforms except for OpenVMS.

Code Generation for Array Aggregates
====================================

Aggregates have a rich syntax and allow the user to specify the values of complex data structures by means of a single construct.  As a result, the code generated for aggregates can be quite complex and involve loops, case statements and multiple assignments.  In the simplest cases, however, the compiler will recognize aggregates whose components and constraints are fully static, and in those cases the compiler will generate little or no executable code.  The following is an outline of the code that GNAT generates for various aggregate constructs.  For further details, you will find it useful to examine the output produced by the -gnatG flag to see the expanded source that is input to the code generator.  You may also want to examine the assembly code generated at various levels of optimization.

The code generated for aggregates depends on the context, the component values, and the type.  In the context of an object declaration the code generated is generally simpler than in the case of an assignment.  As a general rule, static component values and static subtypes also lead to simpler code.

Static constant aggregates with static bounds
---------------------------------------------

For the declarations:

         type One_Dim is array (1..10) of integer;
         ar0 : constant One_Dim := (1, 2, 3, 4, 5, 6, 7, 8, 9, 0);

GNAT generates no executable code: the constant ar0 is placed in static memory. The same is true for constant aggregates with named associations:

         Cr1 : constant One_Dim := (4 => 16, 2 => 4, 3 => 9, 1 => 1, 5 .. 10 => 0);
         Cr3 : constant One_Dim := (others => 7777);

The same is true for multidimensional constant arrays such as:

         type two_dim is array (1..3, 1..3) of integer;
         Unit : constant two_dim := ( (1,0,0), (0,1,0), (0,0,1));

The same is true for arrays of one-dimensional arrays: the following are static:

     type ar1b  is array (1..3) of boolean;
     type ar_ar is array (1..3) of ar1b;
     None  : constant ar1b := (others => false);     --  fully static
     None2 : constant ar_ar := (1..3 => None);       --  fully static

However, for multidimensional aggregates with named associations, GNAT will generate assignments and loops, even if all associations are static.  The following two declarations generate a loop for the first dimension, and individual component assignments for the second dimension:

     Zero1: constant two_dim := (1..3 => (1..3 => 0));
     Zero2: constant two_dim := (others => (others => 0));

Constant aggregates with unconstrained nominal types
----------------------------------------------------

In such cases the aggregate itself establishes the subtype, so that associations with others cannot be used.  GNAT determines the bounds for the actual subtype of the aggregate, and allocates the aggregate statically as well.  No code is generated for the following:

         type One_Unc is array (natural range <>) of integer;
         Cr_Unc : constant One_Unc := (12,24,36);

Aggregates with static bounds
-----------------------------

In all previous examples the aggregate was the initial (and immutable) value of a constant.  If the aggregate initializes a variable, then code is generated for it as a combination of individual assignments and loops over the target object.  The declarations

            Cr_Var1 : One_Dim := (2, 5, 7, 11, 0, 0, 0, 0, 0, 0);
            Cr_Var2 : One_Dim := (others > -1);

generate the equivalent of

            Cr_Var1 (1) := 2;
            Cr_Var1 (2) := 3;
            Cr_Var1 (3) := 5;
            Cr_Var1 (4) := 11;

            for I in Cr_Var2'range loop
               Cr_Var2 (I) := -1;
            end loop;

Aggregates with non-static bounds
---------------------------------

If the bounds of the aggregate are not statically compatible with the bounds of the nominal subtype  of the target, then constraint checks have to be generated on the bounds.  For a multidimensional array, constraint checks may have to be applied to sub-arrays individually, if they do not have statically compatible subtypes.

Aggregates in assignment statements
-----------------------------------

In general, aggregate assignment requires the construction of a temporary, and a copy from the temporary to the target of the assignment.  This is because it is not always possible to convert the assignment into a series of individual component assignments.  For example, consider the simple case:

             A := (A(2), A(1));

This cannot be converted into:

             A(1) := A(2);
             A(2) := A(1);

So the aggregate has to be built first in a separate location, and then copied into the target.  GNAT recognizes simple cases where this intermediate step is not required, and the assignments can be performed in place, directly into the target.  The following sufficient criteria are applied:

  * The bounds of the aggregate are static, and the associations are static.

  * The components of the aggregate are static constants, names of simple variables that are not renamings, or expressions not involving indexed components whose operands obey these rules.

If any of these conditions are violated, the aggregate will be built in a temporary (created either by the front-end or the code generator) and then that temporary will be copied onto the target.

The Size of Discriminated Records with Default Discriminants
============================================================

If a discriminated type T has discriminants with default values, it is possible to declare an object of this type without providing an explicit constraint:

     type Size is range 1..100;

     type Rec (D : Size := 15) is record
        Name : String (1..D);
     end T;

     Word : Rec;

Such an object is said to be unconstrained. The discriminant of the object can be modified by a full assignment to the object, as long as it preserves the relation between the value of the discriminant, and the value of the components that depend on it:

     Word := (3, "yes");

     Word := (5, "maybe");

     Word := (5, "no"); -- raises Constraint_Error

In order to support this behavior efficiently, an unconstrained object is given the maximum size that any value of the type requires. In the case above, Word has storage for the discriminant and for a String of length 100. It is important to note that unconstrained objects do not require dynamic allocation. It would be an improper implementation to place on the heap those components whose size depends on discriminants. (This improper implementation was used by some Ada83 compilers, where the Name component above would have been stored as a pointer to a dynamic string). Following the principle that dynamic storage management should never be introduced implicitly, an Ada compiler should reserve the full size for an unconstrained declared object, and place it on the stack.

This maximum size approach has been a source of surprise to some users, who expect the default values of the discriminants to determine the size reserved for an unconstrained object: ``If the default is 15, why should the object occupy a larger size?'' The answer, of course, is that the discriminant may be later modified, and its full range of values must be taken into account. This is why the declaration:

     type Rec (D : Positive := 15) is record
        Name : String (1..D);
     end record;

     Too_Large : Rec;

is flagged by the compiler with a warning: an attempt to create Too_Large will raise Storage_Error, because the required size includes Positive'Last bytes. As the first example indicates, the proper approach is to declare an index type of ``reasonable'' range so that unconstrained objects are not too large.

One final wrinkle: if the object is declared to be aliased, or if it is created in the heap by means of an allocator, then it is not unconstrained: it is constrained by the default values of the discriminants, and those values cannot be modified by full assignment. This is because in the presence of aliasing all views of the object (which may be manipulated by different tasks, say) must be consistent, so it is imperative that the object, once created, remain invariant.

Strict Conformance to the Ada Reference Manual
==============================================

The dynamic semantics defined by the Ada Reference Manual impose a set of run-time checks to be generated. By default, the GNAT compiler will insert many run-time checks into the compiled code, including most of those required by the Ada Reference Manual. However, there are three checks that are not enabled in the default mode for efficiency reasons: arithmetic overflow checking for integer operations (including division by zero), checks for access before elaboration on subprogram calls, and stack overflow checking (most operating systems do not perform this check by default).

Strict conformance to the Ada Reference Manual can be achieved by adding three compiler options for overflow checking for integer operations (-gnato), dynamic checks for access-before-elaboration on subprogram calls and generic instantiations (-gnatE), and stack overflow checking (-fstack-check).

Note that the result of a floating point arithmetic operation in overflow and invalid situations, when the Machine_Overflows attribute of the result type is False, is to generate IEEE NaN and infinite values. This is the case for machines compliant with the IEEE floating-point standard, but on machines that are not fully compliant with this standard, such as Alpha, the -mieee compiler flag must be used for achieving IEEE confirming behavior (although at the cost of a significant performance penalty), so infinite and NaN values are properly generated.

Implementation of Ada 2012 Features
***********************************

This chapter contains a complete list of Ada 2012 features that have been implemented as of GNAT version 6.4. Generally, these features are only available if the -gnat12 (Ada 2012 features enabled) flag is set or if the configuration pragma Ada_2012 is used. However, new pragmas, attributes, and restrictions are unconditionally available, since the Ada 95 standard allows the addition of new pragmas, attributes, and restrictions (there are exceptions, which are documented in the individual descriptions), and also certain packages were made available in earlier versions of Ada.

An ISO date (YYYY-MM-DD) appears in parentheses on the description line. This date shows the implementation date of the feature. Any wavefront subsequent to this date will contain the indicated feature, as will any subsequent releases. A date of 0000-00-00 means that GNAT has always implemented the feature, or implemented it as soon as it appeared as a binding interpretation.

Each feature corresponds to an Ada Issue (``AI'') approved by the Ada standardization group (ISO/IEC JTC1/SC22/WG9) for inclusion in Ada 2012. The features are ordered based on the relevant sections of the Ada Reference Manual (``RM'').  When a given AI relates to multiple points in the RM, the earliest is used.

A complete description of the AIs may be found in www.ada-auth.org/ai05-summary.html.

  * AI-0176 Quantified expressions (2010-09-29)

     Both universally and existentially quantified expressions are implemented. They use the new syntax for iterators proposed in AI05-139-2, as well as the standard Ada loop syntax.

     RM References:  1.01.04 (12)   2.09 (2/2)   4.04 (7)   4.05.09 (0)

  * AI-0079 Allow other_format characters in source (2010-07-10)

     Wide characters in the unicode category other_format are now allowed in source programs between tokens, but not within a token such as an identifier.

     RM References:  2.01 (4/2)   2.02 (7)

  * AI-0091 Do not allow other_format in identifiers (0000-00-00)

     Wide characters in the unicode category other_format are not permitted within  an identifier, since this can be a security problem. The error message for this case has been improved to be more specific, but GNAT has never allowed such characters to appear in identifiers.

     RM References:  2.03 (3.1/2)   2.03 (4/2)   2.03 (5/2)   2.03 (5.1/2)   2.03 (5.2/2)   2.03 (5.3/2)   2.09 (2/2)

  * AI-0100 Placement of pragmas  (2010-07-01)

     This AI is an earlier version of AI-163. It simplifies the rules for legal placement of pragmas. In the case of lists that allow pragmas, if the list may have no elements, then the list may consist solely of pragmas.

     RM References:  2.08 (7)

  * AI-0163 Pragmas in place of null (2010-07-01)

     A statement sequence may be composed entirely of pragmas. It is no longer necessary to add a dummy null statement to make the sequence legal.

     RM References:  2.08 (7)   2.08 (16)

  * AI-0080 ``View of'' not needed if clear from context (0000-00-00)

     This is an editorial change only, described as non-testable in the AI.

     RM References:  3.01 (7)

  * AI-0183 Aspect specifications (2010-08-16)

     Aspect specifications have been fully implemented except for pre and post- conditions, and type invariants, which have their own separate AI's. All forms of declarations listed in the AI are supported. The following is a list of the aspects supported (with GNAT implementation aspects marked)

     Ada_2005                        -- GNAT
     Ada_2012                        -- GNAT
     Address
     Alignment
     Atomic
     Atomic_Components
     Bit_Order
     Component_Size
     Discard_Names
     External_Tag
     Favor_Top_Level                 -- GNAT
     Inline
     Inline_Always                   -- GNAT
     Invariant
     Machine_Radix
     No_Return
     Object_Size                     -- GNAT
     Pack
     Persistent_BSS                  -- GNAT
     Post
     Pre
     Predicate
     Preelaborable_Initialization
     Pure_Function                   -- GNAT
     Shared                          -- GNAT
     Size
     Storage_Pool
     Storage_Size
     Stream_Size
     Suppress
     Suppress_Debug_Info             -- GNAT
     Test_Case                       -- GNAT
     Unchecked_Union
     Universal_Aliasing              -- GNAT
     Unmodified                      -- GNAT
     Unreferenced                    -- GNAT
     Unreferenced_Objects            -- GNAT
     Unsuppress
     Value_Size                      -- GNAT
     Volatile
     Volatile_Components
     Warnings                        -- GNAT

     Note that for aspects with an expression, e.g. Size, the expression is treated like a default expression (visibility is analyzed at the point of occurrence of the aspect, but evaluation of the expression occurs at the freeze point of the entity involved.

     RM References:  3.02.01 (3)   3.02.02 (2)   3.03.01 (2/2)   3.08 (6)
     3.09.03 (1.1/2)   6.01 (2/2)   6.07 (2/2)   9.05.02 (2/2)   7.01 (3)   7.03
     (2)   7.03 (3)   9.01 (2/2)   9.01 (3/2)   9.04 (2/2)   9.04 (3/2)
     9.05.02 (2/2)   11.01 (2)   12.01 (3)   12.03 (2/2)   12.04 (2/2)   12.05 (2)
     12.06 (2.1/2)   12.06 (2.2/2)   12.07 (2)   13.01 (0.1/2)   13.03 (5/1)
     13.03.01 (0)

  * AI-0128 Inequality is a primitive operation (0000-00-00)

     If an equality operator ("=") is declared for a type, then the implicitly declared inequality operator ("/=") is a primitive operation of the type. This is the only reasonable interpretation, and is the one always implemented by GNAT, but the RM was not entirely clear in making this point.

     RM References:  3.02.03 (6)   6.06 (6)

  * AI-0003 Qualified expressions as names (2010-07-11)

     In Ada 2012, a qualified expression is considered to be syntactically a name, meaning that constructs such as A'(F(X)).B are now legal. This is useful in disambiguating some cases of overloading.

     RM References:  3.03 (11)   3.03 (21)   4.01 (2)   4.04 (7)   4.07 (3) 5.04 (7)

  * AI-0120 Constant instance of protected object (0000-00-00)

     This is an RM editorial change only. The section that lists objects that are constant failed to include the current instance of a protected object within a protected function. This has always been treated as a constant in GNAT.

     RM References:  3.03 (21)

  * AI-0008 General access to constrained objects (0000-00-00)

     The wording in the RM implied that if you have a general access to a constrained object, it could be used to modify the discriminants. This was obviously not intended. Constraint_Error should be raised, and GNAT has always done so in this situation.

     RM References:  3.03 (23)   3.10.02 (26/2)   4.01 (9)   6.04.01 (17)   8.05.01 (5/2)

  * AI-0093 Additional rules use immutably limited (0000-00-00)

     This is an editorial change only, to make more widespread use of the Ada 2012 ``immutably limited''.

     RM References:  3.03 (23.4/3)

  * AI-0096 Deriving from formal private types (2010-07-20)

     In general it is illegal for a type derived from a formal limited type to be nonlimited.  This AI makes an exception to this rule: derivation is legal if it appears in the private part of the generic, and the formal type is not tagged. If the type is tagged, the legality check must be applied to the private part of the package.

     RM References:  3.04 (5.1/2)   6.02 (7)

  * AI-0181 Soft hyphen is a non-graphic character (2010-07-23)

     From Ada 2005 on, soft hyphen is considered a non-graphic character, which means that it has a special name (SOFT_HYPHEN) in conjunction with the Image and Value attributes for the character types. Strictly speaking this is an inconsistency with Ada 95, but in practice the use of these attributes is so obscure that it will not cause problems.

     RM References:  3.05.02 (2/2)   A.01 (35/2)   A.03.03 (21)

  * AI-0182 Additional forms for Character'Value (0000-00-00)

     This AI allows Character'Value to accept the string '?' where ? is any character including non-graphic control characters. GNAT has always accepted such strings. It also allows strings such as HEX_00000041 to be accepted, but GNAT does not take advantage of this permission and raises Constraint_Error, as is certainly still permitted.

     RM References:  3.05 (56/2)

  * AI-0214 Defaulted discriminants for limited tagged (2010-10-01)

     Ada 2012 relaxes the restriction that forbids discriminants of tagged types to have default expressions by allowing them when the type is limited. It is often useful to define a default value for a discriminant even though it can't be changed by assignment.

     RM References:  3.07 (9.1/2)   3.07.02 (3)

  * AI-0102 Some implicit conversions are illegal (0000-00-00)

     It is illegal to assign an anonymous access constant to an anonymous access variable. The RM did not have a clear rule to prevent this, but GNAT has always generated an error for this usage.

     RM References:  3.07 (16)   3.07.01 (9)   6.04.01 (6)   8.06 (27/2)

  * AI-0158 Generalizing membership tests (2010-09-16)

     This AI extends the syntax of membership tests to simplify complex conditions that can be expressed as membership in a subset of values of any type. It introduces syntax for a list of expressions that may be used in loop contexts as well.

     RM References:  3.08.01 (5)   4.04 (3)   4.05.02 (3)   4.05.02 (5)   4.05.02 (27)

  * AI-0173 Testing if tags represent abstract types (2010-07-03)

     The function Ada.Tags.Type_Is_Abstract returns True if invoked with the tag of an abstract type, and False otherwise.

     RM References:  3.09 (7.4/2)   3.09 (12.4/2)

  * AI-0076 function with controlling result (0000-00-00)

     This is an editorial change only. The RM defines calls with controlling results, but uses the term ``function with controlling result'' without an explicit definition.

     RM References:  3.09.02 (2/2)

  * AI-0126 Dispatching with no declared operation (0000-00-00)

     This AI clarifies dispatching rules, and simply confirms that dispatching executes the operation of the parent type when there is no explicitly or implicitly declared operation for the descendant type. This has always been the case in all versions of GNAT.

     RM References:  3.09.02 (20/2)   3.09.02 (20.1/2)   3.09.02 (20.2/2)

  * AI-0097 Treatment of abstract null extension (2010-07-19)

     The RM as written implied that in some cases it was possible to create an object of an abstract type, by having an abstract extension inherit a non- abstract constructor from its parent type. This mistake has been corrected in GNAT and in the RM, and this construct is now illegal.

     RM References:  3.09.03 (4/2)

  * AI-0203 Extended return cannot be abstract (0000-00-00)

     A return_subtype_indication cannot denote an abstract subtype. GNAT has never permitted such usage.

     RM References:  3.09.03 (8/3)

  * AI-0198 Inheriting abstract operators  (0000-00-00)

     This AI resolves a conflict between two rules involving inherited abstract operations and predefined operators. If a derived numeric type inherits an abstract operator, it overrides the predefined one. This interpretation was always the one implemented in GNAT.

     RM References:  3.09.03 (4/3)

  * AI-0073 Functions returning abstract types (2010-07-10)

     This AI covers a number of issues regarding returning abstract types. In particular generic functions cannot have abstract result types or access result types designated an abstract type. There are some other cases which are detailed in the AI. Note that this binding interpretation has not been retrofitted to operate before Ada 2012 mode, since it caused a significant number of regressions.

     RM References:  3.09.03 (8)   3.09.03 (10)   6.05 (8/2)

  * AI-0070 Elaboration of interface types (0000-00-00)

     This is an editorial change only, there are no testable consequences short of checking for the absence of generated code for an interface declaration.

     RM References:  3.09.04 (18/2)

  * AI-0208 Characteristics of incomplete views (0000-00-00)

     The wording in the Ada 2005 RM concerning characteristics of incomplete views was incorrect and implied that some programs intended to be legal were now illegal. GNAT had never considered such programs illegal, so it has always implemented the intent of this AI.

     RM References:  3.10.01 (2.4/2)   3.10.01 (2.6/2)

  * AI-0162 Incomplete type completed by partial view (2010-09-15)

     Incomplete types are made more useful by allowing them to be completed by private types and private extensions.

     RM References:  3.10.01 (2.5/2)   3.10.01 (2.6/2)   3.10.01 (3)   3.10.01 (4/2)

  * AI-0098 Anonymous subprogram access restrictions (0000-00-00)

     An unintentional omission in the RM implied some inconsistent restrictions on the use of anonymous access to subprogram values. These restrictions were not intentional, and have never been enforced by GNAT.

     RM References:  3.10.01 (6)   3.10.01 (9.2/2)

  * AI-0199 Aggregate with anonymous access components (2010-07-14)

     A choice list in a record aggregate can include several components of (distinct) anonymous access types as long as they have matching designated subtypes.

     RM References:  4.03.01 (16)

  * AI-0220 Needed components for aggregates (0000-00-00)

     This AI addresses a wording problem in the RM that appears to permit some complex cases of aggregates with non-static discriminants. GNAT has always implemented the intended semantics.

     RM References:  4.03.01 (17)

  * AI-0147 Conditional expressions (2009-03-29)

     Conditional expressions are permitted. The form of such an expression is:

              (if expr then expr {elsif expr then expr} [else expr])

     The parentheses can be omitted in contexts where parentheses are present anyway, such as subprogram arguments and pragma arguments. If the else clause is omitted, else True is assumed; thus (if A then B) is a way to conveniently represent (A implies B) in standard logic.

     RM References:  4.03.03 (15)   4.04 (1)   4.04 (7)   4.05.07 (0)   4.07 (2)
     4.07 (3)   4.09 (12)   4.09 (33)   5.03 (3)   5.03 (4)   7.05 (2.1/2)

  * AI-0037 Out-of-range box associations in aggregate (0000-00-00)

     This AI confirms that an association of the form Indx => <> in an array aggregate must raise Constraint_Error if Indx is out of range. The RM specified a range check on other associations, but not when the value of the association was defaulted. GNAT has always inserted a constraint check on the index value.

     RM References:  4.03.03 (29)

  * AI-0123 Composability of equality (2010-04-13)

     Equality of untagged record composes, so that the predefined equality for a composite type that includes a component of some untagged record type R uses the equality operation of R (which may be user-defined or predefined). This makes the behavior of untagged records identical to that of tagged types in this respect.

     This change is an incompatibility with previous versions of Ada, but it corrects a non-uniformity that was often a source of confusion. Analysis of a large number of industrial programs indicates that in those rare cases where a composite type had an untagged record component with a user-defined equality, either there was no use of the composite equality, or else the code expected the same composability as for tagged types, and thus had a bug that would be fixed by this change.

     RM References:  4.05.02 (9.7/2)   4.05.02 (14)   4.05.02 (15)   4.05.02 (24) 8.05.04 (8)

  * AI-0088 The value of exponentiation (0000-00-00)

     This AI clarifies the equivalence rule given for the dynamic semantics of exponentiation: the value of the operation can be obtained by repeated multiplication, but the operation can be implemented otherwise (for example using the familiar divide-by-two-and-square algorithm, even if this is less accurate), and does not imply repeated reads of a volatile base.

     RM References:  4.05.06 (11)

  * AI-0188 Case expressions (2010-01-09)

     Case expressions are permitted. This allows use of constructs such as:

            X := (case Y is when 1 => 2, when 2 => 3, when others => 31)

     RM References:  4.05.07 (0)   4.05.08 (0)   4.09 (12)   4.09 (33)

  * AI-0104 Null exclusion and uninitialized allocator (2010-07-15)

     The assignment Ptr := new not null Some_Ptr; will raise Constraint_Error because the default value of the allocated object is null. This useless construct is illegal in Ada 2012.

     RM References:  4.08 (2)

  * AI-0157 Allocation/Deallocation from empty pool (2010-07-11)

     Allocation and Deallocation from an empty storage pool (i.e. allocation or deallocation of a pointer for which a static storage size clause of zero has been given) is now illegal and is detected as such. GNAT previously gave a warning but not an error.

     RM References:  4.08 (5.3/2)   13.11.02 (4)   13.11.02 (17)

  * AI-0179 Statement not required after label (2010-04-10)

     It is not necessary to have a statement following a label, so a label can appear at the end of a statement sequence without the need for putting a null statement afterwards, but it is not allowable to have only labels and no real statements in a statement sequence.

     RM References:  5.01 (2)

  * AI-139-2 Syntactic sugar for iterators (2010-09-29)

     The new syntax for iterating over arrays and containers is now implemented. Iteration over containers is for now limited to read-only iterators. Only default iterators are supported, with the syntax:  for Elem of C.

     RM References:  5.05

  * AI-0134 Profiles must match for full conformance (0000-00-00)

     For full conformance, the profiles of anonymous-access-to-subprogram parameters must match. GNAT has always enforced this rule.

     RM References:  6.03.01 (18)

  * AI-0207 Mode conformance and access constant (0000-00-00)

     This AI confirms that access_to_constant indication must match for mode conformance. This was implemented in GNAT when the qualifier was originally introduced in Ada 2005.

     RM References:  6.03.01 (16/2)

  * AI-0046 Null exclusion match for full conformance (2010-07-17)

     For full conformance, in the case of access parameters, the null exclusion must match (either both or neither must have not null).

     RM References:  6.03.02 (18)

  * AI-0118 The association of parameter associations (0000-00-00)

     This AI clarifies the rules for named associations in subprogram calls and generic instantiations. The rules have been in place since Ada 83.

     RM References:  6.04.01 (2)   12.03 (9)

  * AI-0196 Null exclusion tests for out parameters (0000-00-00)

     Null exclusion checks are not made for out parameters when evaluating the actual parameters. GNAT has never generated these checks.

     RM References:  6.04.01 (13)

  * AI-0015 Constant return objects (0000-00-00)

     The return object declared in an extended_return_statement may be declared constant. This was always intended, and GNAT has always allowed it.

     RM References:  6.05 (2.1/2)   3.03 (10/2)   3.03 (21)   6.05 (5/2) 6.05 (5.7/2)

  * AI-0032 Extended return for class-wide functions (0000-00-00)

     If a function returns a class-wide type, the object of an extended return statement can be declared with a specific type that is covered by the class- wide type. This has been implemented in GNAT since the introduction of extended returns. Note AI-0103 complements this AI by imposing matching rules for constrained return types.

     RM References:  6.05 (5.2/2)   6.05 (5.3/2)   6.05 (5.6/2)   6.05 (5.8/2) 6.05 (8/2)

  * AI-0103 Static matching for extended return (2010-07-23)

     If the return subtype of a function is an elementary type or a constrained type, the subtype indication in an extended return statement must match statically this return subtype.

     RM References:  6.05 (5.2/2)

  * AI-0058 Abnormal completion of an extended return (0000-00-00)

     The RM had some incorrect wording implying wrong treatment of abnormal completion in an extended return. GNAT has always implemented the intended correct semantics as described by this AI.

     RM References:  6.05 (22/2)

  * AI-0050 Raising Constraint_Error early for function call (0000-00-00)

     The implementation permissions for raising Constraint_Error early on a function call when it was clear an exception would be raised were over-permissive and allowed mishandling of discriminants in some cases. GNAT did not take advantage of these incorrect permissions in any case.

     RM References:  6.05 (24/2)

  * AI-0125 Nonoverridable operations of an ancestor (2010-09-28)

     In Ada 2012, the declaration of a primitive operation of a type extension or private extension can also override an inherited primitive that is not visible at the point of this declaration.

     RM References:  7.03.01 (6)   8.03 (23)   8.03.01 (5/2)   8.03.01 (6/2)

  * AI-0062 Null exclusions and deferred constants (0000-00-00)

     A full constant may have a null exclusion even if its associated deferred constant does not. GNAT has always allowed this.

     RM References:  7.04 (6/2)   7.04 (7.1/2)

  * AI-0178 Incomplete views are limited (0000-00-00)

     This AI clarifies the role of incomplete views and plugs an omission in the RM. GNAT always correctly restricted the use of incomplete views and types.

     RM References:  7.05 (3/2)   7.05 (6/2)

  * AI-0087 Actual for formal nonlimited derived type (2010-07-15)

     The actual for a formal nonlimited derived type cannot be limited. In particular, a formal derived type that extends a limited interface but which is not explicitly limited cannot be instantiated with a limited type.

     RM References:  7.05 (5/2)   12.05.01 (5.1/2)

  * AI-0099 Tag determines whether finalization needed (0000-00-00)

     This AI clarifies that ``needs finalization'' is part of dynamic semantics, and therefore depends on the run-time characteristics of an object (i.e. its tag) and not on its nominal type. As the AI indicates: ``we do not expect this to affect any implementation''.

     RM References:  7.06.01 (6)   7.06.01 (7)   7.06.01 (8)   7.06.01 (9/2)

  * AI-0064 Redundant finalization rule (0000-00-00)

     This is an editorial change only. The intended behavior is already checked by an existing ACATS test, which GNAT has always executed correctly.

     RM References:  7.06.01 (17.1/1)

  * AI-0026 Missing rules for Unchecked_Union (2010-07-07)

     Record representation clauses concerning Unchecked_Union types cannot mention the discriminant of the type. The type of a component declared in the variant part of an Unchecked_Union cannot be controlled, have controlled components, nor have protected or task parts. If an Unchecked_Union type is declared within the body of a generic unit or its descendants, then the type of a component declared in the variant part cannot be a formal private type or a formal private extension declared within the same generic unit.

     RM References:  7.06 (9.4/2)   B.03.03 (9/2)   B.03.03 (10/2)

  * AI-0205 Extended return declares visible name (0000-00-00)

     This AI corrects a simple omission in the RM. Return objects have always been visible within an extended return statement.

     RM References:  8.03 (17)

  * AI-0042 Overriding versus implemented-by (0000-00-00)

     This AI fixes a wording gap in the RM. An operation of a synchronized interface can be implemented by a protected or task entry, but the abstract operation is not being overridden in the usual sense, and it must be stated separately that this implementation is legal. This has always been the case in GNAT.

     RM References:  9.01 (9.2/2)   9.04 (11.1/2)

  * AI-0030 Requeue on synchronized interfaces (2010-07-19)

     Requeue is permitted to a protected, synchronized or task interface primitive providing it is known that the overriding operation is an entry. Otherwise the requeue statement has the same effect as a procedure call. Use of pragma Implemented provides a way to impose a static requirement on the overriding operation by adhering to one of the implementation kinds: entry, protected procedure or any of the above.

     RM References:  9.05 (9)   9.05.04 (2)   9.05.04 (3)   9.05.04 (5)
     9.05.04 (6)   9.05.04 (7)   9.05.04 (12)

  * AI-0201 Independence of atomic object components (2010-07-22)

     If an Atomic object has a pragma Pack or a Component_Size attribute, then individual components may not be addressable by independent tasks. However, if the representation clause has no effect (is confirming), then independence is not compromised. Furthermore, in GNAT, specification of other appropriately addressable component sizes (e.g. 16 for 8-bit characters) also preserves independence. GNAT now gives very clear warnings both for the declaration of such a type, and for any assignment to its components.

     RM References:  9.10 (1/3)   C.06 (22/2)   C.06 (23/2)

  * AI-0009 Pragma Independent[_Components] (2010-07-23)

     This AI introduces the new pragmas Independent and Independent_Components, which control guaranteeing independence of access to objects and components. The AI also requires independence not unaffected by confirming rep clauses.

     RM References:  9.10 (1)   13.01 (15/1)   13.02 (9)   13.03 (13)   C.06 (2)
     C.06 (4)   C.06 (6)   C.06 (9)   C.06 (13)   C.06 (14)

  * AI-0072 Task signalling using 'Terminated (0000-00-00)

     This AI clarifies that task signalling for reading 'Terminated only occurs if the result is True. GNAT semantics has always been consistent with this notion of task signalling.

     RM References:  9.10 (6.1/1)

  * AI-0108 Limited incomplete view and discriminants (0000-00-00)

     This AI confirms that an incomplete type from a limited view does not have discriminants. This has always been the case in GNAT.

     RM References:  10.01.01 (12.3/2)

  * AI-0129 Limited views and incomplete types (0000-00-00)

     This AI clarifies the description of limited views: a limited view of a package includes only one view of a type that has an incomplete declaration and a full declaration (there is no possible ambiguity in a client package). This AI also fixes an omission: a nested package in the private part has no limited view. GNAT always implemented this correctly.

     RM References:  10.01.01 (12.2/2)   10.01.01 (12.3/2)

  * AI-0077 Limited withs and scope of declarations (0000-00-00)

     This AI clarifies that a declaration does not include a context clause, and confirms that it is illegal to have a context in which both a limited and a nonlimited view of a package are accessible. Such double visibility was always rejected by GNAT.

     RM References:  10.01.02 (12/2)   10.01.02 (21/2)   10.01.02 (22/2)

  * AI-0122 Private with and children of generics (0000-00-00)

     This AI clarifies the visibility of private children of generic units within instantiations of a parent. GNAT has always handled this correctly.

     RM References:  10.01.02 (12/2)

  * AI-0040 Limited with clauses on descendant (0000-00-00)

     This AI confirms that a limited with clause in a child unit cannot name an ancestor of the unit. This has always been checked in GNAT.

     RM References:  10.01.02 (20/2)

  * AI-0132 Placement of library unit pragmas (0000-00-00)

     This AI fills a gap in the description of library unit pragmas. The pragma clearly must apply to a library unit, even if it does not carry the name of the enclosing unit. GNAT has always enforced the required check.

     RM References:  10.01.05 (7)

  * AI-0034 Categorization of limited views (0000-00-00)

     The RM makes certain limited with clauses illegal because of categorization considerations, when the corresponding normal with would be legal. This is not intended, and GNAT has always implemented the recommended behavior.

     RM References:  10.02.01 (11/1)   10.02.01 (17/2)

  * AI-0035 Inconsistencies with Pure units (0000-00-00)

     This AI remedies some inconsistencies in the legality rules for Pure units. Derived access types are legal in a pure unit (on the assumption that the rule for a zero storage pool size has been enforced on the ancestor type). The rules are enforced in generic instances and in subunits. GNAT has always implemented the recommended behavior.

     RM References:  10.02.01 (15.1/2)   10.02.01 (15.4/2)   10.02.01 (15.5/2)   10.02.01 (17/2)

  * AI-0219 Pure permissions and limited parameters (2010-05-25)

     This AI refines the rules for the cases with limited parameters which do not allow the implementations to omit ``redundant''. GNAT now properly conforms to the requirements of this binding interpretation.

     RM References:  10.02.01 (18/2)

  * AI-0043 Rules about raising exceptions (0000-00-00)

     This AI covers various omissions in the RM regarding the raising of exceptions. GNAT has always implemented the intended semantics.

     RM References:  11.04.01 (10.1/2)   11 (2)

  * AI-0200 Mismatches in formal package declarations (0000-00-00)

     This AI plugs a gap in the RM which appeared to allow some obviously intended illegal instantiations. GNAT has never allowed these instantiations.

     RM References:  12.07 (16)

  * AI-0112 Detection of duplicate pragmas (2010-07-24)

     This AI concerns giving names to various representation aspects, but the practical effect is simply to make the use of duplicate Atomic[_Components], Volatile[_Components] and Independent[_Components] pragmas illegal, and GNAT now performs this required check.

     RM References:  13.01 (8)

  * AI-0106 No representation pragmas on generic formals (0000-00-00)

     The RM appeared to allow representation pragmas on generic formal parameters, but this was not intended, and GNAT has never permitted this usage.

     RM References:  13.01 (9.1/1)

  * AI-0012 Pack/Component_Size for aliased/atomic (2010-07-15)

     It is now illegal to give an inappropriate component size or a pragma Pack that attempts to change the component size in the case of atomic or aliased components. Previously GNAT ignored such an attempt with a warning.

     RM References:  13.02 (6.1/2)   13.02 (7)   C.06 (10)   C.06 (11)   C.06 (21)

  * AI-0039 Stream attributes cannot be dynamic (0000-00-00)

     The RM permitted the use of dynamic expressions (such as ptr.all) for stream attributes, but these were never useful and are now illegal. GNAT has always regarded such expressions as illegal.

     RM References:  13.03 (4)   13.03 (6)   13.13.02 (38/2)

  * AI-0095 Address of intrinsic subprograms (0000-00-00)

     The prefix of 'Address cannot statically denote a subprogram with convention Intrinsic. The use of the Address attribute raises Program_Error if the prefix denotes a subprogram with convention Intrinsic.

     RM References:  13.03 (11/1)

  * AI-0116 Alignment of class-wide objects (0000-00-00)

     This AI requires that the alignment of a class-wide object be no greater than the alignment of any type in the class. GNAT has always followed this recommendation.

     RM References:  13.03 (29)   13.11 (16)

  * AI-0146 Type invariants (2009-09-21)

     Type invariants may be specified for private types using the aspect notation. Aspect Invariant may be specified for any private type, Invariant'Class can only be specified for tagged types, and is inherited by any descendent of the tagged types. The invariant is a boolean expression that is tested for being true in the following situations: conversions to the private type, object declarations for the private type that are default initialized, and [in] out parameters and returned result on return from any primitive operation for the type that is visible to a client.

     RM References:  13.03.03 (00)

  * AI-0078 Relax Unchecked_Conversion alignment rules (0000-00-00)

     In Ada 2012, compilers are required to support unchecked conversion where the target alignment is a multiple of the source alignment. GNAT always supported this case (and indeed all cases of differing alignments, doing copies where required if the alignment was reduced).

     RM References:  13.09 (7)

  * AI-0195 Invalid value handling is implementation defined (2010-07-03)

     The handling of invalid values is now designated to be implementation defined. This is a documentation change only, requiring Annex M in the GNAT Reference Manual to document this handling. In GNAT, checks for invalid values are made only when necessary to avoid erroneous behavior. Operations like assignments which cannot cause erroneous behavior ignore the possibility of invalid values and do not do a check. The date given above applies only to the documentation change, this behavior has always been implemented by GNAT.

     RM References:  13.09.01 (10)

  * AI-0193 Alignment of allocators (2010-09-16)

     This AI introduces a new attribute Max_Alignment_For_Allocation, analogous to Max_Size_In_Storage_Elements, but for alignment instead of size.

     RM References:  13.11 (16)   13.11 (21)   13.11.01 (0)   13.11.01 (1)
     13.11.01 (2)   13.11.01 (3)

  * AI-0177 Parameterized expressions (2010-07-10)

     The new Ada 2012 notion of parameterized expressions is implemented. The form is:

            function specification is (expression)

     This is exactly equivalent to the corresponding function body that returns the expression, but it can appear in a package spec. Note that the expression must be parenthesized.

     RM References:  13.11.01 (3/2)

  * AI-0033 Attach/Interrupt_Handler in generic (2010-07-24)

     Neither of these two pragmas may appear within a generic template, because the generic might be instantiated at other than the library level.

     RM References:  13.11.02 (16)   C.03.01 (7/2)   C.03.01 (8/2)

  * AI-0161 Restriction No_Default_Stream_Attributes (2010-09-11)

     A new restriction No_Default_Stream_Attributes prevents the use of any of the default stream attributes for elementary types. If this restriction is in force, then it is necessary to provide explicit subprograms for any stream attributes used.

     RM References:  13.12.01 (4/2)   13.13.02 (40/2)   13.13.02 (52/2)

  * AI-0194 Value of Stream_Size attribute (0000-00-00)

     The Stream_Size attribute returns the default number of bits in the stream representation of the given type. This value is not affected by the presence of stream subprogram attributes for the type. GNAT has always implemented this interpretation.

     RM References:  13.13.02 (1.2/2)

  * AI-0109 Redundant check in S'Class'Input (0000-00-00)

     This AI is an editorial change only. It removes the need for a tag check that can never fail.

     RM References:  13.13.02 (34/2)

  * AI-0007 Stream read and private scalar types (0000-00-00)

     The RM as written appeared to limit the possibilities of declaring read attribute procedures for private scalar types. This limitation was not intended, and has never been enforced by GNAT.

     RM References:  13.13.02 (50/2)   13.13.02 (51/2)

  * AI-0065 Remote access types and external streaming (0000-00-00)

     This AI clarifies the fact that all remote access types support external streaming. This fixes an obvious oversight in the definition of the language, and GNAT always implemented the intended correct rules.

     RM References:  13.13.02 (52/2)

  * AI-0019 Freezing of primitives for tagged types (0000-00-00)

     The RM suggests that primitive subprograms of a specific tagged type are frozen when the tagged type is frozen. This would be an incompatible change and is not intended. GNAT has never attempted this kind of freezing and its behavior is consistent with the recommendation of this AI.

     RM References:  13.14 (2)   13.14 (3/1)   13.14 (8.1/1)   13.14 (10)   13.14 (14)   13.14 (15.1/2)

  * AI-0017 Freezing and incomplete types (0000-00-00)

     So-called ``Taft-amendment types'' (i.e., types that are completed in package bodies) are not frozen by the occurrence of bodies in the enclosing declarative part. GNAT always implemented this properly.

     RM References:  13.14 (3/1)

  * AI-0060 Extended definition of remote access types (0000-00-00)

     This AI extends the definition of remote access types to include access to limited, synchronized, protected or task class-wide interface types. GNAT already implemented this extension.

     RM References:  A (4)   E.02.02 (9/1)   E.02.02 (9.2/1)   E.02.02 (14/2)   E.02.02 (18)

  * AI-0114 Classification of letters (0000-00-00)

     The code points 170 (FEMININE ORDINAL INDICATOR), 181 (MICRO SIGN), and 186 (MASCULINE ORDINAL INDICATOR) are technically considered lower case letters by Unicode. However, they are not allowed in identifiers, and they return False to Ada.Characters.Handling.Is_Letter/Is_Lower. This behavior is consistent with that defined in Ada 95.

     RM References:  A.03.02 (59)   A.04.06 (7)

  * AI-0185 Ada.Wide_[Wide_]Characters.Handling (2010-07-06)

     Two new packages Ada.Wide_[Wide_]Characters.Handling provide classification functions for Wide_Character and Wide_Wide_Character, as well as providing case folding routines for Wide_[Wide_]Character and Wide_[Wide_]String.

     RM References:  A.03.05 (0)   A.03.06 (0)

  * AI-0031 Add From parameter to Find_Token (2010-07-25)

     A new version of Find_Token is added to all relevant string packages, with an extra parameter From. Instead of starting at the first character of the string, the search for a matching Token starts at the character indexed by the value of From. These procedures are available in all versions of Ada but if used in versions earlier than Ada 2012 they will generate a warning that an Ada 2012 subprogram is being used.

     RM References:  A.04.03 (16)   A.04.03 (67)   A.04.03 (68/1)   A.04.04 (51) A.04.05 (46)

  * AI-0056 Index on null string returns zero (0000-00-00)

     The wording in the Ada 2005 RM implied an incompatible handling of the Index functions, resulting in raising an exception instead of returning zero in some situations. This was not intended and has been corrected. GNAT always returned zero, and is thus consistent with this AI.

     RM References:  A.04.03 (56.2/2)   A.04.03 (58.5/2)

  * AI-0137 String encoding package (2010-03-25)

     The packages Ada.Strings.UTF_Encoding, together with its child packages, Conversions, Strings, Wide_Strings, and Wide_Wide_Strings have been implemented. These packages (whose documentation can be found in the spec files a-stuten.ads, a-suenco.ads, a-suenst.ads, a-suewst.ads, a-suezst.ads) allow encoding and decoding of String, Wide_String, and Wide_Wide_String values using UTF coding schemes (including UTF-8, UTF-16LE, UTF-16BE, and UTF-16), as well as conversions between the different UTF encodings. With the exception of Wide_Wide_Strings, these packages are available in Ada 95 and Ada 2005 mode as well as Ada 2012 mode. The Wide_Wide_Strings package is available in Ada 2005 mode as well as Ada 2012 mode (but not in Ada 95 mode since it uses Wide_Wide_Character).

     RM References:  A.04.11

  * AI-0038 Minor errors in Text_IO (0000-00-00)

     These are minor errors in the description on three points. The intent on all these points has always been clear, and GNAT has always implemented the correct intended semantics.

     RM References:  A.10.05 (37)   A.10.07 (8/1)   A.10.07 (10)   A.10.07 (12)   A.10.08 (10)   A.10.08 (24)

  * AI-0044 Restrictions on container instantiations (0000-00-00)

     This AI places restrictions on allowed instantiations of generic containers. These restrictions are not checked by the compiler, so there is nothing to change in the implementation. This affects only the RM documentation.

     RM References:  A.18 (4/2)   A.18.02 (231/2)   A.18.03 (145/2)   A.18.06 (56/2)   A.18.08 (66/2)   A.18.09 (79/2)   A.18.26 (5/2)   A.18.26 (9/2)

  * AI-0127 Adding Locale Capabilities (2010-09-29)

     This package provides an interface for identifying the current locale.

     RM References:  A.19    A.19.01    A.19.02    A.19.03    A.19.05    A.19.06
     A.19.07    A.19.08    A.19.09    A.19.10    A.19.11    A.19.12    A.19.13

  * AI-0002 Export C with unconstrained arrays (0000-00-00)

     The compiler is not required to support exporting an Ada subprogram with convention C if there are parameters or a return type of an unconstrained array type (such as String). GNAT allows such declarations but generates warnings. It is possible, but complicated, to write the corresponding C code and certainly such code would be specific to GNAT and non-portable.

     RM References:  B.01 (17)   B.03 (62)   B.03 (71.1/2)

  * AI-0216 No_Task_Hierarchy forbids local tasks (0000-00-00)

     It is clearly the intention that No_Task_Hierarchy is intended to forbid tasks declared locally within subprograms, or functions returning task objects, and that is the implementation that GNAT has always provided. However the language in the RM was not sufficiently clear on this point. Thus this is a documentation change in the RM only.

     RM References:  D.07 (3/3)

  * AI-0211 No_Relative_Delays forbids Set_Handler use (2010-07-09)

     The restriction No_Relative_Delays forbids any calls to the subprogram Ada.Real_Time.Timing_Events.Set_Handler.

     RM References:  D.07 (5)   D.07 (10/2)   D.07 (10.4/2)   D.07 (10.7/2)

  * AI-0190 pragma Default_Storage_Pool (2010-09-15)

     This AI introduces a new pragma Default_Storage_Pool, which can be used to control storage pools globally. In particular, you can force every access type that is used for allocation (new) to have an explicit storage pool, or you can declare a pool globally to be used for all access types that lack an explicit one.

     RM References:  D.07 (8)

  * AI-0189 No_Allocators_After_Elaboration (2010-01-23)

     This AI introduces a new restriction No_Allocators_After_Elaboration, which says that no dynamic allocation will occur once elaboration is completed. In general this requires a run-time check, which is not required, and which GNAT does not attempt. But the static cases of allocators in a task body or in the body of the main program are detected and flagged at compile or bind time.

     RM References:  D.07 (19.1/2)   H.04 (23.3/2)

  * AI-0171 Pragma CPU and Ravenscar Profile (2010-09-24)

     A new package System.Multiprocessors is added, together with the definition of pragma CPU for controlling task affinity. A new no dependence restriction, on System.Multiprocessors.Dispatching_Domains, is added to the Ravenscar profile.

     RM References:  D.13.01 (4/2)   D.16

  * AI-0210 Correct Timing_Events metric (0000-00-00)

     This is a documentation only issue regarding wording of metric requirements, that does not affect the implementation of the compiler.

     RM References:  D.15 (24/2)

  * AI-0206 Remote types packages and preelaborate (2010-07-24)

     Remote types packages are now allowed to depend on preelaborated packages. This was formerly considered illegal.

     RM References:  E.02.02 (6)

  * AI-0152 Restriction No_Anonymous_Allocators (2010-09-08)

     Restriction No_Anonymous_Allocators prevents the use of allocators where the type of the returned value is an anonymous access type.

     RM References:  H.04 (8/1)

Obsolescent Features
********************

This chapter describes features that are provided by GNAT, but are considered obsolescent since there are preferred ways of achieving the same effect. These features are provided solely for historical compatibility purposes.

pragma No_Run_Time
==================

The pragma No_Run_Time is used to achieve an affect similar to the use of the "Zero Foot Print" configurable run time, but without requiring a specially configured run time. The result of using this pragma, which must be used for all units in a partition, is to restrict the use of any language features requiring run-time support code. The preferred usage is to use an appropriately configured run-time that includes just those features that are to be made accessible.

pragma Ravenscar
================

The pragma Ravenscar has exactly the same effect as pragma Profile (Ravenscar). The latter usage is preferred since it is part of the new Ada 2005 standard.

pragma Restricted_Run_Time
==========================

The pragma Restricted_Run_Time has exactly the same effect as pragma Profile (Restricted). The latter usage is preferred since the Ada 2005 pragma Profile is intended for this kind of implementation dependent addition.

     [file fdl.texi not included]