     [file gcc-common.texi not included]

This file documents the use and the internals of the GNU Fortran compiler, (gfortran).

Published by the Free Software Foundation 51 Franklin Street, Fifth Floor Boston, MA 02110-1301 USA

Copyright © 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010 Free Software Foundation, Inc.

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 the Invariant Sections being ``Funding Free Software'', the Front-Cover Texts being (a) (see below), and with the Back-Cover Texts being (b) (see below).  A copy of the license is included in the section entitled ``GNU Free Documentation License''.

(a) The FSF's Front-Cover Text is:

A GNU Manual

(b) The FSF's Back-Cover Text is:

You have freedom to copy and modify this GNU Manual, like GNU software.  Copies published by the Free Software Foundation raise funds for GNU development.

Introduction
************

This manual documents the use of gfortran, the GNU Fortran compiler.  You can find in this manual how to invoke gfortran, as well as its features and incompatibilities.

Introduction
************

The GNU Fortran compiler front end was designed initially as a free replacement for, or alternative to, the unix f95 command; gfortran is the command you'll use to invoke the compiler.

About GNU Fortran
=================

The GNU Fortran compiler supports the Fortran 77, 90 and 95 standards completely, parts of the Fortran 2003 and Fortran 2008 standards, and several vendor extensions.  The development goal is to provide the following features:

  * Read a user's program, stored in a file and containing instructions written in Fortran 77, Fortran 90, Fortran 95, Fortran 2003 or Fortran 2008. This file contains source code.

  * Translate the user's program into instructions a computer can carry out more quickly than it takes to translate the instructions in the first place.  The result after compilation of a program is machine code, code designed to be efficiently translated and processed by a machine such as your computer. Humans usually aren't as good writing machine code as they are at writing Fortran (or C++, Ada, or Java), because it is easy to make tiny mistakes writing machine code.

  * Provide the user with information about the reasons why the compiler is unable to create a binary from the source code. Usually this will be the case if the source code is flawed. The Fortran 90 standard requires that the compiler can point out mistakes to the user. An incorrect usage of the language causes an error message.

     The compiler will also attempt to diagnose cases where the user's program contains a correct usage of the language, but instructs the computer to do something questionable. This kind of diagnostics message is called a warning message.

  * Provide optional information about the translation passes from the source code to machine code. This can help a user of the compiler to find the cause of certain bugs which may not be obvious in the source code, but may be more easily found at a lower level compiler output. It also helps developers to find bugs in the compiler itself.

  * Provide information in the generated machine code that can make it easier to find bugs in the program (using a debugging tool, called a debugger, such as the GNU Debugger gdb).

  * Locate and gather machine code already generated to perform actions requested by statements in the user's program. This machine code is organized into modules and is located and linked to the user program.

The GNU Fortran compiler consists of several components:

  * A version of the gcc command (which also might be installed as the system's cc command) that also understands and accepts Fortran source code. The gcc command is the driver program for all the languages in the GNU Compiler Collection (GCC); With gcc, you can compile the source code of any language for which a front end is available in GCC.

  * The gfortran command itself, which also might be installed as the system's f95 command. gfortran is just another driver program, but specifically for the Fortran compiler only. The difference with gcc is that gfortran will automatically link the correct libraries to your program.

  * A collection of run-time libraries. These libraries contain the machine code needed to support capabilities of the Fortran language that are not directly provided by the machine code generated by the gfortran compilation phase, such as intrinsic functions and subroutines, and routines for interaction with files and the operating system.

  * The Fortran compiler itself, (f951). This is the GNU Fortran parser and code generator, linked to and interfaced with the GCC backend library. f951 ``translates'' the source code to assembler code.  You would typically not use this program directly; instead, the gcc or gfortran driver programs will call it for you.

GNU Fortran and GCC
===================

GNU Fortran is a part of GCC, the GNU Compiler Collection.  GCC consists of a collection of front ends for various languages, which translate the source code into a language-independent form called GENERIC.  This is then processed by a common middle end which provides optimization, and then passed to one of a collection of back ends which generate code for different computer architectures and operating systems.

Functionally, this is implemented with a driver program (gcc) which provides the command-line interface for the compiler.  It calls the relevant compiler front-end program (e.g., f951 for Fortran) for each file in the source code, and then calls the assembler and linker as appropriate to produce the compiled output.  In a copy of GCC which has been compiled with Fortran language support enabled, gcc will recognize files with .f, .for, .ftn, .f90, .f95, .f03 and .f08 extensions as Fortran source code, and compile it accordingly.  A gfortran driver program is also provided, which is identical to gcc except that it automatically links the Fortran runtime libraries into the compiled program.

Source files with .f, .for, .fpp, .ftn, .F, .FOR, .FPP, and .FTN extensions are treated as fixed form. Source files with .f90, .f95, .f03, .f08, .F90, .F95, .F03 and .F08 extensions are treated as free form.  The capitalized versions of either form are run through preprocessing.  Source files with the lower case .fpp extension are also run through preprocessing.

This manual specifically documents the Fortran front end, which handles the programming language's syntax and semantics.  The aspects of GCC which relate to the optimization passes and the back-end code generation are documented in the GCC manual; see Introduction (Using the GNU Compiler Collection (GCC)). The two manuals together provide a complete reference for the GNU Fortran compiler.

Preprocessing and conditional compilation
=========================================

Many Fortran compilers including GNU Fortran allow passing the source code through a C preprocessor (CPP; sometimes also called the Fortran preprocessor, FPP) to allow for conditional compilation.  In the case of GNU Fortran, this is the GNU C Preprocessor in the traditional mode.  On systems with case-preserving file names, the preprocessor is automatically invoked if the filename extension is .F, .FOR, .FTN, .fpp, .FPP, .F90, .F95, .F03 or .F08.  To manually invoke the preprocessor on any file, use -cpp, to disable preprocessing on files where the preprocessor is run automatically, use -nocpp.

If a preprocessed file includes another file with the Fortran INCLUDE statement, the included file is not preprocessed.  To preprocess included files, use the equivalent preprocessor statement #include.

If GNU Fortran invokes the preprocessor, __GFORTRAN__ is defined and __GNUC__, __GNUC_MINOR__ and __GNUC_PATCHLEVEL__ can be used to determine the version of the compiler.  See Overview (The C Preprocessor) for details.

While CPP is the de-facto standard for preprocessing Fortran code, Part 3 of the Fortran 95 standard (ISO/IEC 1539-3:1998) defines Conditional Compilation, which is not widely used and not directly supported by the GNU Fortran compiler.  You can use the program coco to preprocess such files (http://www.daniellnagle.com/coco.html).

GNU Fortran and G77
===================

The GNU Fortran compiler is the successor to g77, the Fortran 77 front end included in GCC prior to version 4.  It is an entirely new program that has been designed to provide Fortran 95 support and extensibility for future Fortran language standards, as well as providing backwards compatibility for Fortran 77 and nearly all of the GNU language extensions supported by g77.

Project Status
==============

     As soon as gfortran can parse all of the statements correctly, it will be in the ``larva'' state. When we generate code, the ``puppa'' state. When gfortran is done, we'll see if it will be a beautiful butterfly, or just a big bug....

     --Andy Vaught, April 2000

The start of the GNU Fortran 95 project was announced on the GCC homepage in March 18, 2000 (even though Andy had already been working on it for a while, of course).

The GNU Fortran compiler is able to compile nearly all standard-compliant Fortran 95, Fortran 90, and Fortran 77 programs, including a number of standard and non-standard extensions, and can be used on real-world programs.  In particular, the supported extensions include OpenMP, Cray-style pointers, and several Fortran 2003 and Fortran 2008 features, including TR 15581.  However, it is still under development and has a few remaining rough edges.

At present, the GNU Fortran compiler passes the NIST Fortran 77 Test Suite (http://www.fortran-2000.com/ArnaudRecipes/fcvs21_f95.html), and produces acceptable results on the LAPACK Test Suite (http://www.netlib.org/lapack/faq.html#1.21). It also provides respectable performance on the Polyhedron Fortran compiler benchmarks (http://www.polyhedron.com/pb05.html) and the Livermore Fortran Kernels test (http://www.llnl.gov/asci_benchmarks/asci/limited/lfk/README.html).  It has been used to compile a number of large real-world programs, including the HIRLAM weather-forecasting code (http://mysite.verizon.net/serveall/moene.pdf) and the Tonto quantum chemistry package (http://www.theochem.uwa.edu.au/tonto/); see http://gcc.gnu.org/wiki/GfortranApps for an extended list.

Among other things, the GNU Fortran compiler is intended as a replacement for G77.  At this point, nearly all programs that could be compiled with G77 can be compiled with GNU Fortran, although there are a few minor known regressions.

The primary work remaining to be done on GNU Fortran falls into three categories: bug fixing (primarily regarding the treatment of invalid code and providing useful error messages), improving the compiler optimizations and the performance of compiled code, and extending the compiler to support future standards---in particular, Fortran 2003 and Fortran 2008.

Standards
=========

The GNU Fortran compiler implements ISO/IEC 1539:1997 (Fortran 95).  As such, it can also compile essentially all standard-compliant Fortran 90 and Fortran 77 programs.   It also supports the ISO/IEC TR-15581 enhancements to allocatable arrays.

In the future, the GNU Fortran compiler will also support ISO/IEC 1539-1:2004 (Fortran 2003), ISO/IEC 1539-1:2010 (Fortran 2008) and future Fortran standards.  Partial support of the Fortran 2003 and Fortran 2008 standard is already provided; the current status of the support is reported in the Fortran 2003 status and Fortran 2008 status sections of the documentation.

Additionally, the GNU Fortran compilers supports the OpenMP specification (version 3.0, http://openmp.org/wp/openmp-specifications/).

Varying Length Character Strings
--------------------------------

The Fortran 95 standard specifies in Part 2 (ISO/IEC 1539-2:2000) varying length character strings.  While GNU Fortran currently does not support such strings directly, there exist two Fortran implementations for them, which work with GNU Fortran.  They can be found at http://www.fortran.com/iso_varying_string.f95 and at ftp://ftp.nag.co.uk/sc22wg5/ISO_VARYING_STRING/.

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Runtime: Influencing runtime behavior with environment variables
****************************************************************

The behavior of the gfortran can be influenced by environment variables.

Malformed environment variables are silently ignored.

GFORTRAN_STDIN_UNIT---Unit number for standard input
====================================================

This environment variable can be used to select the unit number preconnected to standard input.  This must be a positive integer. The default value is 5.

GFORTRAN_STDOUT_UNIT---Unit number for standard output
======================================================

This environment variable can be used to select the unit number preconnected to standard output.  This must be a positive integer. The default value is 6.

GFORTRAN_STDERR_UNIT---Unit number for standard error
=====================================================

This environment variable can be used to select the unit number preconnected to standard error.  This must be a positive integer. The default value is 0.

GFORTRAN_USE_STDERR---Send library output to standard error
===========================================================

This environment variable controls where library output is sent. If the first letter is y, Y or 1, standard error is used.  If the first letter is n, N or 0, standard output is used.

GFORTRAN_TMPDIR---Directory for scratch files
=============================================

This environment variable controls where scratch files are created.  If this environment variable is missing, GNU Fortran searches for the environment variable TMP, then TEMP. If these are missing, the default is /tmp.

GFORTRAN_UNBUFFERED_ALL---Don't buffer I/O on all units
=======================================================

This environment variable controls whether all I/O is unbuffered.  If the first letter is y, Y or 1, all I/O is unbuffered.  This will slow down small sequential reads and writes.  If the first letter is n, N or 0, I/O is buffered. This is the default.

GFORTRAN_UNBUFFERED_PRECONNECTED---Don't buffer I/O on preconnected units
=========================================================================

The environment variable named GFORTRAN_UNBUFFERED_PRECONNECTED controls whether I/O on a preconnected unit (i.e. STDOUT or STDERR) is unbuffered.  If the first letter is y, Y or 1, I/O is unbuffered.  This will slow down small sequential reads and writes.  If the first letter is n, N or 0, I/O is buffered.  This is the default.

GFORTRAN_SHOW_LOCUS---Show location for runtime errors
======================================================

If the first letter is y, Y or 1, filename and line numbers for runtime errors are printed.  If the first letter is n, N or 0, don't print filename and line numbers for runtime errors.  The default is to print the location.

GFORTRAN_OPTIONAL_PLUS---Print leading + where permitted
========================================================

If the first letter is y, Y or 1, a plus sign is printed where permitted by the Fortran standard.  If the first letter is n, N or 0, a plus sign is not printed in most cases.  Default is not to print plus signs.

GFORTRAN_DEFAULT_RECL---Default record length for new files
===========================================================

This environment variable specifies the default record length, in bytes, for files which are opened without a RECL tag in the OPEN statement.  This must be a positive integer.  The default value is 1073741824 bytes (1 GB).

GFORTRAN_LIST_SEPARATOR---Separator for list output
===================================================

This environment variable specifies the separator when writing list-directed output.  It may contain any number of spaces and at most one comma.  If you specify this on the command line, be sure to quote spaces, as in

     $ GFORTRAN_LIST_SEPARATOR='  ,  ' ./a.out

when a.out is the compiled Fortran program that you want to run. Default is a single space.

GFORTRAN_CONVERT_UNIT---Set endianness for unformatted I/O
==========================================================

By setting the GFORTRAN_CONVERT_UNIT variable, it is possible to change the representation of data for unformatted files. The syntax for the GFORTRAN_CONVERT_UNIT variable is:

     GFORTRAN_CONVERT_UNIT: mode | mode ';' exception | exception ;
     mode: 'native' | 'swap' | 'big_endian' | 'little_endian' ;
     exception: mode ':' unit_list | unit_list ;
     unit_list: unit_spec | unit_list unit_spec ;
     unit_spec: INTEGER | INTEGER '-' INTEGER ;

The variable consists of an optional default mode, followed by a list of optional exceptions, which are separated by semicolons from the preceding default and each other.  Each exception consists of a format and a comma-separated list of units.  Valid values for the modes are the same as for the CONVERT specifier:

  NATIVE Use the native format.  This is the default.

  SWAP Swap between little- and big-endian.

  LITTLE_ENDIAN Use the little-endian format for unformatted files.

  BIG_ENDIAN Use the big-endian format for unformatted files.

A missing mode for an exception is taken to mean BIG_ENDIAN. Examples of values for GFORTRAN_CONVERT_UNIT are:

  'big_endian'  Do all unformatted I/O in big_endian mode.

  'little_endian;native:10-20,25'  Do all unformatted I/O in little_endian mode, except for units 10 to 20 and 25, which are in native format.

  '10-20'  Units 10 to 20 are big-endian, the rest is native.

Setting the environment variables should be done on the command line or via the export command for sh-compatible shells and via setenv for csh-compatible shells.

Example for sh:

     $ gfortran foo.f90
     $ GFORTRAN_CONVERT_UNIT='big_endian;native:10-20' ./a.out

Example code for csh:

     % gfortran foo.f90
     % setenv GFORTRAN_CONVERT_UNIT 'big_endian;native:10-20'
     % ./a.out

Using anything but the native representation for unformatted data carries a significant speed overhead.  If speed in this area matters to you, it is best if you use this only for data that needs to be portable.

See CONVERT specifier, for an alternative way to specify the data representation for unformatted files.  See Runtime Options, for setting a default data representation for the whole program.  The CONVERT specifier overrides the -fconvert compile options.

Note that the values specified via the GFORTRAN_CONVERT_UNIT environment variable will override the CONVERT specifier in the open statement.  This is to give control over data formats to users who do not have the source code of their program available.

GFORTRAN_ERROR_DUMPCORE---Dump core on run-time errors
======================================================

If the GFORTRAN_ERROR_DUMPCORE variable is set to y, Y or 1 (only the first letter is relevant) then library run-time errors cause core dumps.  To disable the core dumps, set the variable to n, N, 0.  Default is not to core dump unless the -fdump-core compile option was used.

GFORTRAN_ERROR_BACKTRACE---Show backtrace on run-time errors
============================================================

If the GFORTRAN_ERROR_BACKTRACE variable is set to y, Y or 1 (only the first letter is relevant) then a backtrace is printed when a run-time error occurs. To disable the backtracing, set the variable to n, N, 0.  Default is not to print a backtrace unless the -fbacktrace compile option was used.

Fortran 2003 and 2008 Status
****************************

Fortran 2003 status
===================

GNU Fortran supports several Fortran 2003 features; an incomplete list can be found below.  See also the wiki page (http://gcc.gnu.org/wiki/Fortran2003) about Fortran 2003.

  * Intrinsics command_argument_count, get_command, get_command_argument, get_environment_variable, and move_alloc.

  * Array constructors using square brackets.  That is, [...] rather than (/.../).  Type-specification for array constructors like (/ some-type :: ... /).

  * FLUSH statement.

  * IOMSG= specifier for I/O statements.

  * Support for the declaration of enumeration constants via the ENUM and ENUMERATOR statements.  Interoperability with gcc is guaranteed also for the case where the -fshort-enums command line option is given.

  * TR 15581:

       * ALLOCATABLE dummy arguments.

       * ALLOCATABLE function results

       * ALLOCATABLE components of derived types

  * The ERRMSG= tag is now supported in ALLOCATE and DEALLOCATE statements.  The SOURCE= tag is supported in an ALLOCATE statement.  An intrinsic-type-spec can be used as the type-spec in an ALLOCATE statement; while the use of a derived-type-name is currently unsupported.

  * The OPEN statement supports the ACCESS='STREAM' specifier, allowing I/O without any record structure.

  * Namelist input/output for internal files.

  * The PROTECTED statement and attribute.

  * The VALUE statement and attribute.

  * The VOLATILE statement and attribute.

  * The IMPORT statement, allowing to import host-associated derived types.

  * USE statement with INTRINSIC and NON_INTRINSIC attribute; supported intrinsic modules: ISO_FORTRAN_ENV, OMP_LIB and OMP_LIB_KINDS.

  * Renaming of operators in the USE statement.

  * Interoperability with C (ISO C Bindings)

  * BOZ as argument of INT, REAL, DBLE and CMPLX.

  * Type-bound procedures with PROCEDURE or GENERIC, and operators bound to a derived-type.

  * Extension of derived-types (the EXTENDS(...) syntax).

  * ABSTRACT derived-types and declaring procedure bindings DEFERRED.

Fortran 2008 status
===================

The latest version of the Fortran standard is ISO/IEC 1539-1:2010, informally known as Fortran 2008.  The official version is available from International Organization for Standardization (ISO) or its national member organizations. The the final draft (FDIS) can be downloaded free of charge from http://www.nag.co.uk/sc22wg5/links.html.  Fortran is developed by the Working Group 5 of Sub-Committee 22 of the Joint Technical Committee 1 of the International Organization for Standardization and the International Electrotechnical Commission (IEC).  This group is known as WG5 (http://www.nag.co.uk/sc22wg5/).

The GNU Fortran supports several of the new features of Fortran 2008; the wiki (http://gcc.gnu.org/wiki/Fortran2008Status) has some information about the current Fortran 2008 implementation status.  In particular, the following is implemented.

  * The -std=f2008 option and support for the file extensions .f08 and .F08.

  * The OPEN statement now supports the NEWUNIT= option, which returns a unique file unit, thus preventing inadvertent use of the same unit in different parts of the program.

  * The g0 format descriptor and unlimited format items.

  * The mathematical intrinsics ASINH, ACOSH, ATANH, ERF, ERFC, GAMMA, LOG_GAMMA, BESSEL_J0, BESSEL_J1, BESSEL_JN, BESSEL_Y0, BESSEL_Y1, BESSEL_YN, HYPOT, NORM2, and ERFC_SCALED.

  * Using complex arguments with TAN, SINH, COSH, TANH, ASIN, ACOS, and ATAN is now possible; ATAN(Y,X) is now an alias for ATAN2(Y,X).

  * Support of the PARITY intrinsic functions.

  * The following bit intrinsics: LEADZ and TRAILZ for counting the number of leading and trailing zero bits, POPCNT and POPPAR for counting the number of one bits and returning the parity; BGE, BGT, BLE, and BLT for bitwise comparisons; DSHIFTL and DSHIFTR for combined left and right shifts, MASKL and MASKR for simple left and right justified masks, MERGE_BITS for a bitwise merge using a mask, SHIFTA, SHIFTL and SHIFTR for shift operations, and the transformational bit intrinsics IALL, IANY and IPARITY.

  * Support of the EXECUTE_COMMAND_LINE intrinsic subroutine.

  * Support for the STORAGE_SIZE intrinsic inquiry function.

  * The INT{8,16,32} and REAL{32,64,128} kind type parameters and the array-valued named constants INTEGER_KINDS, LOGICAL_KINDS, REAL_KINDS and CHARACTER_KINDS of the intrinsic module ISO_FORTRAN_ENV.

  * The module procedures C_SIZEOF of the intrinsic module ISO_C_BINDINGS and COMPILER_VERSION and COMPILER_OPTIONS of ISO_FORTRAN_ENV.

  * Experimental coarray support (for one image only), use the -fcoarray=single flag to enable it.

  * The BLOCK construct is supported.

  * The STOP and the new ERROR STOP statements now support all constant expressions.

  * Support for the CONTIGUOUS attribute.

  * Support for ALLOCATE with MOLD.

  * Support for the IMPURE attribute for procedures, which allows for ELEMENTAL procedures without the restrictions of PURE.

  * Null pointers (including NULL()) and not-allocated variables can be used as actual argument to optional non-pointer, non-allocatable dummy arguments, denoting an absent argument.

  * Non-pointer variables with TARGET attribute can be used as actual argument to POINTER dummies with INTENT(IN).

  * Pointers including procedure pointers and those in a derived type (pointer components) can now be initialized by a target instead of only by NULL.

  * The EXIT statement (with construct-name) can be now be used to leave not only the DO but also the ASSOCIATE, BLOCK, IF, SELECT CASE and SELECT TYPE constructs.

  * Internal procedures can now be used as actual argument.

  * Minor features: obsolesce diagnostics for ENTRY with -std=f2008; a line may start with a semicolon; for internal and module procedures END can be used instead of END SUBROUTINE and END FUNCTION; SELECTED_REAL_KIND now also takes a RADIX argument; intrinsic types are supported for TYPE(INTRINSIC-TYPE-SPEC); multiple type-bound procedures can be declared in a single PROCEDURE statement; implied-shape arrays are supported for named constants (PARAMETER).

Compiler Characteristics
************************

This chapter describes certain characteristics of the GNU Fortran compiler, that are not specified by the Fortran standard, but which might in some way or another become visible to the programmer.

KIND Type Parameters
====================

The KIND type parameters supported by GNU Fortran for the primitive data types are:

INTEGER
     1, 2, 4, 8*, 16*, default: 4 (1)

LOGICAL
     1, 2, 4, 8*, 16*, default: 4 (1)

REAL
     4, 8, 10**, 16**, default: 4 (2)

COMPLEX
     4, 8, 10**, 16**, default: 4 (2)

CHARACTER
     1, 4, default: 1

* = not available on all systems

** = not available on all systems; additionally 10 and 16 are never available at the same time

(1) Unless -fdefault-integer-8 is used

(2) Unless -fdefault-real-8 is used

The KIND value matches the storage size in bytes, except for COMPLEX where the storage size is twice as much (or both real and imaginary part are a real value of the given size).  It is recommended to use the SELECT_*_KIND intrinsics instead of the concrete values.

Internal representation of LOGICAL variables
============================================

The Fortran standard does not specify how variables of LOGICAL type are represented, beyond requiring that LOGICAL variables of default kind have the same storage size as default INTEGER and REAL variables.  The GNU Fortran internal representation is as follows.

A LOGICAL(KIND=N) variable is represented as an INTEGER(KIND=N) variable, however, with only two permissible values: 1 for .TRUE. and 0 for .FALSE..  Any other integer value results in undefined behavior.

Note that for mixed-language programming using the ISO_C_BINDING feature, there is a C_BOOL kind that can be used to create LOGICAL(KIND=C_BOOL) variables which are interoperable with the C99 _Bool type.  The C99 _Bool type has an internal representation described in the C99 standard, which is identical to the above description, i.e. with 1 for true and 0 for false being the only permissible values.  Thus the internal representation of LOGICAL variables in GNU Fortran is identical to C99 _Bool, except for a possible difference in storage size depending on the kind.

Extensions
**********

The two sections below detail the extensions to standard Fortran that are implemented in GNU Fortran, as well as some of the popular or historically important extensions that are not (or not yet) implemented. For the latter case, we explain the alternatives available to GNU Fortran users, including replacement by standard-conforming code or GNU extensions.

Extensions implemented in GNU Fortran
=====================================

GNU Fortran implements a number of extensions over standard Fortran.  This chapter contains information on their syntax and meaning.  There are currently two categories of GNU Fortran extensions, those that provide functionality beyond that provided by any standard, and those that are supported by GNU Fortran purely for backward compatibility with legacy compilers.  By default, -std=gnu allows the compiler to accept both types of extensions, but to warn about the use of the latter.  Specifying either -std=f95, -std=f2003 or -std=f2008 disables both types of extensions, and -std=legacy allows both without warning.

Old-style kind specifications
-----------------------------

GNU Fortran allows old-style kind specifications in declarations.  These look like:

           TYPESPEC*size x,y,z

where TYPESPEC is a basic type (INTEGER, REAL, etc.), and where size is a byte count corresponding to the storage size of a valid kind for that type.  (For COMPLEX variables, size is the total size of the real and imaginary parts.)  The statement then declares x, y and z to be of type TYPESPEC with the appropriate kind.  This is equivalent to the standard-conforming declaration

           TYPESPEC(k) x,y,z

where k is the kind parameter suitable for the intended precision.  As kind parameters are implementation-dependent, use the KIND, SELECTED_INT_KIND and SELECTED_REAL_KIND intrinsics to retrieve the correct value, for instance REAL*8 x can be replaced by:

     INTEGER, PARAMETER :: dbl = KIND(1.0d0)
     REAL(KIND=dbl) :: x

Old-style variable initialization
---------------------------------

GNU Fortran allows old-style initialization of variables of the form:

           INTEGER i/1/,j/2/
           REAL x(2,2) /3*0.,1./

The syntax for the initializers is as for the DATA statement, but unlike in a DATA statement, an initializer only applies to the variable immediately preceding the initialization.  In other words, something like INTEGER I,J/2,3/ is not valid.  This style of initialization is only allowed in declarations without double colons (::); the double colons were introduced in Fortran 90, which also introduced a standard syntax for initializing variables in type declarations.

Examples of standard-conforming code equivalent to the above example are:

     ! Fortran 90
           INTEGER :: i = 1, j = 2
           REAL :: x(2,2) = RESHAPE((/0.,0.,0.,1./),SHAPE(x))
     ! Fortran 77
           INTEGER i, j
           REAL x(2,2)
           DATA i/1/, j/2/, x/3*0.,1./

Note that variables which are explicitly initialized in declarations or in DATA statements automatically acquire the SAVE attribute.

Extensions to namelist
----------------------

GNU Fortran fully supports the Fortran 95 standard for namelist I/O including array qualifiers, substrings and fully qualified derived types. The output from a namelist write is compatible with namelist read.  The output has all names in upper case and indentation to column 1 after the namelist name.  Two extensions are permitted:

Old-style use of $ instead of &

     $MYNML
      X(:)%Y(2) = 1.0 2.0 3.0
      CH(1:4) = "abcd"
     $END

It should be noted that the default terminator is / rather than &END.

Querying of the namelist when inputting from stdin.  After at least one space, entering ? sends to stdout the namelist name and the names of the variables in the namelist:

      ?

     &mynml
      x
      x%y
      ch
     &end

Entering =? outputs the namelist to stdout, as if WRITE(*,NML = mynml) had been called:

     =?

     &MYNML
      X(1)%Y=  0.000000    ,  1.000000    ,  0.000000    ,
      X(2)%Y=  0.000000    ,  2.000000    ,  0.000000    ,
      X(3)%Y=  0.000000    ,  3.000000    ,  0.000000    ,
      CH=abcd,  /

To aid this dialog, when input is from stdin, errors send their messages to stderr and execution continues, even if IOSTAT is set.

PRINT namelist is permitted.  This causes an error if -std=f95 is used.

     PROGRAM test_print
       REAL, dimension (4)  ::  x = (/1.0, 2.0, 3.0, 4.0/)
       NAMELIST /mynml/ x
       PRINT mynml
     END PROGRAM test_print

Expanded namelist reads are permitted.  This causes an error if -std=f95 is used.  In the following example, the first element of the array will be given the value 0.00 and the two succeeding elements will be given the values 1.00 and 2.00.

     &MYNML
       X(1,1) = 0.00 , 1.00 , 2.00
     /

X format descriptor without count field
---------------------------------------

To support legacy codes, GNU Fortran permits the count field of the X edit descriptor in FORMAT statements to be omitted. When omitted, the count is implicitly assumed to be one.

            PRINT 10, 2, 3
     10     FORMAT (I1, X, I1)

Commas in FORMAT specifications
-------------------------------

To support legacy codes, GNU Fortran allows the comma separator to be omitted immediately before and after character string edit descriptors in FORMAT statements.

            PRINT 10, 2, 3
     10     FORMAT ('FOO='I1' BAR='I2)

Missing period in FORMAT specifications
---------------------------------------

To support legacy codes, GNU Fortran allows missing periods in format specifications if and only if -std=legacy is given on the command line.  This is considered non-conforming code and is discouraged.

            REAL :: value
            READ(*,10) value
     10     FORMAT ('F4')

I/O item lists
--------------

To support legacy codes, GNU Fortran allows the input item list of the READ statement, and the output item lists of the WRITE and PRINT statements, to start with a comma.

BOZ literal constants
---------------------

Besides decimal constants, Fortran also supports binary (b), octal (o) and hexadecimal (z) integer constants.  The syntax is: prefix quote digits quote, were the prefix is either b, o or z, quote is either ' or " and the digits are for binary 0 or 1, for octal between 0 and 7, and for hexadecimal between 0 and F.  (Example: b'01011101'.)

Up to Fortran 95, BOZ literals were only allowed to initialize integer variables in DATA statements.  Since Fortran 2003 BOZ literals are also allowed as argument of REAL, DBLE, INT and CMPLX; the result is the same as if the integer BOZ literal had been converted by TRANSFER to, respectively, real, double precision, integer or complex. As GNU Fortran extension the intrinsic procedures FLOAT, DFLOAT, COMPLEX and DCMPLX are treated alike.

As an extension, GNU Fortran allows hexadecimal BOZ literal constants to be specified using the X prefix, in addition to the standard Z prefix.  The BOZ literal can also be specified by adding a suffix to the string, for example, Z'ABC' and 'ABC'Z are equivalent.

Furthermore, GNU Fortran allows using BOZ literal constants outside DATA statements and the four intrinsic functions allowed by Fortran 2003. In DATA statements, in direct assignments, where the right-hand side only contains a BOZ literal constant, and for old-style initializers of the form integer i /o'0173'/, the constant is transferred as if TRANSFER had been used; for COMPLEX numbers, only the real part is initialized unless CMPLX is used.  In all other cases, the BOZ literal constant is converted to an INTEGER value with the largest decimal representation.  This value is then converted numerically to the type and kind of the variable in question. (For instance, real :: r = b'0000001' + 1 initializes r with 2.0.) As different compilers implement the extension differently, one should be careful when doing bitwise initialization of non-integer variables.

Note that initializing an INTEGER variable with a statement such as DATA i/Z'FFFFFFFF'/ will give an integer overflow error rather than the desired result of -1 when i is a 32-bit integer on a system that supports 64-bit integers.  The -fno-range-check option can be used as a workaround for legacy code that initializes integers in this manner.

Real array indices
------------------

As an extension, GNU Fortran allows the use of REAL expressions or variables as array indices.

Unary operators
---------------

As an extension, GNU Fortran allows unary plus and unary minus operators to appear as the second operand of binary arithmetic operators without the need for parenthesis.

            X = Y * -Z

Implicitly convert LOGICAL and INTEGER values
---------------------------------------------

As an extension for backwards compatibility with other compilers, GNU Fortran allows the implicit conversion of LOGICAL values to INTEGER values and vice versa.  When converting from a LOGICAL to an INTEGER, .FALSE. is interpreted as zero, and .TRUE. is interpreted as one.  When converting from INTEGER to LOGICAL, the value zero is interpreted as .FALSE. and any nonzero value is interpreted as .TRUE..

             LOGICAL :: l
             l = 1

             INTEGER :: i
             i = .TRUE.

However, there is no implicit conversion of INTEGER values in if-statements, nor of LOGICAL or INTEGER values in I/O operations.

Hollerith constants support
---------------------------

GNU Fortran supports Hollerith constants in assignments, function arguments, and DATA and ASSIGN statements.  A Hollerith constant is written as a string of characters preceded by an integer constant indicating the character count, and the letter H or h, and stored in bytewise fashion in a numeric (INTEGER, REAL, or complex) or LOGICAL variable.  The constant will be padded or truncated to fit the size of the variable in which it is stored.

Examples of valid uses of Hollerith constants:

           complex*16 x(2)
           data x /16Habcdefghijklmnop, 16Hqrstuvwxyz012345/
           x(1) = 16HABCDEFGHIJKLMNOP
           call foo (4h abc)

Invalid Hollerith constants examples:

           integer*4 a
           a = 8H12345678 ! Valid, but the Hollerith constant will be truncated.
           a = 0H         ! At least one character is needed.

In general, Hollerith constants were used to provide a rudimentary facility for handling character strings in early Fortran compilers, prior to the introduction of CHARACTER variables in Fortran 77; in those cases, the standard-compliant equivalent is to convert the program to use proper character strings.  On occasion, there may be a case where the intent is specifically to initialize a numeric variable with a given byte sequence.  In these cases, the same result can be obtained by using the TRANSFER statement, as in this example.

           INTEGER(KIND=4) :: a
           a = TRANSFER ("abcd", a)     ! equivalent to: a = 4Habcd

Cray pointers
-------------

Cray pointers are part of a non-standard extension that provides a C-like pointer in Fortran.  This is accomplished through a pair of variables: an integer "pointer" that holds a memory address, and a "pointee" that is used to dereference the pointer.

Pointer/pointee pairs are declared in statements of the form:

             pointer ( <pointer> , <pointee> )

or,

             pointer ( <pointer1> , <pointee1> ), ( <pointer2> , <pointee2> ), ...

The pointer is an integer that is intended to hold a memory address. The pointee may be an array or scalar.  A pointee can be an assumed size array---that is, the last dimension may be left unspecified by using a * in place of a value---but a pointee cannot be an assumed shape array.  No space is allocated for the pointee.

The pointee may have its type declared before or after the pointer statement, and its array specification (if any) may be declared before, during, or after the pointer statement.  The pointer may be declared as an integer prior to the pointer statement.  However, some machines have default integer sizes that are different than the size of a pointer, and so the following code is not portable:

             integer ipt
             pointer (ipt, iarr)

If a pointer is declared with a kind that is too small, the compiler will issue a warning; the resulting binary will probably not work correctly, because the memory addresses stored in the pointers may be truncated.  It is safer to omit the first line of the above example; if explicit declaration of ipt's type is omitted, then the compiler will ensure that ipt is an integer variable large enough to hold a pointer.

Pointer arithmetic is valid with Cray pointers, but it is not the same as C pointer arithmetic.  Cray pointers are just ordinary integers, so the user is responsible for determining how many bytes to add to a pointer in order to increment it.  Consider the following example:

             real target(10)
             real pointee(10)
             pointer (ipt, pointee)
             ipt = loc (target)
             ipt = ipt + 1

The last statement does not set ipt to the address of target(1), as it would in C pointer arithmetic.  Adding 1 to ipt just adds one byte to the address stored in ipt.

Any expression involving the pointee will be translated to use the value stored in the pointer as the base address.

To get the address of elements, this extension provides an intrinsic function LOC().  The LOC() function is equivalent to the & operator in C, except the address is cast to an integer type:

             real ar(10)
             pointer(ipt, arpte(10))
             real arpte
             ipt = loc(ar)  ! Makes arpte is an alias for ar
             arpte(1) = 1.0 ! Sets ar(1) to 1.0

The pointer can also be set by a call to the MALLOC intrinsic (see MALLOC).

Cray pointees often are used to alias an existing variable.  For example:

             integer target(10)
             integer iarr(10)
             pointer (ipt, iarr)
             ipt = loc(target)

As long as ipt remains unchanged, iarr is now an alias for target.  The optimizer, however, will not detect this aliasing, so it is unsafe to use iarr and target simultaneously.  Using a pointee in any way that violates the Fortran aliasing rules or assumptions is illegal.  It is the user's responsibility to avoid doing this; the compiler works under the assumption that no such aliasing occurs.

Cray pointers will work correctly when there is no aliasing (i.e., when they are used to access a dynamically allocated block of memory), and also in any routine where a pointee is used, but any variable with which it shares storage is not used.  Code that violates these rules may not run as the user intends.  This is not a bug in the optimizer; any code that violates the aliasing rules is illegal.  (Note that this is not unique to GNU Fortran; any Fortran compiler that supports Cray pointers will ``incorrectly'' optimize code with illegal aliasing.)

There are a number of restrictions on the attributes that can be applied to Cray pointers and pointees.  Pointees may not have the ALLOCATABLE, INTENT, OPTIONAL, DUMMY, TARGET, INTRINSIC, or POINTER attributes.  Pointers may not have the DIMENSION, POINTER, TARGET, ALLOCATABLE, EXTERNAL, or INTRINSIC attributes, nor may they be function results.  Pointees may not occur in more than one pointer statement.  A pointee cannot be a pointer.  Pointees cannot occur in equivalence, common, or data statements.

A Cray pointer may also point to a function or a subroutine.  For example, the following excerpt is valid:

       implicit none
       external sub
       pointer (subptr,subpte)
       external subpte
       subptr = loc(sub)
       call subpte()
       [...]
       subroutine sub
       [...]
       end subroutine sub

A pointer may be modified during the course of a program, and this will change the location to which the pointee refers.  However, when pointees are passed as arguments, they are treated as ordinary variables in the invoked function.  Subsequent changes to the pointer will not change the base address of the array that was passed.

CONVERT specifier
-----------------

GNU Fortran allows the conversion of unformatted data between little- and big-endian representation to facilitate moving of data between different systems.  The conversion can be indicated with the CONVERT specifier on the OPEN statement. See GFORTRAN_CONVERT_UNIT, for an alternative way of specifying the data format via an environment variable.

Valid values for CONVERT are:

  CONVERT='NATIVE' Use the native format.  This is the default.

  CONVERT='SWAP' Swap between little- and big-endian.

  CONVERT='LITTLE_ENDIAN' Use the little-endian representation for unformatted files.

  CONVERT='BIG_ENDIAN' Use the big-endian representation for unformatted files.

Using the option could look like this:

       open(file='big.dat',form='unformatted',access='sequential', &
            convert='big_endian')

The value of the conversion can be queried by using INQUIRE(CONVERT=ch).  The values returned are 'BIG_ENDIAN' and 'LITTLE_ENDIAN'.

CONVERT works between big- and little-endian for INTEGER values of all supported kinds and for REAL on IEEE systems of kinds 4 and 8.  Conversion between different ``extended double'' types on different architectures such as m68k and x86_64, which GNU Fortran supports as REAL(KIND=10) and REAL(KIND=16), will probably not work.

Note that the values specified via the GFORTRAN_CONVERT_UNIT environment variable will override the CONVERT specifier in the open statement.  This is to give control over data formats to users who do not have the source code of their program available.

Using anything but the native representation for unformatted data carries a significant speed overhead.  If speed in this area matters to you, it is best if you use this only for data that needs to be portable.

OpenMP
------

OpenMP (Open Multi-Processing) is an application programming interface (API) that supports multi-platform shared memory multiprocessing programming in C/C++ and Fortran on many architectures, including Unix and Microsoft Windows platforms. It consists of a set of compiler directives, library routines, and environment variables that influence run-time behavior.

GNU Fortran strives to be compatible to the OpenMP Application Program Interface v3.0 (http://www.openmp.org/mp-documents/spec30.pdf).

To enable the processing of the OpenMP directive !$omp in free-form source code; the c$omp, *$omp and !$omp directives in fixed form; the !$ conditional compilation sentinels in free form; and the c$, *$ and !$ sentinels in fixed form, gfortran needs to be invoked with the -fopenmp.  This also arranges for automatic linking of the GNU OpenMP runtime library libgomp (GNU OpenMP runtime library).

The OpenMP Fortran runtime library routines are provided both in a form of a Fortran 90 module named omp_lib and in a form of a Fortran include file named omp_lib.h.

An example of a parallelized loop taken from Appendix A.1 of the OpenMP Application Program Interface v2.5:

     SUBROUTINE A1(N, A, B)
       INTEGER I, N
       REAL B(N), A(N)
     !$OMP PARALLEL DO !I is private by default
       DO I=2,N
         B(I) = (A(I) + A(I-1)) / 2.0
       ENDDO
     !$OMP END PARALLEL DO
     END SUBROUTINE A1

Please note:

  * -fopenmp implies -frecursive, i.e., all local arrays will be allocated on the stack.  When porting existing code to OpenMP, this may lead to surprising results, especially to segmentation faults if the stacksize is limited.

  * On glibc-based systems, OpenMP enabled applications cannot be statically linked due to limitations of the underlying pthreads-implementation.  It might be possible to get a working solution if -Wl,--whole-archive -lpthread -Wl,--no-whole-archive is added to the command line.  However, this is not supported by gcc and thus not recommended.

Argument list functions %VAL, %REF and %LOC
-------------------------------------------

GNU Fortran supports argument list functions %VAL, %REF and %LOC statements, for backward compatibility with g77. It is recommended that these should be used only for code that is accessing facilities outside of GNU Fortran, such as operating system or windowing facilities.  It is best to constrain such uses to isolated portions of a program--portions that deal specifically and exclusively with low-level, system-dependent facilities.  Such portions might well provide a portable interface for use by the program as a whole, but are themselves not portable, and should be thoroughly tested each time they are rebuilt using a new compiler or version of a compiler.

%VAL passes a scalar argument by value, %REF passes it by reference and %LOC passes its memory location.  Since gfortran already passes scalar arguments by reference, %REF is in effect a do-nothing.  %LOC has the same effect as a Fortran pointer.

An example of passing an argument by value to a C subroutine foo.:

     C
     C prototype      void foo_ (float x);
     C
           external foo
           real*4 x
           x = 3.14159
           call foo (%VAL (x))
           end

For details refer to the g77 manual http://gcc.gnu.org/onlinedocs/gcc-3.4.6/g77/index.html#Top.

Also, c_by_val.f and its partner c_by_val.c of the GNU Fortran testsuite are worth a look.

Extensions not implemented in GNU Fortran
=========================================

The long history of the Fortran language, its wide use and broad userbase, the large number of different compiler vendors and the lack of some features crucial to users in the first standards have lead to the existence of a number of important extensions to the language.  While some of the most useful or popular extensions are supported by the GNU Fortran compiler, not all existing extensions are supported.  This section aims at listing these extensions and offering advice on how best make code that uses them running with the GNU Fortran compiler.

STRUCTURE and RECORD
--------------------

Structures are user-defined aggregate data types; this functionality was standardized in Fortran 90 with an different syntax, under the name of ``derived types''.  Here is an example of code using the non portable structure syntax:

     ! Declaring a structure named ``item'' and containing three fields:
     ! an integer ID, an description string and a floating-point price.
     STRUCTURE /item/
       INTEGER id
       CHARACTER(LEN=200) description
       REAL price
     END STRUCTURE

     ! Define two variables, an single record of type ``item''
     ! named ``pear'', and an array of items named ``store_catalog''
     RECORD /item/ pear, store_catalog(100)

     ! We can directly access the fields of both variables
     pear.id = 92316
     pear.description = "juicy D'Anjou pear"
     pear.price = 0.15
     store_catalog(7).id = 7831
     store_catalog(7).description = "milk bottle"
     store_catalog(7).price = 1.2

     ! We can also manipulate the whole structure
     store_catalog(12) = pear
     print *, store_catalog(12)

This code can easily be rewritten in the Fortran 90 syntax as following:

     ! ``STRUCTURE /name/ ... END STRUCTURE'' becomes
     ! ``TYPE name ... END TYPE''
     TYPE item
       INTEGER id
       CHARACTER(LEN=200) description
       REAL price
     END TYPE

     ! ``RECORD /name/ variable'' becomes ``TYPE(name) variable''
     TYPE(item) pear, store_catalog(100)

     ! Instead of using a dot (.) to access fields of a record, the
     ! standard syntax uses a percent sign (%)
     pear%id = 92316
     pear%description = "juicy D'Anjou pear"
     pear%price = 0.15
     store_catalog(7)%id = 7831
     store_catalog(7)%description = "milk bottle"
     store_catalog(7)%price = 1.2

     ! Assignments of a whole variable don't change
     store_catalog(12) = pear
     print *, store_catalog(12)

ENCODE and DECODE statements
----------------------------

GNU Fortran doesn't support the ENCODE and DECODE statements.  These statements are best replaced by READ and WRITE statements involving internal files (CHARACTER variables and arrays), which have been part of the Fortran standard since Fortran 77.  For example, replace a code fragment like

           INTEGER*1 LINE(80)
           REAL A, B, C
     c     ... Code that sets LINE
           DECODE (80, 9000, LINE) A, B, C
      9000 FORMAT (1X, 3(F10.5))

with the following:

           CHARACTER(LEN=80) LINE
           REAL A, B, C
     c     ... Code that sets LINE
           READ (UNIT=LINE, FMT=9000) A, B, C
      9000 FORMAT (1X, 3(F10.5))

Similarly, replace a code fragment like

           INTEGER*1 LINE(80)
           REAL A, B, C
     c     ... Code that sets A, B and C
           ENCODE (80, 9000, LINE) A, B, C
      9000 FORMAT (1X, 'OUTPUT IS ', 3(F10.5))

with the following:

           CHARACTER(LEN=80) LINE
           REAL A, B, C
     c     ... Code that sets A, B and C
           WRITE (UNIT=LINE, FMT=9000) A, B, C
      9000 FORMAT (1X, 'OUTPUT IS ', 3(F10.5))

Variable FORMAT expressions
---------------------------

A variable FORMAT expression is format statement which includes angle brackets enclosing a Fortran expression: FORMAT(I<N>).  GNU Fortran does not support this legacy extension.  The effect of variable format expressions can be reproduced by using the more powerful (and standard) combination of internal output and string formats.  For example, replace a code fragment like this:

           WRITE(6,20) INT1
      20   FORMAT(I<N+1>)

with the following:

     c     Variable declaration
           CHARACTER(LEN=20) FMT
     c
     c     Other code here...
     c
           WRITE(FMT,'("(I", I0, ")")') N+1
           WRITE(6,FMT) INT1

or with:

     c     Variable declaration
           CHARACTER(LEN=20) FMT
     c
     c     Other code here...
     c
           WRITE(FMT,*) N+1
           WRITE(6,"(I" // ADJUSTL(FMT) // ")") INT1

Alternate complex function syntax
---------------------------------

Some Fortran compilers, including g77, let the user declare complex functions with the syntax COMPLEX FUNCTION name*16(), as well as COMPLEX*16 FUNCTION name().  Both are non-standard, legacy extensions.  gfortran accepts the latter form, which is more common, but not the former.

Mixed-Language Programming
**************************

This chapter is about mixed-language interoperability, but also applies if one links Fortran code compiled by different compilers.  In most cases, use of the C Binding features of the Fortran 2003 standard is sufficient, and their use is highly recommended.

Interoperability with C
=======================

Since Fortran 2003 (ISO/IEC 1539-1:2004(E)) there is a standardized way to generate procedure and derived-type declarations and global variables which are interoperable with C (ISO/IEC 9899:1999).  The bind(C) attribute has been added to inform the compiler that a symbol shall be interoperable with C; also, some constraints are added.  Note, however, that not all C features have a Fortran equivalent or vice versa.  For instance, neither C's unsigned integers nor C's functions with variable number of arguments have an equivalent in Fortran.

Note that array dimensions are reversely ordered in C and that arrays in C always start with index 0 while in Fortran they start by default with 1.  Thus, an array declaration A(n,m) in Fortran matches A[m][n] in C and accessing the element A(i,j) matches A[j-1][i-1].  The element following A(i,j) (C: A[j-1][i-1]; assuming i < n) in memory is A(i+1,j) (C: A[j-1][i]).

Intrinsic Types
---------------

In order to ensure that exactly the same variable type and kind is used in C and Fortran, the named constants shall be used which are defined in the ISO_C_BINDING intrinsic module.  That module contains named constants for kind parameters and character named constants for the escape sequences in C.  For a list of the constants, see ISO_C_BINDING.

Derived Types and struct
------------------------

For compatibility of derived types with struct, one needs to use the BIND(C) attribute in the type declaration.  For instance, the following type declaration

      USE ISO_C_BINDING
      TYPE, BIND(C) :: myType
        INTEGER(C_INT) :: i1, i2
        INTEGER(C_SIGNED_CHAR) :: i3
        REAL(C_DOUBLE) :: d1
        COMPLEX(C_FLOAT_COMPLEX) :: c1
        CHARACTER(KIND=C_CHAR) :: str(5)
      END TYPE

matches the following struct declaration in C

      struct {
        int i1, i2;
        /* Note: "char" might be signed or unsigned.  */
        signed char i3;
        double d1;
        float _Complex c1;
        char str[5];
      } myType;

Derived types with the C binding attribute shall not have the sequence attribute, type parameters, the extends attribute, nor type-bound procedures.  Every component must be of interoperable type and kind and may not have the pointer or allocatable attribute.  The names of the variables are irrelevant for interoperability.

As there exist no direct Fortran equivalents, neither unions nor structs with bit field or variable-length array members are interoperable.

Interoperable Global Variables
------------------------------

Variables can be made accessible from C using the C binding attribute, optionally together with specifying a binding name.  Those variables have to be declared in the declaration part of a MODULE, be of interoperable type, and have neither the pointer nor the allocatable attribute.

       MODULE m
         USE myType_module
         USE ISO_C_BINDING
         integer(C_INT), bind(C, name="_MyProject_flags") :: global_flag
         type(myType), bind(C) :: tp
       END MODULE

Here, _MyProject_flags is the case-sensitive name of the variable as seen from C programs while global_flag is the case-insensitive name as seen from Fortran.  If no binding name is specified, as for TP, the C binding name is the (lowercase) Fortran binding name. If a binding name is specified, only a single variable may be after the double colon.  Note of warning: You cannot use a global variable to access ERRNO of the C library as the C standard allows it to be a macro.  Use the IERRNO intrinsic (GNU extension) instead.

Interoperable Subroutines and Functions
---------------------------------------

Subroutines and functions have to have the BIND(C) attribute to be compatible with C.  The dummy argument declaration is relatively straightforward.  However, one needs to be careful because C uses call-by-value by default while Fortran behaves usually similar to call-by-reference.  Furthermore, strings and pointers are handled differently.  Note that only explicit size and assumed-size arrays are supported but not assumed-shape or allocatable arrays.

To pass a variable by value, use the VALUE attribute. Thus the following C prototype

     int func(int i, int *j)

matches the Fortran declaration

       integer(c_int) function func(i,j)
         use iso_c_binding, only: c_int
         integer(c_int), VALUE :: i
         integer(c_int) :: j

Note that pointer arguments also frequently need the VALUE attribute, see Working with Pointers.

Strings are handled quite differently in C and Fortran.  In C a string is a NUL-terminated array of characters while in Fortran each string has a length associated with it and is thus not terminated (by e.g. NUL).  For example, if one wants to use the following C function,

       #include <stdio.h>
       void print_C(char *string) /* equivalent: char string[]  */
       {
          printf("%s\n", string);
       }

to print ``Hello World'' from Fortran, one can call it using

       use iso_c_binding, only: C_CHAR, C_NULL_CHAR
       interface
         subroutine print_c(string) bind(C, name="print_C")
           use iso_c_binding, only: c_char
           character(kind=c_char) :: string(*)
         end subroutine print_c
       end interface
       call print_c(C_CHAR_"Hello World"//C_NULL_CHAR)

As the example shows, one needs to ensure that the string is NUL terminated.  Additionally, the dummy argument STRING of print_C is a length-one assumed-size array; using character(len=*) is not allowed.  The example above uses c_char_"Hello World" to ensure the string literal has the right type; typically the default character kind and c_char are the same and thus "Hello World" is equivalent.  However, the standard does not guarantee this.

The use of strings is now further illustrated using the C library function strncpy, whose prototype is

       char *strncpy(char *restrict s1, const char *restrict s2, size_t n);

The function strncpy copies at most N characters from string S2 to S1 and returns S1.  In the following example, we ignore the return value:

       use iso_c_binding
       implicit none
       character(len=30) :: str,str2
       interface
         ! Ignore the return value of strncpy -> subroutine
         ! "restrict" is always assumed if we do not pass a pointer
         subroutine strncpy(dest, src, n) bind(C)
           import
           character(kind=c_char),  intent(out) :: dest(*)
           character(kind=c_char),  intent(in)  :: src(*)
           integer(c_size_t), value, intent(in) :: n
         end subroutine strncpy
       end interface
       str = repeat('X',30) ! Initialize whole string with 'X'
       call strncpy(str, c_char_"Hello World"//C_NULL_CHAR, &
                    len(c_char_"Hello World",kind=c_size_t))
       print '(a)', str ! prints: "Hello WorldXXXXXXXXXXXXXXXXXXX"
       end

The intrinsic procedures are described in Intrinsic Procedures.

Working with Pointers
---------------------

C pointers are represented in Fortran via the special opaque derived type type(c_ptr) (with private components).  Thus one needs to use intrinsic conversion procedures to convert from or to C pointers. For example,

       use iso_c_binding
       type(c_ptr) :: cptr1, cptr2
       integer, target :: array(7), scalar
       integer, pointer :: pa(:), ps
       cptr1 = c_loc(array(1)) ! The programmer needs to ensure that the
                               ! array is contiguous if required by the C
                               ! procedure
       cptr2 = c_loc(scalar)
       call c_f_pointer(cptr2, ps)
       call c_f_pointer(cptr2, pa, shape=[7])

When converting C to Fortran arrays, the one-dimensional SHAPE argument has to be passed.

If a pointer is a dummy-argument of an interoperable procedure, it usually has to be declared using the VALUE attribute.  void* matches TYPE(C_PTR), VALUE, while TYPE(C_PTR) alone matches void**.

Procedure pointers are handled analogously to pointers; the C type is TYPE(C_FUNPTR) and the intrinsic conversion procedures are C_F_PROCPOINTER and C_FUNLOC.

Let's consider two examples of actually passing a procedure pointer from C to Fortran and vice versa.  Note that these examples are also very similar to passing ordinary pointers between both languages. First, consider this code in C:

     /* Procedure implemented in Fortran.  */
     void get_values (void (*)(double));

     /* Call-back routine we want called from Fortran.  */
     void
     print_it (double x)
     {
       printf ("Number is %f.\n", x);
     }

     /* Call Fortran routine and pass call-back to it.  */
     void
     foobar ()
     {
       get_values (&print_it);
     }

A matching implementation for get_values in Fortran, that correctly receives the procedure pointer from C and is able to call it, is given in the following MODULE:

     MODULE m
       IMPLICIT NONE

       ! Define interface of call-back routine.
       ABSTRACT INTERFACE
         SUBROUTINE callback (x)
           USE, INTRINSIC :: ISO_C_BINDING
           REAL(KIND=C_DOUBLE), INTENT(IN), VALUE :: x
         END SUBROUTINE callback
       END INTERFACE

     CONTAINS

       ! Define C-bound procedure.
       SUBROUTINE get_values (cproc) BIND(C)
         USE, INTRINSIC :: ISO_C_BINDING
         TYPE(C_FUNPTR), INTENT(IN), VALUE :: cproc

         PROCEDURE(callback), POINTER :: proc

         ! Convert C to Fortran procedure pointer.
         CALL C_F_PROCPOINTER (cproc, proc)

         ! Call it.
         CALL proc (1.0_C_DOUBLE)
         CALL proc (-42.0_C_DOUBLE)
         CALL proc (18.12_C_DOUBLE)
       END SUBROUTINE get_values

     END MODULE m

Next, we want to call a C routine that expects a procedure pointer argument and pass it a Fortran procedure (which clearly must be interoperable!). Again, the C function may be:

     int
     call_it (int (*func)(int), int arg)
     {
       return func (arg);
     }

It can be used as in the following Fortran code:

     MODULE m
       USE, INTRINSIC :: ISO_C_BINDING
       IMPLICIT NONE

       ! Define interface of C function.
       INTERFACE
         INTEGER(KIND=C_INT) FUNCTION call_it (func, arg) BIND(C)
           USE, INTRINSIC :: ISO_C_BINDING
           TYPE(C_FUNPTR), INTENT(IN), VALUE :: func
           INTEGER(KIND=C_INT), INTENT(IN), VALUE :: arg
         END FUNCTION call_it
       END INTERFACE

     CONTAINS

       ! Define procedure passed to C function.
       ! It must be interoperable!
       INTEGER(KIND=C_INT) FUNCTION double_it (arg) BIND(C)
         INTEGER(KIND=C_INT), INTENT(IN), VALUE :: arg
         double_it = arg + arg
       END FUNCTION double_it

       ! Call C function.
       SUBROUTINE foobar ()
         TYPE(C_FUNPTR) :: cproc
         INTEGER(KIND=C_INT) :: i

         ! Get C procedure pointer.
         cproc = C_FUNLOC (double_it)

         ! Use it.
         DO i = 1_C_INT, 10_C_INT
           PRINT *, call_it (cproc, i)
         END DO
       END SUBROUTINE foobar

     END MODULE m

Further Interoperability of Fortran with C
------------------------------------------

Assumed-shape and allocatable arrays are passed using an array descriptor (dope vector).  The internal structure of the array descriptor used by GNU Fortran is not yet documented and will change.  There will also be a Technical Report (TR 29113) which standardizes an interoperable array descriptor.  Until then, you can use the Chasm Language Interoperability Tools, http://chasm-interop.sourceforge.net/, which provide an interface to GNU Fortran's array descriptor.

The technical report 29113 will presumably also include support for C-interoperable OPTIONAL and for assumed-rank and assumed-type dummy arguments.  However, the TR has neither been approved nor implemented in GNU Fortran; therefore, these features are not yet available.

GNU Fortran Compiler Directives
===============================

The Fortran standard standard describes how a conforming program shall behave; however, the exact implementation is not standardized.  In order to allow the user to choose specific implementation details, compiler directives can be used to set attributes of variables and procedures which are not part of the standard.  Whether a given attribute is supported and its exact effects depend on both the operating system and on the processor; see C Extensions (Using the GNU Compiler Collection (GCC)) for details.

For procedures and procedure pointers, the following attributes can be used to change the calling convention:

  * CDECL -- standard C calling convention

  * STDCALL -- convention where the called procedure pops the stack

  * FASTCALL -- part of the arguments are passed via registers instead using the stack

Besides changing the calling convention, the attributes also influence the decoration of the symbol name, e.g., by a leading underscore or by a trailing at-sign followed by the number of bytes on the stack.  When assigning a procedure to a procedure pointer, both should use the same calling convention.

On some systems, procedures and global variables (module variables and COMMON blocks) need special handling to be accessible when they are in a shared library.  The following attributes are available:

  * DLLEXPORT -- provide a global pointer to a pointer in the DLL

  * DLLIMPORT -- reference the function or variable using a global pointer

The attributes are specified using the syntax

!GCC$ ATTRIBUTES ATTRIBUTE-LIST :: VARIABLE-LIST

where in free-form source code only whitespace is allowed before !GCC$ and in fixed-form source code !GCC$, cGCC$ or *GCC$ shall start in the first column.

For procedures, the compiler directives shall be placed into the body of the procedure; for variables and procedure pointers, they shall be in the same declaration part as the variable or procedure pointer.

Non-Fortran Main Program
========================

Even if you are doing mixed-language programming, it is very likely that you do not need to know or use the information in this section.  Since it is about the internal structure of GNU Fortran, it may also change in GCC minor releases.

When you compile a PROGRAM with GNU Fortran, a function with the name main (in the symbol table of the object file) is generated, which initializes the libgfortran library and then calls the actual program which uses the name MAIN__, for historic reasons.  If you link GNU Fortran compiled procedures to, e.g., a C or C++ program or to a Fortran program compiled by a different compiler, the libgfortran library is not initialized and thus a few intrinsic procedures do not work properly, e.g. those for obtaining the command-line arguments.

Therefore, if your PROGRAM is not compiled with GNU Fortran and the GNU Fortran compiled procedures require intrinsics relying on the library initialization, you need to initialize the library yourself.  Using the default options, gfortran calls _gfortran_set_args and _gfortran_set_options.  The initialization of the former is needed if the called procedures access the command line (and for backtracing); the latter sets some flags based on the standard chosen or to enable backtracing.  In typical programs, it is not necessary to call any initialization function.

If your PROGRAM is compiled with GNU Fortran, you shall not call any of the following functions.  The libgfortran initialization functions are shown in C syntax but using C bindings they are also accessible from Fortran.

_gfortran_set_args --- Save command-line arguments
--------------------------------------------------

Description:
     _gfortran_set_args saves the command-line arguments; this initialization is required if any of the command-line intrinsics is called.  Additionally, it shall be called if backtracing is enabled (see _gfortran_set_options).

Syntax:
     void _gfortran_set_args (int argc, char *argv[])

Arguments:

     ARGC   number of command line argument strings
     ARGV   the command-line argument strings; argv[0]
     is the pathname of the executable itself.

Example:

          int main (int argc, char *argv[])
          {
            /* Initialize libgfortran.  */
            _gfortran_set_args (argc, argv);
            return 0;
          }

_gfortran_set_options --- Set library option flags
--------------------------------------------------

Description:
     _gfortran_set_options sets several flags related to the Fortran standard to be used, whether backtracing or core dumps should be enabled and whether range checks should be performed.  The syntax allows for upward compatibility since the number of passed flags is specified; for non-passed flags, the default value is used.  See also Code Gen Options.  Please note that not all flags are actually used.

Syntax:
     void _gfortran_set_options (int num, int options[])

Arguments:

     NUM   number of options passed
     ARGV   The list of flag values

option flag list:

     OPTION[0]   Allowed standard; can give run-time errors
     if e.g. an input-output edit descriptor is invalid in a given standard.
     Possible values are (bitwise or-ed) GFC_STD_F77 (1),
     GFC_STD_F95_OBS (2), GFC_STD_F95_DEL (4), GFC_STD_F95
     (8), GFC_STD_F2003 (16), GFC_STD_GNU (32),
     GFC_STD_LEGACY (64), GFC_STD_F2008 (128), and
     GFC_STD_F2008_OBS (256).  Default: GFC_STD_F95_OBS
     | GFC_STD_F95_DEL | GFC_STD_F95 | GFC_STD_F2003 | GFC_STD_F2008
     | GFC_STD_F2008_OBS | GFC_STD_F77 | GFC_STD_GNU | GFC_STD_LEGACY.
     OPTION[1]   Standard-warning flag; prints a warning to
     standard error.  Default: GFC_STD_F95_DEL | GFC_STD_LEGACY.
     OPTION[2]   If non zero, enable pedantic checking.
     Default: off.
     OPTION[3]   If non zero, enable core dumps on run-time
     errors.  Default: off.
     OPTION[4]   If non zero, enable backtracing on run-time
     errors.  Default: off.
     Note: Installs a signal handler and requires command-line
     initialization using _gfortran_set_args.
     OPTION[5]   If non zero, supports signed zeros.
     Default: enabled.
     OPTION[6]   Enables run-time checking.  Possible values
     are (bitwise or-ed): GFC_RTCHECK_BOUNDS (1), GFC_RTCHECK_ARRAY_TEMPS (2),
     GFC_RTCHECK_RECURSION (4), GFC_RTCHECK_DO (16), GFC_RTCHECK_POINTER (32).
     Default: disabled.
     OPTION[7]   If non zero, range checking is enabled.
     Default: enabled.  See -frange-check (see Code Gen Options).

Example:

            /* Use gfortran 4.5 default options.  */
            static int options[] = {68, 255, 0, 0, 0, 1, 0, 1};
            _gfortran_set_options (8, &options);

_gfortran_set_convert --- Set endian conversion
-----------------------------------------------

Description:
     _gfortran_set_convert set the representation of data for unformatted files.

Syntax:
     void _gfortran_set_convert (int conv)

Arguments:

     CONV   Endian conversion, possible values:
     GFC_CONVERT_NATIVE (0, default), GFC_CONVERT_SWAP (1),
     GFC_CONVERT_BIG (2), GFC_CONVERT_LITTLE (3).

Example:

          int main (int argc, char *argv[])
          {
            /* Initialize libgfortran.  */
            _gfortran_set_args (argc, argv);
            _gfortran_set_convert (1);
            return 0;
          }

_gfortran_set_record_marker --- Set length of record markers
------------------------------------------------------------

Description:
     _gfortran_set_record_marker sets the length of record markers for unformatted files.

Syntax:
     void _gfortran_set_record_marker (int val)

Arguments:

     VAL   Length of the record marker; valid values
     are 4 and 8.  Default is 4.

Example:

          int main (int argc, char *argv[])
          {
            /* Initialize libgfortran.  */
            _gfortran_set_args (argc, argv);
            _gfortran_set_record_marker (8);
            return 0;
          }

_gfortran_set_fpe --- Set when a Floating Point Exception should be raised
--------------------------------------------------------------------------

Description:
     _gfortran_set_fpe sets the IEEE exceptions for which a Floating Point Exception (FPE) should be raised.  On most systems, this will result in a SIGFPE signal being sent and the program being interrupted.

Syntax:
     void _gfortran_set_fpe (int val)

Arguments:

     OPTION[0]   IEEE exceptions.  Possible values are
     (bitwise or-ed) zero (0, default) no trapping,
     GFC_FPE_INVALID (1), GFC_FPE_DENORMAL (2),
     GFC_FPE_ZERO (4), GFC_FPE_OVERFLOW (8),
     GFC_FPE_UNDERFLOW (16), and GFC_FPE_PRECISION (32).

Example:

          int main (int argc, char *argv[])
          {
            /* Initialize libgfortran.  */
            _gfortran_set_args (argc, argv);
            /* FPE for invalid operations such as SQRT(-1.0).  */
            _gfortran_set_fpe (1);
            return 0;
          }

_gfortran_set_max_subrecord_length --- Set subrecord length
-----------------------------------------------------------

Description:
     _gfortran_set_max_subrecord_length set the maximum length for a subrecord.  This option only makes sense for testing and debugging of unformatted I/O.

Syntax:
     void _gfortran_set_max_subrecord_length (int val)

Arguments:

     VAL   the maximum length for a subrecord;
     the maximum permitted value is 2147483639, which is also
     the default.

Example:

          int main (int argc, char *argv[])
          {
            /* Initialize libgfortran.  */
            _gfortran_set_args (argc, argv);
            _gfortran_set_max_subrecord_length (8);
            return 0;
          }

     [file intrinsic.texi not included]

Contributing
************

Free software is only possible if people contribute to efforts to create it. We're always in need of more people helping out with ideas and comments, writing documentation and contributing code.

If you want to contribute to GNU Fortran, have a look at the long lists of projects you can take on. Some of these projects are small, some of them are large; some are completely orthogonal to the rest of what is happening on GNU Fortran, but others are ``mainstream'' projects in need of enthusiastic hackers. All of these projects are important! We'll eventually get around to the things here, but they are also things doable by someone who is willing and able.

Contributors to GNU Fortran
===========================

Most of the parser was hand-crafted by Andy Vaught, who is also the initiator of the whole project.  Thanks Andy! Most of the interface with GCC was written by Paul Brook.

The following individuals have contributed code and/or ideas and significant help to the GNU Fortran project (in alphabetical order):

  - Janne Blomqvist

  - Steven Bosscher

  - Paul Brook

  - Tobias Burnus

  - François-Xavier Coudert

  - Bud Davis

  - Jerry DeLisle

  - Erik Edelmann

  - Bernhard Fischer

  - Daniel Franke

  - Richard Guenther

  - Richard Henderson

  - Katherine Holcomb

  - Jakub Jelinek

  - Niels Kristian Bech Jensen

  - Steven Johnson

  - Steven G. Kargl

  - Thomas Koenig

  - Asher Langton

  - H. J. Lu

  - Toon Moene

  - Brooks Moses

  - Andrew Pinski

  - Tim Prince

  - Christopher D. Rickett

  - Richard Sandiford

  - Tobias Schlüter

  - Roger Sayle

  - Paul Thomas

  - Andy Vaught

  - Feng Wang

  - Janus Weil

  - Daniel Kraft

The following people have contributed bug reports, smaller or larger patches, and much needed feedback and encouragement for the GNU Fortran project:

  - Bill Clodius

  - Dominique d'Humières

  - Kate Hedstrom

  - Erik Schnetter

  - Joost VandeVondele

Many other individuals have helped debug, test and improve the GNU Fortran compiler over the past few years, and we welcome you to do the same! If you already have done so, and you would like to see your name listed in the list above, please contact us.

Projects
========

Help build the test suite
     Solicit more code for donation to the test suite: the more extensive the testsuite, the smaller the risk of breaking things in the future! We can keep code private on request.

Bug hunting/squishing
     Find bugs and write more test cases! Test cases are especially very welcome, because it allows us to concentrate on fixing bugs instead of isolating them.  Going through the bugzilla database at http://gcc.gnu.org/bugzilla/ to reduce testcases posted there and add more information (for example, for which version does the testcase work, for which versions does it fail?) is also very helpful.

Proposed Extensions
===================

Here's a list of proposed extensions for the GNU Fortran compiler, in no particular order.  Most of these are necessary to be fully compatible with existing Fortran compilers, but they are not part of the official J3 Fortran 95 standard.

Compiler extensions:
--------------------

  * User-specified alignment rules for structures.

  * Automatically extend single precision constants to double.

  * Compile code that conserves memory by dynamically allocating common and module storage either on stack or heap.

  * Compile flag to generate code for array conformance checking (suggest -CC).

  * User control of symbol names (underscores, etc).

  * Compile setting for maximum size of stack frame size before spilling parts to static or heap.

  * Flag to force local variables into static space.

  * Flag to force local variables onto stack.

Environment Options
-------------------

  * Pluggable library modules for random numbers, linear algebra. LA should use BLAS calling conventions.

  * Environment variables controlling actions on arithmetic exceptions like overflow, underflow, precision loss---Generate NaN, abort, default. action.

  * Set precision for fp units that support it (i387).

  * Variable for setting fp rounding mode.

  * Variable to fill uninitialized variables with a user-defined bit pattern.

  * Environment variable controlling filename that is opened for that unit number.

  * Environment variable to clear/trash memory being freed.

  * Environment variable to control tracing of allocations and frees.

  * Environment variable to display allocated memory at normal program end.

  * Environment variable for filename for * IO-unit.

  * Environment variable for temporary file directory.

  * Environment variable forcing standard output to be line buffered (unix).

     [file gpl_v3.texi not included]

     [file fdl.texi not included]

     [file funding.texi not included]

Option Index
************

gfortran's command line options are indexed here without any initial - or --.  Where an option has both positive and negative forms (such as -foption and -fno-option), relevant entries in the manual are indexed under the most appropriate form; it may sometimes be useful to look up both forms.