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\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

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@c Use with @@smallbook.

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@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

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@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.

Malformed environment variables are silently ignored.

@menu

* 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

@node GFORTRAN_STDIN_UNIT

@section 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.

@node GFORTRAN_STDOUT_UNIT

@section 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.

@node GFORTRAN_STDERR_UNIT

@section 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.

@node GFORTRAN_USE_STDERR

@section 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.

@node GFORTRAN_TMPDIR

@section GFORTRAN_TMPDIR -- Directory for scratch files

This environment variable controls where scratch files are

created.  Default is '/tmp'.

@node GFORTRAN_UNBUFFERED_ALL

@section GFORTRAN_UNBUFFERED_ALL -- Don't buffer output

This environment variable controls wether all output is unbuffered.

If the first letter is 'y', 'Y' or '1', all output is unbuffered.

This will slow down large writes.  If the first letter is 'n', 'N' or '0',

output is bufferred.  This is the default.

@node GFORTRAN_SHOW_LOCUS

@section 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.

@node GFORTRAN_OPTIONAL_PLUS

@section 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 lettter

is 'n', 'N' or '0', a plus sign is not printed in most cases.

Default is not to print plus signs.

@node GFORTRAN_DEFAULT_RECL

@section GFORTRAN_DEFAULT_RECL -- Default record lenght for new files

This environment variable specifies the default record length 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.

@node GFORTRAN_LIST_SEPARATOR

@section 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

@smallexample

$ GFORTRAN_LIST_SEPARATOR='  ,  ' ./a.out

@end smallexample

when @code{a.out} is the gfortran program that you want to run.

Default is a single space.

@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