gfortran.texi   [plain text]

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@settitle The GNU Fortran Compiler

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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

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

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

@end ifinfo

@setchapternewpage odd
@title Using GNU Fortran
@author The @t{gfortran} team
@vskip 0pt plus 1filll
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
@end titlepage

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@c ---------------------------------------------------------------------
@c TexInfo table of contents.
@c ---------------------------------------------------------------------

@node Top
@top Introduction
@cindex Introduction

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

@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 GNU Fortran compiler.
@end ifset

@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.
* Introduction::

Part I: Invoking GNU Fortran
* Invoking GNU Fortran:: Command options supported by @command{gfortran}.
* Runtime::              Influencing runtime behavior with environment variables.

Part II: Language Reference
* Fortran 2003 status::  Fortran 2003 features supported by GNU Fortran.
* Extensions::           Language extensions implemented by GNU Fortran.
* Intrinsic Procedures:: Intrinsic procedures supported by GNU Fortran.

* Contributing::         How you can help.
* 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.
* Option Index::         Index of command line options
* Keyword Index::        Index of concepts
@end menu
@end ifnottex

@c ---------------------------------------------------------------------
@c Introduction
@c ---------------------------------------------------------------------

@node Introduction
@chapter Introduction

@c The following duplicates the text on the TexInfo table of contents.
This manual documents the use of @command{gfortran}, the GNU Fortran
compiler. You can find in this manual how to invoke @command{gfortran},
as well as its features and incompatibilities.

@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 GNU Fortran
@end ifset
@end iftex

The GNU Fortran compiler front end was
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.

* About GNU Fortran::    What you should know about the GNU Fortran compiler.
* GNU Fortran and GCC::  You can compile Fortran, C, or other programs.
* GNU Fortran and G77::  Why we chose to start from scratch.
* Project Status::       Status of GNU Fortran, roadmap, proposed extensions.
* Standards::	         Standards supported by GNU Fortran.
@end menu

@c ---------------------------------------------------------------------
@c About GNU Fortran
@c ---------------------------------------------------------------------

@node About GNU Fortran
@section About GNU Fortran

The GNU Fortran compiler is still in an early state of development.
It can generate code for most constructs and expressions,
but much work remains to be done.

When the GNU Fortran compiler is finished,
it will do everything you expect from any decent compiler: 

@itemize @bullet
Read a user's program,
stored in a file and containing instructions written
in Fortran 77, Fortran 90, Fortran 95 or Fortran 2003.
This file contains @dfn{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
@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.

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}.

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 @dfn{debugger}, such as the GNU Debugger @command{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 @dfn{modules} and is located
and @dfn{linked} to the user program. 
@end itemize

The GNU Fortran compiler consists of several components:

@itemize @bullet
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.

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 compiler only.
The difference with @command{gcc} is that @command{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
@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.

The Fortran compiler itself, (@command{f951}).
This is the GNU Fortran 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 GNU Fortran and GCC
@c ---------------------------------------------------------------------

@node GNU Fortran and GCC
@section GNU Fortran and GCC
@cindex GNU Compiler Collection
@cindex GCC

GNU Fortran is a part of GCC, the @dfn{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
@dfn{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 (@command{gcc})
which provides the command-line interface for the compiler.  It calls
the relevant compiler front-end program (e.g., @command{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,
@command{gcc} will recognize files with @file{.f}, @file{.f90}, @file{.f95},
and @file{.f03} extensions as Fortran source code, and compile it
accordingly.  A @command{gfortran} driver program is also provided,
which is identical to @command{gcc} except that it automatically links
the Fortran runtime libraries into the compiled program.

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 
@ref{Top,,Introduction,gcc,Using the GNU Compiler Collection (GCC)}.
The two manuals together provide a complete reference for the GNU
Fortran compiler.

@c ---------------------------------------------------------------------
@c GNU Fortran and G77
@c ---------------------------------------------------------------------

@node GNU Fortran and G77
@section GNU Fortran and G77
@cindex Fortran 77
@cindex @command{g77}

The GNU Fortran compiler is the successor to @command{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 @command{g77}.

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

@node Project Status
@section Project Status

As soon as @command{gfortran} can parse all of the statements correctly,
it will be in the ``larva'' state.
When we generate code, the ``puppa'' state.
When @command{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).

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 features
such as enumeration, stream I/O, and some of the enhancements to
allocatable array support from 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}, and produces acceptable results on the
@uref{, LAPACK Test Suite}.
It also provides respectable performance on 
the @uref{, Polyhedron Fortran
compiler benchmarks} and the
Livermore Fortran Kernels test}.  It has been used to compile a number of
large real-world programs, including
@uref{, the HIRLAM
weather-forecasting code} and
@uref{, the Tonto quantum 
chemistry package}; see @url{} 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

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.

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

@node Standards
@section Standards
@cindex 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, and
the @uref{,
OpenMP Application Program Interface v2.5} specification.

In the future, the GNU Fortran compiler may also support other standard 
variants of and extensions to the Fortran language.  These include
ISO/IEC 1539-1:2004 (Fortran 2003).

@c =====================================================================
@c =====================================================================

\part{I}{Invoking GNU Fortran}
@end tex

@c ---------------------------------------------------------------------
@c Compiler Options
@c ---------------------------------------------------------------------

@include invoke.texi

@c ---------------------------------------------------------------------
@c Runtime
@c ---------------------------------------------------------------------

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

The behavior of the @command{gfortran} can be influenced by
environment variables.

Malformed environment variables are silently ignored.

* GFORTRAN_STDIN_UNIT:: Unit number for standard input
* GFORTRAN_STDOUT_UNIT:: Unit number for standard output
* GFORTRAN_STDERR_UNIT:: Unit number for standard error
* GFORTRAN_USE_STDERR:: Send library output to standard error
* GFORTRAN_TMPDIR:: Directory for scratch files
* GFORTRAN_UNBUFFERED_ALL:: Don't buffer output
* GFORTRAN_SHOW_LOCUS::  Show location for runtime errors
* GFORTRAN_OPTIONAL_PLUS:: Print leading + where permitted
* GFORTRAN_DEFAULT_RECL:: Default record length for new files
* GFORTRAN_LIST_SEPARATOR::  Separator for list output
* GFORTRAN_CONVERT_UNIT::  Set endianness for unformatted I/O
@end menu

@section @env{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.

@section @env{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.

@section @env{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.

@section @env{GFORTRAN_USE_STDERR}---Send library output to standard error

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

@section @env{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 @env{TMP}.  If
this is also missing, the default is @file{/tmp}.

@section @env{GFORTRAN_UNBUFFERED_ALL}---Don't buffer output

This environment variable controls whether all output is unbuffered.
If the first letter is @samp{y}, @samp{Y} or @samp{1}, all output is
unbuffered. This will slow down large writes.  If the first letter is
@samp{n}, @samp{N}  or @samp{0}, output is buffered.  This is the

@section @env{GFORTRAN_SHOW_LOCUS}---Show location for runtime errors

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

@section @env{GFORTRAN_OPTIONAL_PLUS}---Print leading + where permitted

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

@section @env{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 @code{RECL} tag in the
@code{OPEN} statement.  This must be a positive integer.  The
default value is 1073741824 bytes (1 GB).

@section @env{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
@end smallexample
when @command{a.out} is the compiled Fortran program that you want to run.
Default is a single space.

@section @env{GFORTRAN_CONVERT_UNIT}---Set endianness for unformatted I/O

By setting the @env{GFORTRAN_CONVERT_UNIT} variable, it is possible
to change the representation of data for unformatted files.
The syntax for the @env{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 ;
@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 @env{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 @command{export}
command for @command{sh}-compatible shells and via @command{setenv}
for @command{csh}-compatible shells.

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

Example code for @command{csh}:
% 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

@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 @option{-fconvert} compile options.

@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
users who do not have the source code of their program available.

@c =====================================================================
@c =====================================================================

\part{II}{Language Reference}
@end tex

@c ---------------------------------------------------------------------
@c Fortran 2003 Status
@c ---------------------------------------------------------------------

@node Fortran 2003 status
@chapter Fortran 2003 Status

Although GNU Fortran focuses on implementing the Fortran 95
standard for the time being, a few Fortran 2003 features are currently

Intrinsics @code{command_argument_count}, @code{get_command},
@code{get_command_argument}, @code{get_environment_variable}, and

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

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

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

@cindex @code{ENUM} statement
@cindex @code{ENUMERATOR} statement
@cindex statement, @code{ENUM}
@cindex statement, @code{ENUMERATOR}
@opindex @code{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.

@cindex TR 15581
TR 15581:
@cindex @code{ALLOCATABLE} dummy arguments
@code{ALLOCATABLE} dummy arguments.
@cindex @code{ALLOCATABLE} function results
@code{ALLOCATABLE} function results
@cindex @code{ALLOCATABLE} components of derived types
@code{ALLOCATABLE} components of derived types
@end itemize

@cindex @code{STREAM} I/O
@cindex @code{ACCESS='STREAM'} I/O
The @code{OPEN} statement supports the @code{ACCESS='STREAM'} specifier,
allowing I/O without any record structure.

@end itemize

@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

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,
@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.

* Old-style kind specifications::
* Old-style variable initialization::
* Extensions to namelist::
* X format descriptor without count field::
* Commas in FORMAT specifications::
* Missing period in FORMAT specifications::
* I/O item lists::
* BOZ literal constants::
* Real array indices::
* Unary operators::
* Implicitly convert LOGICAL and INTEGER values::
* Hollerith constants support::
* Cray pointers::
* CONVERT specifier::
* OpenMP::
@end menu

@node Old-style kind specifications
@section Old-style kind specifications
@cindex kind, old-style

GNU Fortran allows old-style kind specifications in declarations. These
look like:
      TYPESPEC*size x,y,z
@end smallexample
where @code{TYPESPEC} is a basic type (@code{INTEGER}, @code{REAL},
etc.), and where @code{size} is a byte count corresponding to the
storage size of a valid kind for that type.  (For @code{COMPLEX}
variables, @code{size} is the total size of the real and imaginary
parts.)  The statement then declares @code{x}, @code{y} and @code{z} to
be of type @code{TYPESPEC} with the appropriate kind.  This is
equivalent to the standard-conforming declaration
      TYPESPEC(k) x,y,z
@end smallexample
where @code{k} is equal to @code{size} for most types, but is equal to
@code{size/2} for the @code{COMPLEX} type.

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

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

Examples of standard-conforming code equivalent to the above example
! 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./
@end smallexample

Note that variables which are explicitly initialized in declarations
or in @code{DATA} statements automatically acquire the @code{SAVE}

@node Extensions to namelist
@section Extensions to namelist
@cindex 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 @samp{$} instead of @samp{&}
 X(:)%Y(2) = 1.0 2.0 3.0
 CH(1:4) = "abcd"
@end smallexample

It should be noted that the default terminator is @samp{/} rather than

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

@end smallexample

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

 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 @code{IOSTAT} is set.

@code{PRINT} namelist is permitted.  This causes an error if
@option{-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
@end smallexample

Expanded namelist reads are permitted.  This causes an error if 
@option{-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.
  X(1,1) = 0.00 , 1.00 , 2.00
@end smallexample

@node X format descriptor without count field
@section @code{X} format descriptor without count field

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

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

@node Commas in FORMAT specifications
@section Commas in @code{FORMAT} specifications

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

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

@node Missing period in FORMAT specifications
@section Missing period in @code{FORMAT} specifications

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

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

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

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

@node BOZ literal constants
@section BOZ literal constants
@cindex BOZ literal constants

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

The Fortran standard restricts the appearance of a BOZ literal constant
to the @code{DATA} statement, and it is expected to be assigned to an
@code{INTEGER} variable.  GNU Fortran permits a BOZ literal to appear in
any initialization expression as well as assignment statements.

Attempts to use a BOZ literal constant to do a bitwise initialization of
a variable can lead to confusion.  A BOZ literal constant is converted
to an @code{INTEGER} value with the kind type with the largest decimal
representation, and this value is then converted numerically to the type
and kind of the variable in question.  Thus, one should not expect a
bitwise copy of the BOZ literal constant to be assigned to a @code{REAL}

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

@node Real array indices
@section Real array indices
@cindex array, indices of type real

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

@node Unary operators
@section Unary operators
@cindex operators, unary

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
@end smallexample

@node Implicitly convert LOGICAL and INTEGER values
@section Implicitly convert @code{LOGICAL} and @code{INTEGER} values
@cindex conversion, to integer
@cindex conversion, to logical

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

       INTEGER :: i = 1
       IF (i) PRINT *, 'True'
@end smallexample

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

GNU Fortran supports Hollerith constants in assignments, function
arguments, and @code{DATA} and @code{ASSIGN} statements.  A Hollerith
constant is written as a string of characters preceded by an integer
constant indicating the character count, and the letter @code{H} or
@code{h}, and stored in bytewise fashion in a numeric (@code{INTEGER},
@code{REAL}, or @code{complex}) or @code{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/
      call foo (4h abc)
@end smallexample

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.
@end smallexample

In general, Hollerith constants were used to provide a rudimentary
facility for handling character strings in early Fortran compilers,
prior to the introduction of @code{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 @code{TRANSFER} statement, as in this example.
      INTEGER(KIND=4) :: a
      a = TRANSFER ("abcd", a)     ! equivalent to: a = 4Habcd
@end smallexample

@node Cray pointers
@section Cray pointers
@cindex pointer, cray

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> )
@end 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 @code{*} 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)
@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 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       
@end smallexample
The last statement does not set @code{ipt} to the address of
@code{target(1)}, as it would in C pointer arithmetic.  Adding @code{1}
to @code{ipt} just adds one byte to the address stored in @code{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 @code{LOC()}.  The @code{LOC()} function is equivalent to the
@code{&} 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
@end smallexample
The pointer can also be set by a call to the @code{MALLOC} intrinsic
(see @ref{MALLOC}).

Cray pointees often are used to alias an existing variable.  For
        integer target(10)
        integer iarr(10)
        pointer (ipt, iarr)
        ipt = loc(target)
@end smallexample
As long as @code{ipt} remains unchanged, @code{iarr} is now an alias for
@code{target}. The optimizer, however, will not detect this aliasing, so
it is unsafe to use @code{iarr} and @code{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

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
@code{ALLOCATABLE}, @code{INTENT}, @code{OPTIONAL}, @code{DUMMY},
@code{TARGET}, @code{INTRINSIC}, or @code{POINTER} attributes. Pointers
may not have the @code{DIMENSION}, @code{POINTER}, @code{TARGET},
@code{ALLOCATABLE}, @code{EXTERNAL}, or @code{INTRINSIC} attributes.
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
@end smallexample

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

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 @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:
  open(file='big.dat',form='unformatted',access='sequential', &
@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 GNU Fortran
supports as @code{REAL(KIND=10)} and @code{REAL(KIND=16)}, 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
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

@node OpenMP
@section OpenMP
@cindex OpenMP

GNU Fortran attempts to be OpenMP Application Program Interface v2.5
compatible when invoked with the @option{-fopenmp} option.  GNU Fortran
then generates parallelized code according to the OpenMP directives
used in the source.  The OpenMP Fortran runtime library
routines are provided both in a form of a Fortran 90 module named
@code{omp_lib} and in a form of a Fortran @code{include} file named

For details refer to the actual
OpenMP Application Program Interface v2.5} specification.

@c ---------------------------------------------------------------------
@c Intrinsic Procedures
@c ---------------------------------------------------------------------

@include intrinsic.texi

@end tex

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

@node Contributing
@unnumbered 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,
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::
* Projects::
* Proposed Extensions::
@end menu

@node Contributors
@section Contributors to GNU Fortran
@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 GNU Fortran project
(in no particular order):

@itemize @minus
@item Andy Vaught
@item Katherine Holcomb
@item Tobias Schl@"uter
@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 Fran@,{c}ois-Xavier Coudert
@item Steven G. Kargl
@item Jerry Delisle
@item Janne Blomqvist
@item Erik Edelmann
@item Thomas Koenig
@item Asher Langton
@item Jakub Jelinek
@item Roger Sayle
@item H.J. Lu
@item Richard Henderson
@item Richard Sandiford
@item Richard Guenther
@item Bernhard Fischer
@end itemize

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

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

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.

@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!

@node Proposed Extensions
@section 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.

@subsection Compiler extensions: 
@itemize @bullet
User-specified alignment rules for structures.

Flag to generate @code{Makefile} info.

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.

Flag for maximum errors before ending compile.

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

@subsection Environment Options
@itemize @bullet
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.

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

Environment variable controlling filename that is opened for that unit

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).

@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 Indices
@c ---------------------------------------------------------------------

@node Option Index
@unnumbered Option Index
@command{gfortran}'s command line options are indexed here without any
initial @samp{-} or @samp{--}. 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.
@printindex op

@node Keyword Index
@unnumbered Keyword Index
@printindex cp