gfortran.texi

理解和实践操作系统的一本好书

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{.for}, @file{.ftn},
@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.

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

This manual specifically documents the Fortran front end, which handles
the programming language's syntax and semantics.  The aspects of GCC
which relate to the optimization passes and the back-end code generation
are documented in the GCC manual; see 
@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 Preprocessing and conditional compilation
@c ---------------------------------------------------------------------

@node Preprocessing and conditional compilation
@section Preprocessing and conditional compilation
@cindex CPP
@cindex FPP
@cindex Conditional compilation
@cindex Preprocessing

Many Fortran compilers including GNU Fortran allow passing the source code
through a C preprocessor (CPP; sometimes also called the Fortran preprocessor,
FPP) to allow for conditional compilation. In the case of GNU Fortran,
this is the GNU C Preprocessor in the traditional mode. On systems with
case-preserving file names, the preprocessor is automatically invoked if the
file extension is @code{.F}, @code{.FOR}, @code{.FTN}, @code{.F90},
@code{.F95} or @code{.F03}; otherwise use for fixed-format code the option
@code{-x f77-cpp-input} and for free-format code @code{-x f95-cpp-input}.
Invocation of the preprocessor can be suppressed using @code{-x f77} or
@code{-x f95}.

If the GNU Fortran invoked the preprocessor, @code{__GFORTRAN__}
is defined and @code{__GNUC__}, @code{__GNUC_MINOR__} and
@code{__GNUC_PATCHLEVEL__} can be used to determine the version of the
compiler. See @ref{Top,,Overview,cpp,The C Preprocessor} for details.

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


@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

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

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

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


@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{http://www.openmp.org/drupal/mp-documents/spec25.pdf,
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: INVOCATION REFERENCE
@c =====================================================================

@tex
\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.

@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 I/O for all units.
* GFORTRAN_UNBUFFERED_PRECONNECTED:: Don't buffer I/O for preconnected units.
* 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
* GFORTRAN_ERROR_DUMPCORE:: Dump core on run-time errors
* GFORTRAN_ERROR_BACKTRACE:: Show backtrace on run-time errors
@end menu

@node GFORTRAN_STDIN_UNIT
@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.

@node GFORTRAN_STDOUT_UNIT
@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.

@node GFORTRAN_STDERR_UNIT
@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.

@node GFORTRAN_USE_STDERR
@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.

@node GFORTRAN_TMPDIR
@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}.

@node GFORTRAN_UNBUFFERED_ALL
@section @env{GFORTRAN_UNBUFFERED_ALL}---Don't buffer I/O on all units

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

@node GFORTRAN_UNBUFFERED_PRECONNECTED
@section @env{GFORTRAN_UNBUFFERED_PRECONNECTED}---Don't buffer I/O on preconnected units

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

@node GFORTRAN_SHOW_LOCUS
@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.

@node GFORTRAN_OPTIONAL_PLUS
@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.

@node GFORTRAN_DEFAULT_RECL
@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).

@node GFORTRAN_LIST_SEPARATOR
@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
@smallexample
$ GFORTRAN_LIST_SEPARATOR='  ,  ' ./a.out
@end smallexample
when @command{a.out} is the compiled Fortran program that you want to run.
Default is a single space.

@node GFORTRAN_CONVERT_UNIT
@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:
@smallexample
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}:
@smallexample
$ gfortran foo.f90
$ GFORTRAN_CONVERT_UNIT='big_endian;native:10-20' ./a.out
@end smallexample

Example code for @command{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 @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.

@node GFORTRAN_ERROR_DUMPCORE
@section @env{GFORTRAN_ERROR_DUMPCORE}---Dump core on run-time errors

If the @env{GFORTRAN_ERROR_DUMPCORE} variable is set to