2006-02-09 Paul Thomas <pault@gcc.gnu.org>

[pf3gnuchains/gcc-fork.git] / gcc / fortran / gfortran.texi

\input texinfo  @c -*-texinfo-*-
@c %**start of header
@setfilename gfortran.info
@set copyrights-gfortran 1999-2005

@include gcc-common.texi

@settitle The GNU Fortran 95 Compiler

@c Create a separate index for command line options
@defcodeindex op
@c Merge the standard indexes into a single one.
@syncodeindex fn cp
@syncodeindex vr cp
@syncodeindex ky cp
@syncodeindex pg cp
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@c %**end of header

@c Use with @@smallbook.

@c %** start of document

@c Cause even numbered pages to be printed on the left hand side of
@c the page and odd numbered pages to be printed on the right hand
@c side of the page.  Using this, you can print on both sides of a
@c sheet of paper and have the text on the same part of the sheet.

@c The text on right hand pages is pushed towards the right hand
@c margin and the text on left hand pages is pushed toward the left
@c hand margin.
@c (To provide the reverse effect, set bindingoffset to -0.75in.)

@c @tex
@c \global\bindingoffset=0.75in
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@c @end tex

@copying
Copyright @copyright{} @value{copyrights-gfortran} 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 1.1 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``GNU General Public License'' and ``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.
@end copying

@ifinfo
@dircategory Programming
@direntry
* gfortran: (gfortran).                  The GNU Fortran 95 Compiler.
@end direntry
This file documents the use and the internals of
the GNU Fortran 95 compiler, (@command{gfortran}).

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

@insertcopying
@end ifinfo


@setchapternewpage odd
@titlepage
@title Using GNU Fortran 95
@sp 2
@center The gfortran team
@page
@vskip 0pt plus 1filll
For the @value{version-GCC} Version*
@sp 1
Published by the Free Software Foundation @*
51 Franklin Street, Fifth Floor@*
Boston, MA 02110-1301, USA@*
@c Last printed ??ber, 19??.@*
@c Printed copies are available for $? each.@*
@c ISBN ???
@sp 1
@insertcopying
@end titlepage
@summarycontents
@contents
@page

@node Top
@top Introduction
@cindex Introduction

This manual documents the use of @command{gfortran}, 
the GNU Fortran 95 compiler. You can find in this manual how to invoke
@command{gfortran}, as well as its features and incompatibilities.

@ifset DEVELOPMENT
@emph{Warning:} This document, and the compiler it describes, are still
under development.  While efforts are made to keep it up-to-date, it might
not accurately reflect the status of the most recent @command{gfortran}.
@end ifset

@comment
@comment  When you add a new menu item, please keep the right hand
@comment  aligned to the same column.  Do not use tabs.  This provides
@comment  better formatting.
@comment
@menu
* Getting Started::      What you should know about @command{gfortran}.
* GFORTRAN and GCC::     You can compile Fortran, C, or other programs.
* GFORTRAN and G77::     Why we chose to start from scratch.
* Invoking GFORTRAN::    Command options supported by @command{gfortran}.
* Project Status::       Status of @command{gfortran}, roadmap, proposed extensions.
* Contributing::         How you can help.
* Standards::            Standards supported by @command{gfortran}
* Runtime::              Influencing runtime behavior with environment variables.
* Extensions::           Language extensions implemented by @command{gfortran}
* Intrinsic Procedures:: Intrinsic procedures supported by @command{gfortran}
* Copying::              GNU General Public License says
                         how you can copy and share GNU Fortran.
* GNU Free Documentation License::
                         How you can copy and share this manual.
* Funding::              How to help assure continued work for free software.
* Index::                Index of this documentation.
@end menu



@c ---------------------------------------------------------------------
@c Getting Started
@c ---------------------------------------------------------------------

@node Getting Started
@chapter Getting Started

Gfortran is the GNU Fortran 95 compiler front end,
designed initially as a free replacement for,
or alternative to, the unix @command{f95} command;
@command{gfortran} is the command you'll use to invoke the compiler.

Gfortran is still in an early state of development.
@command{gfortran} can generate code for most constructs and expressions,
but much work remains to be done.

When @command{gfortran} is finished,
it will do everything you expect from any decent compiler: 

@itemize @bullet
@item
Read a user's program,
stored in a file and containing instructions written
in Fortran 77, Fortran 90 or Fortran 95.
This file contains @dfn{source code}.

@item
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
@dfn{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 is easy to make tiny mistakes writing machine code.

@item
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.
When writing Fortran, it is easy to make big mistakes.
The Fortran 90 requires that the compiler can point out
mistakes to the user.
An incorrect usage of the language causes an @dfn{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 @dfn{warning message}.

@item
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.

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

@item
Locate and gather machine code already generated to
perform actions requested by statements in the user's program.
This machine code is organized into @dfn{modules} and is located
and @dfn{linked} to the user program. 
@end itemize

Gfortran consists of several components:

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

@item
The @command{gfortran} command itself,
which also might be installed as the
system's @command{f95} command.
@command{gfortran} is just another driver program,
but specifically for the Fortran 95 compiler only.
The difference with @command{gcc} is that @command{gfortran}
will automatically link the correct libraries to your program.

@item
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
@command{gfortran} compilation phase,
such as intrinsic functions and subroutines,
and routines for interaction with files and the operating system.
@c and mechanisms to spawn,
@c unleash and pause threads in parallelized code.

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



@c ---------------------------------------------------------------------
@c GFORTRAN and GCC
@c ---------------------------------------------------------------------

@node GFORTRAN and GCC
@chapter GFORTRAN and GCC
@cindex GNU Compiler Collection

GCC used to be the GNU ``C'' Compiler,
but is now known as the @dfn{GNU Compiler Collection}.
GCC provides the GNU system with a very versatile
compiler middle end (shared optimization passes),
and back ends (code generators) for many different
computer architectures and operating systems.
The code of the middle end and back end are shared by all
compiler front ends that are in the GNU Compiler Collection.

A GCC front end is essentially a source code parser
and an intermediate code generator.  The code generator translates the
semantics of the source code into a language independent form called
@dfn{GENERIC}.

The parser takes a source file written in a
particular computer language, reads and parses it,
and tries to make sure that the source code conforms to
the language rules.
Once the correctness of a program has been established,
the compiler will build a data structure known as the
@dfn{Abstract Syntax tree},
or just @dfn{AST} or ``tree'' for short.
This data structure represents the whole program
or a subroutine or a function.
The ``tree'' is passed to the GCC middle end,
which will perform optimization passes on it.  The optimized AST is then 
handed off too the back end which assembles the program unit.

Different phases in this translation process can be,
and in fact @emph{are} merged in many compiler front ends.
GNU Fortran 95 has a strict separation between the
parser and code generator.

The goal of the gfortran project is to build a new front end for GCC.
Specifically, a Fortran 95 front end.
In a non-gfortran installation,
@command{gcc} will not be able to compile Fortran 95 source code
(only the ``C'' front end has to be compiled if you want to build GCC,
all other languages are optional).
If you build GCC with gfortran, @command{gcc} will recognize
@file{.f/.f90/.f95} source files and accepts Fortran 95 specific
command line options.



@c ---------------------------------------------------------------------
@c GFORTRAN and G77
@c ---------------------------------------------------------------------

@node GFORTRAN and G77
@chapter GFORTRAN and G77
@cindex Fortran 77
@cindex G77

Why do we write a compiler front end from scratch? 
There's a fine Fortran 77 compiler in the
GNU Compiler Collection that accepts some features
of the Fortran 90 standard as extensions.
Why not start from there and revamp it?

One of the reasons is that Craig Burley, the author of G77,
has decided to stop working on the G77 front end.
On @uref{http://world.std.com/~burley/g77-why.html,
Craig explains the reasons for his decision to stop working on G77}
in one of the pages in his homepage.
Among the reasons is a lack of interest in improvements to
@command{g77}.
Users appear to be quite satisfied with @command{g77} as it is.
While @command{g77} is still being maintained (by Toon Moene),
it is unlikely that sufficient people will be willing
to completely rewrite the existing code. 

But there are other reasons to start from scratch.
Many people, including Craig Burley,
no longer agreed with certain design decisions in the G77 front end.
Also, the interface of @command{g77} to the back end is written in
a style which is confusing and not up to date on recommended practice.
In fact, a full rewrite had already been planned for GCC 3.0.

When Craig decided to stop,
it just seemed to be a better idea to start a new project from scratch,
because it was expected to be easier to maintain code we
develop ourselves than to do a major overhaul of @command{g77} first,
and then build a Fortran 95 compiler out of it.

@include invoke.texi

@c ---------------------------------------------------------------------
@c Project Status
@c ---------------------------------------------------------------------

@node Project Status
@chapter Project Status

@quotation
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
@end quotation

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).

Gfortran is currently reaching the stage where is is able to compile real
world programs.  However it is still under development and has many rough
edges.

@menu
* Compiler Status::
* Library Status::
* Proposed Extensions::
@end menu

@node Compiler Status
@section Compiler Status

@table @emph
@item Front end
This is the part of gfortran which parses a source file, verifies that it
is valid Fortran 95, performs compile time replacement of constants
(PARAMETER variables) and reads and generate module files. This is
almost complete. Every Fortran 95 source should be accepted, and most
none-Fortran 95 source should be rejected. If you find a source file where
this is not true, please tell us. You can use the -fsyntax-only switch to
make gfortran quit after running the front end, effectively reducing it to
a syntax checker.

@item Middle end interface
These are the parts of gfortran that take the parse tree generated by the
front end and translate it to the GENERIC form required by the GCC back
end. Work is ongoing in these parts of gfortran, but a large part has
already been completed.
@end table

@node Library Status
@section Library Status

Some intrinsic functions map directly to library functions, and in most
cases the name of the library function used depends on the type of the
arguments.  For some intrinsics we generate inline code, and for others,
such as sin, cos and sqrt, we rely on the backend to use special
instructions in the floating point unit of the CPU if available, or to
fall back to a call to libm if these are not available.

Implementation of some non-elemental intrinsic functions (eg. DOT_PRODUCT,
AVERAGE) is not yet optimal. This is hard because we have to make decisions
whether to use inline code (good for small arrays as no function call
overhead occurs) or generate function calls (good for large arrays as it
allows use of hand-optimized assembly routines, SIMD instructions, etc.)

The IO library is in a mostly usable state.  Unformatted I/O for
@code{REAL(KIND=10)} variables is currently not recommended.

Array intrinsics mostly work.

@node Proposed Extensions
@section Proposed Extensions

Here's a list of proposed extensions for @command{gfortran}, 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.

@subsection Compiler extensions: 
@itemize @bullet
@item
Flag for defining the kind number for default logicals.

@item
User-specified alignment rules for structures.
@item
Flag to generate @code{Makefile} info.

@item
Automatically extend single precision constants to double.

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

@item
Flag to cause the compiler to distinguish between upper and lower case
names.  The Fortran 95 standard does not distinguish them.

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

@item
User control of symbol names (underscores, etc).

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

@item
Flag to force local variables into static space.

@item
Flag to force local variables onto stack.

@item
Flag to compile lines beginning with ``D''.

@item
Flag to ignore lines beginning with ``D''.

@item
Flag for maximum errors before ending compile.

@item
Generate code to check for null pointer dereferences -- prints locus of
dereference instead of segfaulting.  There was some discussion about this
option in the g95 development mailing list.

@item
Allow setting the default unit number.

@item
Option to initialize otherwise uninitialized integer and floating
point variables.

@item
Support for OpenMP directives.  This also requires support from the runtime
library and the rest of the compiler.

@item
Support for Fortran 200x. This includes several new features including
floating point exceptions, extended use of allocatable arrays, C
interoperability, Parameterizer data types and function pointers.
@end itemize


@subsection Environment Options
@itemize @bullet
@item
Pluggable library modules for random numbers, linear algebra.
LA should use BLAS calling conventions.

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

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

@item
Variable for setting fp rounding mode.

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

@item
Environment variable controlling filename that is opened for that unit
number.

@item
Environment variable to clear/trash memory being freed.

@item
Environment variable to control tracing of allocations and frees.

@item
Environment variable to display allocated memory at normal program end.

@item
Environment variable for filename for * IO-unit.

@item
Environment variable for temporary file directory.

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

@end itemize

@node Runtime
@chapter Runtime:  Influencing runtime behavior with environment variables
@cindex Runtime

The behaviour of the @command{gfortran} can be influenced by
environment variables.
@menu
* GFORTRAN_CONVERT_UNIT::  Set endianness for unformatted I/O
@end menu

@node GFORTRAN_CONVERT_UNIT
@section GFORTRAN_CONVERT_UNIT --- Set endianness for unformatted I/O

By setting the @code{GFORTRAN_CONVERT_UNIT variable}, it is possible
to change the representation of data for unformatted files.
The syntax for the @code{GFORTRAN_CONVERT_UNIT} variable is:
@smallexample
GFORTRAN_CONVERT_UNIT: mode | mode ';' 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 ;
@end smallexample
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 @code{CONVERT} specifier:

@itemize @w{}
@item @code{NATIVE} Use the native format.  This is the default.
@item @code{SWAP} Swap between little- and big-endian.
@item @code{LITTLE_ENDIAN} Use the little-endian format
        for unformatted files.
@item @code{BIG_ENDIAN} Use the big-endian format for unformatted files.
@end itemize
A missing mode for an exception is taken to mean @code{BIG_ENDIAN}.
Examples of values for @code{GFORTRAN_CONVERT_UNIT} are:
@itemize @w{}
@item @code{'big_endian'}  Do all unformatted I/O in big_endian mode.
@item @code{'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.
@item @code{'10-20'}  Units 10 to 20 are big-endian, the rest is native.
@end itemize

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

Example for @code{sh}:
@smallexample
$ gfortran foo.f90
$ GFORTRAN_CONVERT_UNIT='big_endian;native:10-20' ./a.out
@end smallexample

Example code for @code{csh}:
@smallexample
% gfortran foo.f90
% setenv GFORTRAN_CONVERT_UNIT 'big_endian;native:10-20'
% ./a.out
@end smallexample

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.

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

@c ---------------------------------------------------------------------
@c Extensions
@c ---------------------------------------------------------------------

@c Maybe this chapter should be merged with the 'Standards' section,
@c whenever that is written :-)

@node Extensions
@chapter Extensions
@cindex Extension

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

@menu
* Old-style kind specifications::
* Old-style variable initialization::
* Extensions to namelist::
* X format descriptor::
* Commas in FORMAT specifications::
* I/O item lists::
* Hexadecimal constants::
* Real array indices::
* Unary operators::
* Implicitly interconvert LOGICAL and INTEGER::
* Hollerith constants support::
* Cray pointers::
* CONVERT specifier::
@end menu

@node Old-style kind specifications
@section Old-style kind specifications
@cindex Kind specifications

@command{gfortran} allows old-style kind specifications in
declarations. These look like:
@smallexample
      TYPESPEC*k x,y,z
@end smallexample
where @code{TYPESPEC} is a basic type, and where @code{k} is a valid kind
number for that type. The statement then declares @code{x}, @code{y}
and @code{z} to be of type @code{TYPESPEC} with kind @code{k}. In
other words, it is equivalent to the standard conforming declaration
@smallexample
      TYPESPEC(k) x,y,z
@end smallexample

@node Old-style variable initialization
@section Old-style variable initialization
@cindex Initialization

@command{gfortran} allows old-style initialization of variables of the
form:
@smallexample
      INTEGER*4 i/1/,j/2/
      REAL*8 x(2,2) /3*0.,1./
@end smallexample
These are only allowed in declarations without double colons
(@code{::}), as these were introduced in Fortran 90 which also
introduced a new syntax for variable initializations. The syntax for
the individual initializers is as for the @code{DATA} statement, but
unlike in a @code{DATA} statement, an initializer only applies to the
variable immediately preceding. In other words, something like
@code{INTEGER I,J/2,3/} is not valid.

Examples of standard conforming code equivalent to the above example, are:
@smallexample
! Fortran 90
      INTEGER(4) :: i = 1, j = 2
      REAL(8) :: x(2,2) = RESHAPE((/0.,0.,0.,1./),SHAPE(x))
! Fortran 77
      INTEGER  i, j
      DOUBLE PRECISION x(2,2)
      DATA i,j,x /1,2,3*0.,1./
@end smallexample

@node Extensions to namelist
@section Extensions to namelist
@cindex Namelist

@command{gfortran} 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 &
@smallexample
$MYNML
 X(:)%Y(2) = 1.0 2.0 3.0
 CH(1:4) = "abcd"
$END
@end smallexample

It should be noticed 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:
@smallexample
?

&mynml
 x
 x%y
 ch
&end
@end smallexample

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

&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,  /
@end smallexample

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

@node X format descriptor
@section X format descriptor
@cindex X format descriptor

To support legacy codes, @command{gfortran} 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.

@smallexample
       PRINT 10, 2, 3
10     FORMAT (I1, X, I1)
@end smallexample

@node Commas in FORMAT specifications
@section Commas in FORMAT specifications
@cindex Commas in FORMAT specifications

To support legacy codes, @command{gfortran} allows the comma separator
to be omitted immediately before and after character string edit
descriptors in FORMAT statements.

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

@node I/O item lists
@section I/O item lists
@cindex I/O item lists

To support legacy codes, @command{gfortran} 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.

@node Hexadecimal constants
@section Hexadecimal constants
@cindex Hexadecimal constants

As a GNU extension, @command{gfortran} allows hexadecimal constants to
be specified using the X prefix, in addition to the standard Z prefix.

@node Real array indices
@section Real array indices
@cindex Real array indices

As a GNU extension, @command{gfortran} allows arrays to be indexed using
real types, whose values are implicitly converted to integers.

@node Unary operators
@section Unary operators
@cindex Unary operators

As a GNU extension, @command{gfortran} allows unary plus and unary
minus operators to appear as the second operand of binary arithmetic
operators without the need for parenthesis.

@smallexample
       X = Y * -Z
@end smallexample

@node Implicitly interconvert LOGICAL and INTEGER
@section Implicitly interconvert LOGICAL and INTEGER
@cindex Implicitly interconvert LOGICAL and INTEGER

As a GNU extension for backwards compatibility with other compilers,
@command{gfortran} allows the implicit conversion of LOGICALs to INTEGERs
and vice versa.  When converting from a LOGICAL to an INTEGER, the numeric
value of @code{.FALSE.} is zero, and that of @code{.TRUE.} is one.  When
converting from INTEGER to LOGICAL, the value zero is interpreted as
@code{.FALSE.} and any nonzero value is interpreted as @code{.TRUE.}.

@smallexample
       INTEGER*4 i
       i = .FALSE.
@end smallexample

@node Hollerith constants support
@section Hollerith constants support
@cindex Hollerith constants

A Hollerith constant is a string of characters preceded by the letter @samp{H}
or @samp{h}, and there must be an literal, unsigned, nonzero default integer
constant indicating the number of characters in the string. Hollerith constants
are stored as byte strings, one character per byte.

@command{gfortran} supports Hollerith constants. They can be used as the right
hands in the @code{DATA} statement and @code{ASSIGN} statement, also as the
arguments. The left hands can be of Integer, Real, Complex and Logical type.
The constant will be padded or truncated to fit the size of left hand.

Valid Hollerith constants examples:
@smallexample
complex*16 x(2)
data x /16Habcdefghijklmnop, 16Hqrstuvwxyz012345/
call foo (4H abc)
x(1) = 16Habcdefghijklmnop
@end smallexample

Invalid Hollerith constants examples:
@smallexample
integer*4 a
a = 8H12345678 ! The Hollerith constant is too long. It will be truncated.
a = 0H         ! At least one character needed.
@end smallexample

@node Cray pointers
@section Cray pointers
@cindex 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:
@smallexample
        pointer ( <pointer> , <pointee> )
@end smallexample
or,
@smallexample
        pointer ( <pointer1> , <pointee1> ), ( <pointer2> , <pointee2> ), ...
@end smallexample
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:
@smallexample
        integer ipt
        pointer (ipt, iarr)
@end smallexample
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:
@smallexample
        real target(10)
        real pointee(10)
        pointer (ipt, pointee)
        ipt = loc (target)
        ipt = ipt + 1       
@end smallexample
The last statement does not set ipt to the address of
@code{target(1)}, as one familiar with C pointer arithmetic might
expect.  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(), loc() is essentially the C '&' operator, except the
address is cast to an integer type:
@smallexample
        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
@end smallexample
The pointer can also be set by a call to a malloc-type
function.  There is no malloc intrinsic implemented as part of the
Cray pointer extension, but it might be a useful future addition to
@command{gfortran}.  Even without an intrinsic malloc function,
dynamic memory allocation can be combined with Cray pointers by
calling a short C function:
@smallexample
mymalloc.c:

        void mymalloc_(void **ptr, int *nbytes)
        @{
            *ptr = malloc(*nbytes);
            return;
        @}

caller.f:

        program caller
        integer ipinfo;
        real*4 data
        pointer (ipdata, data(1024))
        call mymalloc(ipdata,4*1024)
        end
@end smallexample
Cray pointees often are used to alias an existing variable.  For
example:
@smallexample
        integer target(10)
        integer iarr(10)
        pointer (ipt, iarr)
        ipt = loc(target)
@end smallexample
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're 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 gfortran; 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
attributes ALLOCATABLE, INTENT, OPTIONAL, DUMMY, TARGET, EXTERNAL,
INTRINSIC, or POINTER.  Pointers may not have the attributes
DIMENSION, POINTER, TARGET, ALLOCATABLE, EXTERNAL, or INTRINSIC.
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 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.

@node CONVERT specifier
@section CONVERT specifier
@cindex CONVERT specifier

gfortran 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 @code{CONVERT} specifier on the @code{OPEN} statement.
@xref{GFORTRAN_CONVERT_UNIT}, for an alternative way of specifying
the data format via an environment variable.

Valid values for @code{CONVERT} are:
@itemize @w{}
@item @code{CONVERT='NATIVE'} Use the native format.  This is the default.
@item @code{CONVERT='SWAP'} Swap between little- and big-endian.
@item @code{CONVERT='LITTLE_ENDIAN'} Use the little-endian representation
        for unformatted files.
@item @code{CONVERT='BIG_ENDIAN'} Use the big-endian representation for
        unformatted files.
@end itemize

Using the option could look like this:
@smallexample
  open(file='big.dat',form='unformatted',access='sequential', &
       convert='big_endian')
@end smallexample

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

@code{CONVERT} works between big- and little-endian for
@code{INTEGER} values of all supported kinds and for @code{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 gfortran
supports as @code{REAL(KIND=10)} will probably not work.

@emph{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
a user who does not have the source code of his 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.

@c ---------------------------------------------------------------------
@include intrinsic.texi
@c ---------------------------------------------------------------------

@c ---------------------------------------------------------------------
@c Contributing
@c ---------------------------------------------------------------------

@node Contributing
@chapter Contributing
@cindex 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 95,
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 @command{gfortran},
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.

@menu
* Contributors::
* Projects::
@end menu


@node Contributors
@section Contributors to GNU Fortran 95
@cindex Contributors
@cindex Credits
@cindex Authors

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

The following individuals have contributed code and/or
ideas and significant help to the gfortran project
(in no particular order): 

@itemize @minus
@item Andy Vaught
@item Katherine Holcomb
@item Tobias Schlüter
@item Steven Bosscher
@item Toon Moene
@item Tim Prince
@item Niels Kristian Bech Jensen
@item Steven Johnson
@item Paul Brook
@item Feng Wang
@item Bud Davis
@item Paul Thomas
@item François-Xavier Coudert
@item Steve Kargl
@item Jerry Delisle
@item Janne Blomqvist
@item Erik Edelmann
@item Thomas Koenig
@item Asher Langton
@end itemize

The following people have contributed bug reports,
smaller or larger patches,
and much needed feedback and encouragement for the
@command{gfortran} project: 

@itemize @minus
@item Erik Schnetter
@item Bill Clodius
@item Kate Hedstrom
@end itemize

Many other individuals have helped debug,
test and improve @command{gfortran} 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.


@node Projects
@section Projects

@table @emph

@item Help build the test suite
Solicit more code for donation to the test suite.
We can keep code private on request.

@item 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.

@item Smaller projects (``bug'' fixes):
  @itemize @minus
  @item Allow init exprs to be numbers raised to integer powers.
  @item Implement correct rounding.
  @item Implement F restrictions on Fortran 95 syntax.
  @item See about making Emacs-parsable error messages.
  @end itemize
@end table

If you wish to work on the runtime libraries,
please contact a project maintainer.
@c TODO: email!


@c ---------------------------------------------------------------------
@c Standards
@c ---------------------------------------------------------------------

@node Standards
@chapter Standards
@cindex Standards

The GNU Fortran 95 Compiler aims to be a conforming implementation of
ISO/IEC 1539:1997 (Fortran 95).

In the future it may also support other variants of and extensions to
the Fortran language.  These include ANSI Fortran 77, ISO Fortran 90,
ISO Fortran 2003 and OpenMP.

@menu
* Fortran 2003 status::
@end menu

@node Fortran 2003 status
@section Fortran 2003 status

Although @command{gfortran} focuses on implementing the Fortran 95
standard for the time being, a few Fortran 2003 features are currently
available.

@itemize
@item 
Intrinsics @code{command_argument_count}, @code{get_command},
@code{get_command_argument}, and @code{get_environment_variable}.

@item 
@cindex Array constructors
@cindex @code{[...]}
Array constructors using square brackets. That is, @code{[...]} rather
than @code{(/.../)}.

@item
@cindex @code{FLUSH} statement
@code{FLUSH} statement.

@item
@cindex @code{IOMSG=} specifier
@code{IOMSG=} specifier for I/O statements.

@item
@cindex @code{ENUM} statement
@cindex @code{ENUMERATOR} statement
@cindex @command{-fshort-enums}
Support for the declaration of enumeration constants via the
@code{ENUM} and @code{ENUMERATOR} statements.  Interoperability with
@command{gcc} is guaranteed also for the case where the
@command{-fshort-enums} command line option is given.

@end itemize


@c ---------------------------------------------------------------------
@c GNU General Public License
@c ---------------------------------------------------------------------

@include gpl.texi



@c ---------------------------------------------------------------------
@c GNU Free Documentation License
@c ---------------------------------------------------------------------

@include fdl.texi



@c ---------------------------------------------------------------------
@c Funding Free Software
@c ---------------------------------------------------------------------

@include funding.texi

@c ---------------------------------------------------------------------
@c Index
@c ---------------------------------------------------------------------

@node Index
@unnumbered Index

@printindex cp

@bye