* COPYING: Update to current

[pf3gnuchains/gcc-fork.git] / gcc / f / g77.texi

\input texinfo  @c -*-texinfo-*-
@c %**start of header
@setfilename g77.info

@set last-update 2000-11-27
@set copyrights-g77 1995,1996,1997,1998,1999,2000

@include root.texi

@c This tells @include'd files that they're part of the overall G77 doc
@c set.  (They might be part of a higher-level doc set too.)
@set DOC-G77

@c @setfilename useg77.info
@c @setfilename portg77.info
@c To produce the full manual, use the "g77.info" setfilename, and
@c make sure the following do NOT begin with '@c' (and the @clear lines DO)
@set INTERNALS
@set USING
@c To produce a user-only manual, use the "useg77.info" setfilename, and
@c make sure the following does NOT begin with '@c':
@c @clear INTERNALS
@c To produce a porter-only manual, use the "portg77.info" setfilename,
@c and make sure the following does NOT begin with '@c':
@c @clear USING

@c 6/27/96 FSF DO wants smallbook fmt for 1st bound edition. (from gcc.texi)
@c @smallbook

@c i also commented out the finalout command, so if there *are* any
@c overfulls, you'll (hopefully) see the rectangle in the right hand
@c margin. -- burley 1999-03-13 (from mew's comment in gcc.texi).
@c @finalout

@ifset INTERNALS
@ifset USING
@settitle Using and Porting GNU Fortran
@end ifset
@end ifset
@c seems reasonable to assume at least one of INTERNALS or USING is set...
@ifclear INTERNALS
@settitle Using GNU Fortran
@end ifclear
@ifclear USING
@settitle Porting GNU Fortran
@end ifclear
@c then again, have some fun
@ifclear INTERNALS
@ifclear USING
@settitle Doing Squat with GNU Fortran
@end ifclear
@end ifclear

@syncodeindex fn cp
@syncodeindex vr cp
@c %**end of header

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

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

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

@ifinfo
@dircategory Programming
@direntry
* g77: (g77).                  The GNU Fortran compiler.
@end direntry
@ifset INTERNALS
@ifset USING
This file documents the use and the internals of the GNU Fortran (@code{g77})
compiler.
It corresponds to the @value{which-g77} version of @code{g77}.
@end ifset
@end ifset
@ifclear USING
This file documents the internals of the GNU Fortran (@code{g77}) compiler.
It corresponds to the @value{which-g77} version of @code{g77}.
@end ifclear
@ifclear INTERNALS
This file documents the use of the GNU Fortran (@code{g77}) compiler.
It corresponds to the @value{which-g77} version of @code{g77}.
@end ifclear

Published by the Free Software Foundation
59 Temple Place - Suite 330
Boston, MA 02111-1307 USA

Copyright (C) @value{copyrights-g77} Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

@ignore
Permission is granted to process this file through Tex and print the
results, provided the printed document carries copying permission
notice identical to this one except for the removal of this paragraph
(this paragraph not being relevant to the printed manual).

@end ignore
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided also that the
sections entitled ``GNU General Public License,'' ``Funding for Free
Software,'' and ``Protect Your Freedom---Fight `Look And Feel'@w{}'' are
included exactly as in the original, and provided that the entire
resulting derived work is distributed under the terms of a permission
notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that the sections entitled ``GNU General Public License,''
``Funding for Free Software,'' and ``Protect Your Freedom---Fight `Look
And Feel'@w{}'', and this permission notice, may be included in
translations approved by the Free Software Foundation instead of in the
original English.
@end ifinfo

Contributed by James Craig Burley (@email{@value{email-burley}}).
Inspired by a first pass at translating @file{g77-0.5.16/f/DOC} that
was contributed to Craig by David Ronis (@email{ronis@@onsager.chem.mcgill.ca}).

@setchapternewpage odd
@c @finalout
@titlepage
@ifset INTERNALS
@ifset USING
@center @titlefont{Using and Porting GNU Fortran}

@end ifset
@end ifset
@ifclear INTERNALS
@title Using GNU Fortran
@end ifclear
@ifclear USING
@title Porting GNU Fortran
@end ifclear
@sp 2
@center James Craig Burley
@sp 3
@center Last updated @value{last-update}
@sp 1
@center for version @value{version-g77}
@page
@vskip 0pt plus 1filll
Copyright @copyright{} @value{copyrights-g77} Free Software Foundation, Inc.
@sp 2
For the @value{which-g77} Version*
@sp 1
Published by the Free Software Foundation @*
59 Temple Place - Suite 330@*
Boston, MA 02111-1307, USA@*
@c Last printed ??ber, 19??.@*
@c Printed copies are available for $? each.@*
@c ISBN ???
@sp 1
Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided also that the
sections entitled ``GNU General Public License,'' ``Funding for Free
Software,'' and ``Protect Your Freedom---Fight `Look And Feel'@w{}'' are
included exactly as in the original, and provided that the entire
resulting derived work is distributed under the terms of a permission
notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that the sections entitled ``GNU General Public License,''
``Funding for Free Software,'' and ``Protect Your Freedom---Fight `Look
And Feel'@w{}'', and this permission notice, may be included in
translations approved by the Free Software Foundation instead of in the
original English.
@end titlepage
@page

@ifinfo

@node Top, Copying,, (DIR)
@top Introduction
@cindex Introduction

@ifset INTERNALS
@ifset USING
This manual documents how to run, install and port @code{g77},
as well as its new features and incompatibilities,
and how to report bugs.
It corresponds to the @value{which-g77} version of @code{g77}.
@end ifset
@end ifset

@ifclear INTERNALS
This manual documents how to run and install @code{g77},
as well as its new features and incompatibilities, and how to report
bugs.
It corresponds to the @value{which-g77} version of @code{g77}.
@end ifclear
@ifclear USING
This manual documents how to port @code{g77},
as well as its new features and incompatibilities,
and how to report bugs.
It corresponds to the @value{which-g77} version of @code{g77}.
@end ifclear

@end ifinfo

@ifset DEVELOPMENT
@emph{Warning:} This document is still under development,
and might not accurately reflect the @code{g77} code base
of which it is a part.
Efforts are made to keep it somewhat up-to-date,
but they are particularly concentrated
on any version of this information
that is distributed as part of a @emph{released} @code{g77}.

In particular, while this document is intended to apply to
the @value{which-g77} version of @code{g77},
only an official @emph{release} of that version
is expected to contain documentation that is
most consistent with the @code{g77} product in that version.
@end ifset

@menu
* Copying::         GNU General Public License says
                    how you can copy and share GNU Fortran.
* Contributors::    People who have contributed to GNU Fortran.
* Funding::         How to help assure continued work for free software.
* Funding GNU Fortran::  How to help assure continued work on GNU Fortran.
* Look and Feel::   Protect your freedom---fight ``look and feel''.
@ifset USING
* Getting Started:: Finding your way around this manual.
* What is GNU Fortran?::  How @code{g77} fits into the universe.
* G77 and GCC::     You can compile Fortran, C, or other programs.
* Invoking G77::    Command options supported by @code{g77}.
* News::            News about recent releases of @code{g77}.
* Changes::         User-visible changes to recent releases of @code{g77}.
* Language::        The GNU Fortran language.
* Compiler::        The GNU Fortran compiler.
* Other Dialects::  Dialects of Fortran supported by @code{g77}.
* Other Compilers:: Fortran compilers other than @code{g77}.
* Other Languages:: Languages other than Fortran.
* Installation::    How to configure, compile and install GNU Fortran.
* Debugging and Interfacing::  How @code{g77} generates code.
* Collected Fortran Wisdom::  How to avoid Trouble.
* Trouble::         If you have trouble with GNU Fortran.
* Open Questions::  Things we'd like to know.
* Bugs::            How, why, and where to report bugs.
* Service::         How to find suppliers of support for GNU Fortran.
@end ifset
@ifset INTERNALS
* Adding Options::  Guidance on teaching @code{g77} about new options.
* Projects::        Projects for @code{g77} internals hackers.
* Front End::       Design and implementation of the @code{g77} front end.
@end ifset

* M: Diagnostics.   Diagnostics produced by @code{g77}.

* Index::           Index of concepts and symbol names.
@end menu
@c yes, the "M: " @emph{is} intentional -- bad.def references it (CMPAMBIG)!

@node Copying
@unnumbered GNU GENERAL PUBLIC LICENSE
@center Version 2, June 1991

@display
Copyright @copyright{} 1989, 1991 Free Software Foundation, Inc.
59 Temple Place - Suite 330, Boston, MA 02111-1307, USA

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
@end display

@unnumberedsec Preamble

  The licenses for most software are designed to take away your
freedom to share and change it.  By contrast, the GNU General Public
License is intended to guarantee your freedom to share and change free
software---to make sure the software is free for all its users.  This
General Public License applies to most of the Free Software
Foundation's software and to any other program whose authors commit to
using it.  (Some other Free Software Foundation software is covered by
the GNU Library General Public License instead.)  You can apply it to
your programs, too.

  When we speak of free software, we are referring to freedom, not
price.  Our General Public Licenses are designed to make sure that you
have the freedom to distribute copies of free software (and charge for
this service if you wish), that you receive source code or can get it
if you want it, that you can change the software or use pieces of it
in new free programs; and that you know you can do these things.

  To protect your rights, we need to make restrictions that forbid
anyone to deny you these rights or to ask you to surrender the rights.
These restrictions translate to certain responsibilities for you if you
distribute copies of the software, or if you modify it.

  For example, if you distribute copies of such a program, whether
gratis or for a fee, you must give the recipients all the rights that
you have.  You must make sure that they, too, receive or can get the
source code.  And you must show them these terms so they know their
rights.

  We protect your rights with two steps: (1) copyright the software, and
(2) offer you this license which gives you legal permission to copy,
distribute and/or modify the software.

  Also, for each author's protection and ours, we want to make certain
that everyone understands that there is no warranty for this free
software.  If the software is modified by someone else and passed on, we
want its recipients to know that what they have is not the original, so
that any problems introduced by others will not reflect on the original
authors' reputations.

  Finally, any free program is threatened constantly by software
patents.  We wish to avoid the danger that redistributors of a free
program will individually obtain patent licenses, in effect making the
program proprietary.  To prevent this, we have made it clear that any
patent must be licensed for everyone's free use or not licensed at all.

  The precise terms and conditions for copying, distribution and
modification follow.

@iftex
@unnumberedsec TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
@end iftex
@ifinfo
@center TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
@end ifinfo

@enumerate 0
@item
This License applies to any program or other work which contains
a notice placed by the copyright holder saying it may be distributed
under the terms of this General Public License.  The ``Program'', below,
refers to any such program or work, and a ``work based on the Program''
means either the Program or any derivative work under copyright law:
that is to say, a work containing the Program or a portion of it,
either verbatim or with modifications and/or translated into another
language.  (Hereinafter, translation is included without limitation in
the term ``modification''.)  Each licensee is addressed as ``you''.

Activities other than copying, distribution and modification are not
covered by this License; they are outside its scope.  The act of
running the Program is not restricted, and the output from the Program
is covered only if its contents constitute a work based on the
Program (independent of having been made by running the Program).
Whether that is true depends on what the Program does.

@item
You may copy and distribute verbatim copies of the Program's
source code as you receive it, in any medium, provided that you
conspicuously and appropriately publish on each copy an appropriate
copyright notice and disclaimer of warranty; keep intact all the
notices that refer to this License and to the absence of any warranty;
and give any other recipients of the Program a copy of this License
along with the Program.

You may charge a fee for the physical act of transferring a copy, and
you may at your option offer warranty protection in exchange for a fee.

@item
You may modify your copy or copies of the Program or any portion
of it, thus forming a work based on the Program, and copy and
distribute such modifications or work under the terms of Section 1
above, provided that you also meet all of these conditions:

@enumerate a
@item
You must cause the modified files to carry prominent notices
stating that you changed the files and the date of any change.

@item
You must cause any work that you distribute or publish, that in
whole or in part contains or is derived from the Program or any
part thereof, to be licensed as a whole at no charge to all third
parties under the terms of this License.

@item
If the modified program normally reads commands interactively
when run, you must cause it, when started running for such
interactive use in the most ordinary way, to print or display an
announcement including an appropriate copyright notice and a
notice that there is no warranty (or else, saying that you provide
a warranty) and that users may redistribute the program under
these conditions, and telling the user how to view a copy of this
License.  (Exception: if the Program itself is interactive but
does not normally print such an announcement, your work based on
the Program is not required to print an announcement.)
@end enumerate

These requirements apply to the modified work as a whole.  If
identifiable sections of that work are not derived from the Program,
and can be reasonably considered independent and separate works in
themselves, then this License, and its terms, do not apply to those
sections when you distribute them as separate works.  But when you
distribute the same sections as part of a whole which is a work based
on the Program, the distribution of the whole must be on the terms of
this License, whose permissions for other licensees extend to the
entire whole, and thus to each and every part regardless of who wrote it.

Thus, it is not the intent of this section to claim rights or contest
your rights to work written entirely by you; rather, the intent is to
exercise the right to control the distribution of derivative or
collective works based on the Program.

In addition, mere aggregation of another work not based on the Program
with the Program (or with a work based on the Program) on a volume of
a storage or distribution medium does not bring the other work under
the scope of this License.

@item
You may copy and distribute the Program (or a work based on it,
under Section 2) in object code or executable form under the terms of
Sections 1 and 2 above provided that you also do one of the following:

@enumerate a
@item
Accompany it with the complete corresponding machine-readable
source code, which must be distributed under the terms of Sections
1 and 2 above on a medium customarily used for software interchange; or,

@item
Accompany it with a written offer, valid for at least three
years, to give any third party, for a charge no more than your
cost of physically performing source distribution, a complete
machine-readable copy of the corresponding source code, to be
distributed under the terms of Sections 1 and 2 above on a medium
customarily used for software interchange; or,

@item
Accompany it with the information you received as to the offer
to distribute corresponding source code.  (This alternative is
allowed only for noncommercial distribution and only if you
received the program in object code or executable form with such
an offer, in accord with Subsection b above.)
@end enumerate

The source code for a work means the preferred form of the work for
making modifications to it.  For an executable work, complete source
code means all the source code for all modules it contains, plus any
associated interface definition files, plus the scripts used to
control compilation and installation of the executable.  However, as a
special exception, the source code distributed need not include
anything that is normally distributed (in either source or binary
form) with the major components (compiler, kernel, and so on) of the
operating system on which the executable runs, unless that component
itself accompanies the executable.

If distribution of executable or object code is made by offering
access to copy from a designated place, then offering equivalent
access to copy the source code from the same place counts as
distribution of the source code, even though third parties are not
compelled to copy the source along with the object code.

@item
You may not copy, modify, sublicense, or distribute the Program
except as expressly provided under this License.  Any attempt
otherwise to copy, modify, sublicense or distribute the Program is
void, and will automatically terminate your rights under this License.
However, parties who have received copies, or rights, from you under
this License will not have their licenses terminated so long as such
parties remain in full compliance.

@item
You are not required to accept this License, since you have not
signed it.  However, nothing else grants you permission to modify or
distribute the Program or its derivative works.  These actions are
prohibited by law if you do not accept this License.  Therefore, by
modifying or distributing the Program (or any work based on the
Program), you indicate your acceptance of this License to do so, and
all its terms and conditions for copying, distributing or modifying
the Program or works based on it.

@item
Each time you redistribute the Program (or any work based on the
Program), the recipient automatically receives a license from the
original licensor to copy, distribute or modify the Program subject to
these terms and conditions.  You may not impose any further
restrictions on the recipients' exercise of the rights granted herein.
You are not responsible for enforcing compliance by third parties to
this License.

@item
If, as a consequence of a court judgment or allegation of patent
infringement or for any other reason (not limited to patent issues),
conditions are imposed on you (whether by court order, agreement or
otherwise) that contradict the conditions of this License, they do not
excuse you from the conditions of this License.  If you cannot
distribute so as to satisfy simultaneously your obligations under this
License and any other pertinent obligations, then as a consequence you
may not distribute the Program at all.  For example, if a patent
license would not permit royalty-free redistribution of the Program by
all those who receive copies directly or indirectly through you, then
the only way you could satisfy both it and this License would be to
refrain entirely from distribution of the Program.

If any portion of this section is held invalid or unenforceable under
any particular circumstance, the balance of the section is intended to
apply and the section as a whole is intended to apply in other
circumstances.

It is not the purpose of this section to induce you to infringe any
patents or other property right claims or to contest validity of any
such claims; this section has the sole purpose of protecting the
integrity of the free software distribution system, which is
implemented by public license practices.  Many people have made
generous contributions to the wide range of software distributed
through that system in reliance on consistent application of that
system; it is up to the author/donor to decide if he or she is willing
to distribute software through any other system and a licensee cannot
impose that choice.

This section is intended to make thoroughly clear what is believed to
be a consequence of the rest of this License.

@item
If the distribution and/or use of the Program is restricted in
certain countries either by patents or by copyrighted interfaces, the
original copyright holder who places the Program under this License
may add an explicit geographical distribution limitation excluding
those countries, so that distribution is permitted only in or among
countries not thus excluded.  In such case, this License incorporates
the limitation as if written in the body of this License.

@item
The Free Software Foundation may publish revised and/or new versions
of the General Public License from time to time.  Such new versions will
be similar in spirit to the present version, but may differ in detail to
address new problems or concerns.

Each version is given a distinguishing version number.  If the Program
specifies a version number of this License which applies to it and ``any
later version'', you have the option of following the terms and conditions
either of that version or of any later version published by the Free
Software Foundation.  If the Program does not specify a version number of
this License, you may choose any version ever published by the Free Software
Foundation.

@item
If you wish to incorporate parts of the Program into other free
programs whose distribution conditions are different, write to the author
to ask for permission.  For software which is copyrighted by the Free
Software Foundation, write to the Free Software Foundation; we sometimes
make exceptions for this.  Our decision will be guided by the two goals
of preserving the free status of all derivatives of our free software and
of promoting the sharing and reuse of software generally.

@iftex
@heading NO WARRANTY
@end iftex
@ifinfo
@center NO WARRANTY
@end ifinfo

@item
BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW.  EXCEPT WHEN
OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES
PROVIDE THE PROGRAM ``AS IS'' WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED
OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  THE ENTIRE RISK AS
TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU.  SHOULD THE
PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING,
REPAIR OR CORRECTION.

@item
IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES,
INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING
OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED
TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY
YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER
PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES.
@end enumerate

@iftex
@heading END OF TERMS AND CONDITIONS
@end iftex
@ifinfo
@center END OF TERMS AND CONDITIONS
@end ifinfo

@page
@unnumberedsec How to Apply These Terms to Your New Programs

  If you develop a new program, and you want it to be of the greatest
possible use to the public, the best way to achieve this is to make it
free software which everyone can redistribute and change under these terms.

  To do so, attach the following notices to the program.  It is safest
to attach them to the start of each source file to most effectively
convey the exclusion of warranty; and each file should have at least
the ``copyright'' line and a pointer to where the full notice is found.

@smallexample
@var{one line to give the program's name and a brief idea of what it does.}
Copyright (C) @var{year}  @var{name of author}

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
@end smallexample

Also add information on how to contact you by electronic and paper mail.

If the program is interactive, make it output a short notice like this
when it starts in an interactive mode:

@smallexample
Gnomovision version 69, Copyright (C) @var{year} @var{name of author}
Gnomovision comes with ABSOLUTELY NO WARRANTY; for details
type `show w'.
This is free software, and you are welcome to redistribute it
under certain conditions; type `show c' for details.
@end smallexample

The hypothetical commands @samp{show w} and @samp{show c} should show
the appropriate parts of the General Public License.  Of course, the
commands you use may be called something other than @samp{show w} and
@samp{show c}; they could even be mouse-clicks or menu items---whatever
suits your program.

You should also get your employer (if you work as a programmer) or your
school, if any, to sign a ``copyright disclaimer'' for the program, if
necessary.  Here is a sample; alter the names:

@smallexample
Yoyodyne, Inc., hereby disclaims all copyright interest in the program
`Gnomovision' (which makes passes at compilers) written by James Hacker.

@var{signature of Ty Coon}, 1 April 1989
Ty Coon, President of Vice
@end smallexample

This General Public License does not permit incorporating your program into
proprietary programs.  If your program is a subroutine library, you may
consider it more useful to permit linking proprietary applications with the
library.  If this is what you want to do, use the GNU Library General
Public License instead of this License.

@node Contributors
@unnumbered Contributors to GNU Fortran
@cindex contributors
@cindex credits

In addition to James Craig Burley, who wrote the front end,
many people have helped create and improve GNU Fortran.

@itemize @bullet
@item
The packaging and compiler portions of GNU Fortran are based largely
on the GNU CC compiler.
@xref{Contributors,,Contributors to GNU CC,gcc,Using and Porting GNU CC},
for more information.

@item
The run-time library used by GNU Fortran is a repackaged version
of the @code{libf2c} library (combined from the @code{libF77} and
@code{libI77} libraries) provided as part of @code{f2c}, available for
free from @code{netlib} sites on the Internet.

@item
Cygnus Support and The Free Software Foundation contributed
significant money and/or equipment to Craig's efforts.

@item
The following individuals served as alpha testers prior to @code{g77}'s
public release.  This work consisted of testing, researching, sometimes
debugging, and occasionally providing small amounts of code and fixes
for @code{g77}, plus offering plenty of helpful advice to Craig:

@itemize @w{}
@item
Jonathan Corbet
@item
Dr.@: Mark Fernyhough
@item
Takafumi Hayashi (The University of Aizu)---@email{takafumi@@u-aizu.ac.jp}
@item
Kate Hedstrom
@item
Michel Kern (INRIA and Rice University)---@email{Michel.Kern@@inria.fr}
@item
Dr.@: A. O. V. Le Blanc
@item
Dave Love
@item
Rick Lutowski
@item
Toon Moene
@item
Rick Niles
@item
Derk Reefman
@item
Wayne K. Schroll
@item
Bill Thorson
@item
Pedro A. M. Vazquez
@item
Ian Watson
@end itemize

@item
Scott Snyder (@email{snyder@@d0sgif.fnal.gov})
provided the patch to add rudimentary support
for @code{INTEGER*1}, @code{INTEGER*2}, and
@code{LOGICAL*1}.
This inspired Craig to add further support,
even though the resulting support
would still be incomplete, because version 0.6 is still
a ways off.

@item
David Ronis (@email{ronis@@onsager.chem.mcgill.ca}) inspired
and encouraged Craig to rewrite the documentation in texinfo
format by contributing a first pass at a translation of the
old @file{g77-0.5.16/f/DOC} file.

@item
Toon Moene (@email{toon@@moene.indiv.nluug.nl}) performed
some analysis of generated code as part of an overall project
to improve @code{g77} code generation to at least be as good
as @code{f2c} used in conjunction with @code{gcc}.
So far, this has resulted in the three, somewhat
experimental, options added by @code{g77} to the @code{gcc}
compiler and its back end.

(These, in turn, had made their way into the @code{egcs}
version of the compiler, and do not exist in @code{gcc}
version 2.8 or versions of @code{g77} based on that version
of @code{gcc}.)

@item
John Carr (@email{jfc@@mit.edu}) wrote the alias analysis improvements.

@item
Thanks to Mary Cortani and the staff at Craftwork Solutions
(@email{support@@craftwork.com}) for all of their support.

@item
Many other individuals have helped debug, test, and improve @code{g77}
over the past several years, and undoubtedly more people
will be doing so in the future.
If you have done so, and would like
to see your name listed in the above list, please ask!
The default is that people wish to remain anonymous.
@end itemize

@node Funding
@chapter Funding Free Software

If you want to have more free software a few years from now, it makes
sense for you to help encourage people to contribute funds for its
development.  The most effective approach known is to encourage
commercial redistributors to donate.

Users of free software systems can boost the pace of development by
encouraging for-a-fee distributors to donate part of their selling price
to free software developers---the Free Software Foundation, and others.

The way to convince distributors to do this is to demand it and expect
it from them.  So when you compare distributors, judge them partly by
how much they give to free software development.  Show distributors
they must compete to be the one who gives the most.

To make this approach work, you must insist on numbers that you can
compare, such as, ``We will donate ten dollars to the Frobnitz project
for each disk sold.''  Don't be satisfied with a vague promise, such as
``A portion of the profits are donated,'' since it doesn't give a basis
for comparison.

Even a precise fraction ``of the profits from this disk'' is not very
meaningful, since creative accounting and unrelated business decisions
can greatly alter what fraction of the sales price counts as profit.
If the price you pay is $50, ten percent of the profit is probably
less than a dollar; it might be a few cents, or nothing at all.

Some redistributors do development work themselves.  This is useful too;
but to keep everyone honest, you need to inquire how much they do, and
what kind.  Some kinds of development make much more long-term
difference than others.  For example, maintaining a separate version of
a program contributes very little; maintaining the standard version of a
program for the whole community contributes much.  Easy new ports
contribute little, since someone else would surely do them; difficult
ports such as adding a new CPU to the GNU C compiler contribute more;
major new features or packages contribute the most.

By establishing the idea that supporting further development is ``the
proper thing to do'' when distributing free software for a fee, we can
assure a steady flow of resources into making more free software.

@display
Copyright (C) 1994 Free Software Foundation, Inc.
Verbatim copying and redistribution of this section is permitted
without royalty; alteration is not permitted.
@end display

@node Funding GNU Fortran
@chapter Funding GNU Fortran
@cindex funding improvements
@cindex improvements, funding

Work on GNU Fortran is still being done mostly by its author,
James Craig Burley (@email{@value{email-burley}}), who is a volunteer
for, not an employee of, the Free Software Foundation (FSF).
(He has a web page at @uref{@value{www-burley}}.)

As with other GNU software, funding is important because it can pay for
needed equipment, personnel, and so on.

@cindex FSF, funding the
@cindex funding the FSF
The FSF provides information on the best way to fund ongoing
development of GNU software (such as GNU Fortran) in documents
such as the ``GNUS Bulletin''.
Email @email{gnu@@gnu.org} for information on funding the FSF.

To fund specific GNU Fortran work in particular, the FSF might
provide a means for that, but the FSF does not provide direct funding
to the author of GNU Fortran to continue his work.  The FSF has
employee salary restrictions that can be incompatible with the
financial needs of some volunteers, who therefore choose to
remain volunteers and thus be able to be free to do contract work
and otherwise make their own schedules for doing GNU work.

Still, funding the FSF at least indirectly benefits work
on specific projects like GNU Fortran because it ensures the
continuing operation of the FSF offices, their workstations, their
network connections, and so on, which are invaluable to volunteers.
(Similarly, hiring Cygnus Support can help a project like GNU
Fortran---Cygnus has been a long-time donor of equipment usage to the author
of GNU Fortran, and this too has been invaluable---see @ref{Contributors}.)

Currently, the only way to directly fund the author of GNU Fortran
in his work on that project is to hire him for the work you want
him to do, or donate money to him.
Several people have done this
already, with the result that he has not needed to immediately find
contract work on a few occasions.
If more people did this, he
would be able to plan on not doing contract work for many months and
could thus devote that time to work on projects (such as the planned
changes for 0.6) that require longer timeframes to complete.
For the latest information on the status of the author, do
@kbd{finger -l burley@@gnu.org} on a UNIX system
(or any system with a command like UNIX @code{finger}).

Another important way to support work on GNU Fortran is to volunteer
to help out.
Work is needed on documentation, testing, porting
to various machines, and in some cases, coding (although major
changes planned for version 0.6 make it difficult to add manpower to this
area).
Email @email{@value{email-general}} to volunteer for this work.

@xref{Funding,,Funding Free Software}, for more information.

@node Look and Feel
@chapter Protect Your Freedom---Fight ``Look And Feel''
@c the above chapter heading overflows onto the next line. --mew 1/26/93

To preserve the ability to write free software, including replacements
for proprietary software, authors must be free to replicate the
user interface to which users of existing software have become
accustomed.

@xref{Look and Feel,,Protect Your Freedom---Fight ``Look And Feel'',
gcc,Using and Porting GNU CC}, for more information.

@node Getting Started
@chapter Getting Started
@cindex getting started
@cindex new users
@cindex newbies
@cindex beginners

If you don't need help getting started reading the portions
of this manual that are most important to you, you should skip
this portion of the manual.

If you are new to compilers, especially Fortran compilers, or
new to how compilers are structured under UNIX and UNIX-like
systems, you'll want to see @ref{What is GNU Fortran?}.

If you are new to GNU compilers, or have used only one GNU
compiler in the past and not had to delve into how it lets
you manage various versions and configurations of @code{gcc},
you should see @ref{G77 and GCC}.

Everyone except experienced @code{g77} users should
see @ref{Invoking G77}.

If you're acquainted with previous versions of @code{g77},
you should see @ref{News,,News About GNU Fortran}.
Further, if you've actually used previous versions of @code{g77},
especially if you've written or modified Fortran code to
be compiled by previous versions of @code{g77}, you
should see @ref{Changes}.

If you intend to write or otherwise compile code that is
not already strictly conforming ANSI FORTRAN 77---and this
is probably everyone---you should see @ref{Language}.

If you don't already have @code{g77} installed on your
system, you must see @ref{Installation}.

If you run into trouble getting Fortran code to compile,
link, run, or work properly, you might find answers
if you see @ref{Debugging and Interfacing},
see @ref{Collected Fortran Wisdom},
and see @ref{Trouble}.
You might also find that the problems you are encountering
are bugs in @code{g77}---see @ref{Bugs}, for information on
reporting them, after reading the other material.

If you need further help with @code{g77}, or with
freely redistributable software in general,
see @ref{Service}.

If you would like to help the @code{g77} project,
see @ref{Funding GNU Fortran}, for information on
helping financially, and see @ref{Projects}, for information
on helping in other ways.

If you're generally curious about the future of
@code{g77}, see @ref{Projects}.
If you're curious about its past,
see @ref{Contributors},
and see @ref{Funding GNU Fortran}.

To see a few of the questions maintainers of @code{g77} have,
and that you might be able to answer,
see @ref{Open Questions}.

@ifset USING
@node What is GNU Fortran?
@chapter What is GNU Fortran?
@cindex concepts, basic
@cindex basic concepts

GNU Fortran, or @code{g77}, is designed initially as a free replacement
for, or alternative to, the UNIX @code{f77} command.
(Similarly, @code{gcc} is designed as a replacement
for the UNIX @code{cc} command.)

@code{g77} also is designed to fit in well with the other
fine GNU compilers and tools.

Sometimes these design goals conflict---in such cases, resolution
often is made in favor of fitting in well with Project GNU.
These cases are usually identified in the appropriate
sections of this manual.

@cindex compilers
As compilers, @code{g77}, @code{gcc}, and @code{f77}
share the following characteristics:

@itemize @bullet
@cindex source code
@cindex file, source
@cindex code, source
@cindex source file
@item
They read a user's program, stored in a file and
containing instructions written in the appropriate
language (Fortran, C, and so on).
This file contains @dfn{source code}.

@cindex translation of user programs
@cindex machine code
@cindex code, machine
@cindex mistakes
@item
They 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.
These instructions are called @dfn{machine code}---code
designed to be efficiently translated and processed
by a machine such as a computer.
Humans usually aren't as good writing machine code
as they are at writing Fortran or C, because
it is easy to make tiny mistakes writing machine code.
When writing Fortran or C, it is easy
to make big mistakes.

@cindex debugger
@cindex bugs, finding
@cindex @code{gdb}, command
@cindex commands, @code{gdb}
@item
They 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 @code{gdb}).

@cindex libraries
@cindex linking
@cindex @code{ld} command
@cindex commands, @code{ld}
@item
They 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{libraries} and is located and gathered
during the @dfn{link} phase of the compilation
process.
(Linking often is thought of as a separate
step, because it can be directly invoked via the
@code{ld} command.
However, the @code{g77} and @code{gcc}
commands, as with most compiler commands, automatically
perform the linking step by calling on @code{ld}
directly, unless asked to not do so by the user.)

@cindex language, incorrect use of
@cindex incorrect use of language
@item
They attempt to diagnose cases where the user's
program contains incorrect usages of the language.
The @dfn{diagnostics} produced by the compiler
indicate the problem and the location in the user's
source file where the problem was first noticed.
The user can use this information to locate and
fix the problem.
@cindex diagnostics, incorrect
@cindex incorrect diagnostics
@cindex error messages, incorrect
@cindex incorrect error messages
(Sometimes an incorrect usage
of the language leads to a situation where the
compiler can no longer make any sense of what
follows---while a human might be able to---and
thus ends up complaining about many ``problems''
it encounters that, in fact, stem from just one
problem, usually the first one reported.)

@cindex warnings
@cindex questionable instructions
@item
They attempt to diagnose cases where the user's
program contains a correct usage of the language,
but instructs the computer to do something questionable.
These diagnostics often are in the form of @dfn{warnings},
instead of the @dfn{errors} that indicate incorrect
usage of the language.
@end itemize

How these actions are performed is generally under the
control of the user.
Using command-line options, the user can specify
how persnickety the compiler is to be regarding
the program (whether to diagnose questionable usage
of the language), how much time to spend making
the generated machine code run faster, and so on.

@cindex components of g77
@cindex @code{g77}, components of
@code{g77} consists of several components:

@cindex @code{gcc}, command
@cindex commands, @code{gcc}
@itemize @bullet
@item
A modified version of the @code{gcc} command, which also might be
installed as the system's @code{cc} command.
(In many cases, @code{cc} refers to the
system's ``native'' C compiler, which
might be a non-GNU compiler, or an older version
of @code{gcc} considered more stable or that is
used to build the operating system kernel.)

@cindex @code{g77}, command
@cindex commands, @code{g77}
@item
The @code{g77} command itself, which also might be installed as the
system's @code{f77} command.

@cindex libg2c library
@cindex libf2c library
@cindex libraries, libf2c
@cindex libraries, libg2c
@cindex run-time, library
@item
The @code{libg2c} run-time library.
This library contains the machine code needed to support
capabilities of the Fortran language that are not directly
provided by the machine code generated by the @code{g77}
compilation phase.

@code{libg2c} is just the unique name @code{g77} gives
to its version of @code{libf2c} to distinguish it from
any copy of @code{libf2c} installed from @code{f2c}
(or versions of @code{g77} that built @code{libf2c} under
that same name)
on the system.

The maintainer of @code{libf2c} currently is
@email{dmg@@bell-labs.com}.

@cindex @code{f771}, program
@cindex programs, @code{f771}
@cindex assembler
@cindex @code{as} command
@cindex commands, @code{as}
@cindex assembly code
@cindex code, assembly
@item
The compiler itself, internally named @code{f771}.

Note that @code{f771} does not generate machine code directly---it
generates @dfn{assembly code} that is a more readable form
of machine code, leaving the conversion to actual machine code
to an @dfn{assembler}, usually named @code{as}.
@end itemize

@code{gcc} is often thought of as ``the C compiler'' only,
but it does more than that.
Based on command-line options and the names given for files
on the command line, @code{gcc} determines which actions to perform, including
preprocessing, compiling (in a variety of possible languages), assembling,
and linking.

@cindex driver, gcc command as
@cindex @code{gcc}, command as driver
@cindex executable file
@cindex files, executable
@cindex cc1 program
@cindex programs, cc1
@cindex preprocessor
@cindex cpp program
@cindex programs, cpp
For example, the command @samp{gcc foo.c} @dfn{drives} the file
@file{foo.c} through the preprocessor @code{cpp}, then
the C compiler (internally named
@code{cc1}), then the assembler (usually @code{as}), then the linker
(@code{ld}), producing an executable program named @file{a.out} (on
UNIX systems).

@cindex cc1plus program
@cindex programs, cc1plus
As another example, the command @samp{gcc foo.cc} would do much the same as
@samp{gcc foo.c}, but instead of using the C compiler named @code{cc1},
@code{gcc} would use the C++ compiler (named @code{cc1plus}).

@cindex @code{f771}, program
@cindex programs, @code{f771}
In a GNU Fortran installation, @code{gcc} recognizes Fortran source
files by name just like it does C and C++ source files.
It knows to use the Fortran compiler named @code{f771}, instead of
@code{cc1} or @code{cc1plus}, to compile Fortran files.

@cindex @code{gcc}, not recognizing Fortran source
@cindex unrecognized file format
@cindex file format not recognized
Non-Fortran-related operation of @code{gcc} is generally
unaffected by installing the GNU Fortran version of @code{gcc}.
However, without the installed version of @code{gcc} being the
GNU Fortran version, @code{gcc} will not be able to compile
and link Fortran programs---and since @code{g77} uses @code{gcc}
to do most of the actual work, neither will @code{g77}!

@cindex @code{g77}, command
@cindex commands, @code{g77}
The @code{g77} command is essentially just a front-end for
the @code{gcc} command.
Fortran users will normally use @code{g77} instead of @code{gcc},
because @code{g77}
knows how to specify the libraries needed to link with Fortran programs
(@code{libg2c} and @code{lm}).
@code{g77} can still compile and link programs and
source files written in other languages, just like @code{gcc}.

@cindex printing version information
@cindex version information, printing
The command @samp{g77 -v} is a quick
way to display lots of version information for the various programs
used to compile a typical preprocessed Fortran source file---this
produces much more output than @samp{gcc -v} currently does.
(If it produces an error message near the end of the output---diagnostics
from the linker, usually @code{ld}---you might
have an out-of-date @code{libf2c} that improperly handles
complex arithmetic.)
In the output of this command, the line beginning @samp{GNU Fortran Front
End} identifies the version number of GNU Fortran; immediately
preceding that line is a line identifying the version of @code{gcc}
with which that version of @code{g77} was built.

@cindex libf2c library
@cindex libraries, libf2c
The @code{libf2c} library is distributed with GNU Fortran for
the convenience of its users, but is not part of GNU Fortran.
It contains the procedures
needed by Fortran programs while they are running.

@cindex in-line code
@cindex code, in-line
For example, while code generated by @code{g77} is likely
to do additions, subtractions, and multiplications @dfn{in line}---in
the actual compiled code---it is not likely to do trigonometric
functions this way.

Instead, operations like trigonometric
functions are compiled by the @code{f771} compiler
(invoked by @code{g77} when compiling Fortran code) into machine
code that, when run, calls on functions in @code{libg2c}, so
@code{libg2c} must be linked with almost every useful program
having any component compiled by GNU Fortran.
(As mentioned above, the @code{g77} command takes
care of all this for you.)

The @code{f771} program represents most of what is unique to GNU Fortran.
While much of the @code{libg2c} component comes from
the @code{libf2c} component of @code{f2c},
a free Fortran-to-C converter distributed by Bellcore (AT&T),
plus @code{libU77}, provided by Dave Love,
and the @code{g77} command is just a small front-end to @code{gcc},
@code{f771} is a combination of two rather
large chunks of code.

@cindex GNU Back End (GBE)
@cindex GBE
@cindex @code{gcc}, back end
@cindex back end, gcc
@cindex code generator
One chunk is the so-called @dfn{GNU Back End}, or GBE,
which knows how to generate fast code for a wide variety of processors.
The same GBE is used by the C, C++, and Fortran compiler programs @code{cc1},
@code{cc1plus}, and @code{f771}, plus others.
Often the GBE is referred to as the ``gcc back end'' or
even just ``gcc''---in this manual, the term GBE is used
whenever the distinction is important.

@cindex GNU Fortran Front End (FFE)
@cindex FFE
@cindex @code{g77}, front end
@cindex front end, @code{g77}
The other chunk of @code{f771} is the
majority of what is unique about GNU Fortran---the code that knows how
to interpret Fortran programs to determine what they are intending to
do, and then communicate that knowledge to the GBE for actual compilation
of those programs.
This chunk is called the @dfn{Fortran Front End} (FFE).
The @code{cc1} and @code{cc1plus} programs have their own front ends,
for the C and C++ languages, respectively.
These fronts ends are responsible for diagnosing
incorrect usage of their respective languages by the
programs the process, and are responsible for most of
the warnings about questionable constructs as well.
(The GBE handles producing some warnings, like those
concerning possible references to undefined variables.)

Because so much is shared among the compilers for various languages,
much of the behavior and many of the user-selectable options for these
compilers are similar.
For example, diagnostics (error messages and
warnings) are similar in appearance; command-line
options like @samp{-Wall} have generally similar effects; and the quality
of generated code (in terms of speed and size) is roughly similar
(since that work is done by the shared GBE).

@node G77 and GCC
@chapter Compile Fortran, C, or Other Programs
@cindex compiling programs
@cindex programs, compiling

@cindex @code{gcc}, command
@cindex commands, @code{gcc}
A GNU Fortran installation includes a modified version of the @code{gcc}
command.

In a non-Fortran installation, @code{gcc} recognizes C, C++,
and Objective-C source files.

In a GNU Fortran installation, @code{gcc} also recognizes Fortran source
files and accepts Fortran-specific command-line options, plus some
command-line options that are designed to cater to Fortran users
but apply to other languages as well.

@xref{G++ and GCC,,Compile C; C++; or Objective-C,gcc,Using and Porting GNU CC},
for information on the way different languages are handled
by the GNU CC compiler (@code{gcc}).

@cindex @code{g77}, command
@cindex commands, @code{g77}
Also provided as part of GNU Fortran is the @code{g77} command.
The @code{g77} command is designed to make compiling and linking Fortran
programs somewhat easier than when using the @code{gcc} command for
these tasks.
It does this by analyzing the command line somewhat and changing it
appropriately before submitting it to the @code{gcc} command.

@cindex -v option
@cindex @code{g77} options, -v
@cindex options, -v
Use the @samp{-v} option with @code{g77}
to see what is going on---the first line of output is the invocation
of the @code{gcc} command.

@node Invoking G77
@chapter GNU Fortran Command Options
@cindex GNU Fortran command options
@cindex command options
@cindex options, GNU Fortran command

The @code{g77} command supports all the options supported by the
@code{gcc} command.
@xref{Invoking GCC,,GNU CC Command Options,gcc,Using and Porting GNU CC},
for information
on the non-Fortran-specific aspects of the @code{gcc} command (and,
therefore, the @code{g77} command).

@cindex options, negative forms
@cindex negative forms of options
All @code{gcc} and @code{g77} options
are accepted both by @code{g77} and by @code{gcc}
(as well as any other drivers built at the same time,
such as @code{g++}),
since adding @code{g77} to the @code{gcc} distribution
enables acceptance of @code{g77}-specific options
by all of the relevant drivers.

In some cases, options have positive and negative forms;
the negative form of @samp{-ffoo} would be @samp{-fno-foo}.
This manual documents only one of these two forms, whichever
one is not the default.

@menu
* Option Summary::      Brief list of all @code{g77} options,
                        without explanations.
* Overall Options::     Controlling the kind of output:
                        an executable, object files, assembler files,
                        or preprocessed source.
* Shorthand Options::   Options that are shorthand for other options.
* Fortran Dialect Options::  Controlling the variant of Fortran language
                             compiled.
* Warning Options::     How picky should the compiler be?
* Debugging Options::   Symbol tables, measurements, and debugging dumps.
* Optimize Options::    How much optimization?
* Preprocessor Options:: Controlling header files and macro definitions.
                         Also, getting dependency information for Make.
* Directory Options::   Where to find header files and libraries.
                        Where to find the compiler executable files.
* Code Gen Options::    Specifying conventions for function calls, data layout
                        and register usage.
* Environment Variables:: Env vars that affect GNU Fortran.
@end menu

@node Option Summary
@section Option Summary

Here is a summary of all the options specific to GNU Fortran, grouped
by type.  Explanations are in the following sections.

@table @emph
@item Overall Options
@xref{Overall Options,,Options Controlling the Kind of Output}.
@smallexample
-fversion  -fset-g77-defaults  -fno-silent
@end smallexample

@item Shorthand Options
@xref{Shorthand Options}.
@smallexample
-ff66  -fno-f66  -ff77  -fno-f77  -fno-ugly
@end smallexample

@item Fortran Language Options
@xref{Fortran Dialect Options,,Options Controlling Fortran Dialect}.
@smallexample
-ffree-form  -fno-fixed-form  -ff90
-fvxt  -fdollar-ok  -fno-backslash
-fno-ugly-args  -fno-ugly-assign  -fno-ugly-assumed
-fugly-comma  -fugly-complex  -fugly-init  -fugly-logint
-fonetrip  -ftypeless-boz
-fintrin-case-initcap  -fintrin-case-upper
-fintrin-case-lower  -fintrin-case-any
-fmatch-case-initcap  -fmatch-case-upper
-fmatch-case-lower  -fmatch-case-any
-fsource-case-upper -fsource-case-lower  -fsource-case-preserve
-fsymbol-case-initcap  -fsymbol-case-upper
-fsymbol-case-lower  -fsymbol-case-any
-fcase-strict-upper  -fcase-strict-lower
-fcase-initcap  -fcase-upper  -fcase-lower  -fcase-preserve
-ff2c-intrinsics-delete  -ff2c-intrinsics-hide
-ff2c-intrinsics-disable  -ff2c-intrinsics-enable
-fbadu77-intrinsics-delete  -fbadu77-intrinsics-hide
-fbadu77-intrinsics-disable  -fbadu77-intrinsics-enable
-ff90-intrinsics-delete  -ff90-intrinsics-hide
-ff90-intrinsics-disable  -ff90-intrinsics-enable
-fgnu-intrinsics-delete  -fgnu-intrinsics-hide
-fgnu-intrinsics-disable  -fgnu-intrinsics-enable
-fmil-intrinsics-delete  -fmil-intrinsics-hide
-fmil-intrinsics-disable  -fmil-intrinsics-enable
-funix-intrinsics-delete  -funix-intrinsics-hide
-funix-intrinsics-disable  -funix-intrinsics-enable
-fvxt-intrinsics-delete  -fvxt-intrinsics-hide
-fvxt-intrinsics-disable  -fvxt-intrinsics-enable
-ffixed-line-length-@var{n}  -ffixed-line-length-none
@end smallexample

@item Warning Options
@xref{Warning Options,,Options to Request or Suppress Warnings}.
@smallexample
-fsyntax-only  -pedantic  -pedantic-errors  -fpedantic
-w  -Wno-globals  -Wimplicit -Wunused  -Wuninitialized
-Wall  -Wsurprising
-Werror  -W
@end smallexample

@item Debugging Options
@xref{Debugging Options,,Options for Debugging Your Program or GCC}.
@smallexample
-g
@end smallexample

@item Optimization Options
@xref{Optimize Options,,Options that Control Optimization}.
@smallexample
-malign-double
-ffloat-store  -fforce-mem  -fforce-addr  -fno-inline
-ffast-math  -fstrength-reduce  -frerun-cse-after-loop
-fexpensive-optimizations  -fdelayed-branch
-fschedule-insns  -fschedule-insn2  -fcaller-saves
-funroll-loops  -funroll-all-loops
-fno-move-all-movables  -fno-reduce-all-givs
-fno-rerun-loop-opt
@end smallexample

@item Directory Options
@xref{Directory Options,,Options for Directory Search}.
@smallexample
-I@var{dir}  -I-
@end smallexample

@item Code Generation Options
@xref{Code Gen Options,,Options for Code Generation Conventions}.
@smallexample
-fno-automatic  -finit-local-zero  -fno-f2c
-ff2c-library  -fno-underscoring  -fno-ident
-fpcc-struct-return  -freg-struct-return
-fshort-double  -fno-common  -fpack-struct
-fzeros  -fno-second-underscore
-fdebug-kludge  -femulate-complex
-falias-check  -fargument-alias
-fargument-noalias  -fno-argument-noalias-global
-fno-globals  -fflatten-arrays
-fbounds-check  -ffortran-bounds-check
@end smallexample
@end table

@menu
* Overall Options::     Controlling the kind of output:
                        an executable, object files, assembler files,
                        or preprocessed source.
* Shorthand Options::   Options that are shorthand for other options.
* Fortran Dialect Options::  Controlling the variant of Fortran language
                             compiled.
* Warning Options::     How picky should the compiler be?
* Debugging Options::   Symbol tables, measurements, and debugging dumps.
* Optimize Options::    How much optimization?
* Preprocessor Options:: Controlling header files and macro definitions.
                         Also, getting dependency information for Make.
* Directory Options::   Where to find header files and libraries.
                        Where to find the compiler executable files.
* Code Gen Options::    Specifying conventions for function calls, data layout
                        and register usage.
@end menu

@node Overall Options
@section Options Controlling the Kind of Output
@cindex overall options
@cindex options, overall

Compilation can involve as many as four stages: preprocessing, code
generation (often what is really meant by the term ``compilation''),
assembly, and linking, always in that order.  The first three
stages apply to an individual source file, and end by producing an
object file; linking combines all the object files (those newly
compiled, and those specified as input) into an executable file.

@cindex file name suffix
@cindex suffixes, file name
@cindex file name extension
@cindex extensions, file name
@cindex file type
@cindex types, file
For any given input file, the file name suffix determines what kind of
program is contained in the file---that is, the language in which the
program is written is generally indicated by the suffix.
Suffixes specific to GNU Fortran are listed below.
@xref{Overall Options,,gcc,Using and Porting GNU CC}, for
information on suffixes recognized by GNU CC.

@table @code
@cindex .f filename suffix
@cindex .for filename suffix
@cindex .FOR filename suffix
@item @var{file}.f
@item @var{file}.for
@item @var{file}.FOR
Fortran source code that should not be preprocessed.

Such source code cannot contain any preprocessor directives, such
as @code{#include}, @code{#define}, @code{#if}, and so on.

You can force @samp{.f} files to be preprocessed by @code{cpp} by using
@samp{-x f77-cpp-input}.
@xref{LEX}.

@cindex preprocessor
@cindex C preprocessor
@cindex cpp preprocessor
@cindex Fortran preprocessor
@cindex cpp program
@cindex programs, cpp
@cindex .F filename suffix
@cindex .fpp filename suffix
@cindex .FPP filename suffix
@item @var{file}.F
@item @var{file}.fpp
@item @var{file}.FPP
Fortran source code that must be preprocessed (by the C preprocessor
@code{cpp}, which is part of GNU CC).

Note that preprocessing is not extended to the contents of
files included by the @code{INCLUDE} directive---the @code{#include}
preprocessor directive must be used instead.

@cindex Ratfor preprocessor
@cindex programs, @code{ratfor}
@cindex @samp{.r} filename suffix
@cindex @code{ratfor}
@item @var{file}.r
Ratfor source code, which must be preprocessed by the @code{ratfor}
command, which is available separately (as it is not yet part of the GNU
Fortran distribution).
One version in Fortran, adapted for use with @code{g77}, is at
@uref{ftp://members.aol.com/n8tm/rat7.uue} (of uncertain copyright
status).  Another, public domain version in C is at
@uref{http://sepwww.stanford.edu/sep/prof/ratfor.shar.2}.
@end table

UNIX users typically use the @file{@var{file}.f} and @file{@var{file}.F}
nomenclature.
Users of other operating systems, especially those that cannot
distinguish upper-case
letters from lower-case letters in their file names, typically use
the @file{@var{file}.for} and @file{@var{file}.fpp} nomenclature.

@cindex #define
@cindex #include
@cindex #if
Use of the preprocessor @code{cpp} allows use of C-like
constructs such as @code{#define} and @code{#include}, but can
lead to unexpected, even mistaken, results due to Fortran's source file
format.
It is recommended that use of the C preprocessor
be limited to @code{#include} and, in
conjunction with @code{#define}, only @code{#if} and related directives,
thus avoiding in-line macro expansion entirely.
This recommendation applies especially
when using the traditional fixed source form.
With free source form,
fewer unexpected transformations are likely to happen, but use of
constructs such as Hollerith and character constants can nevertheless
present problems, especially when these are continued across multiple
source lines.
These problems result, primarily, from differences between the way
such constants are interpreted by the C preprocessor and by a Fortran
compiler.

Another example of a problem that results from using the C preprocessor
is that a Fortran comment line that happens to contain any
characters ``interesting'' to the C preprocessor,
such as a backslash at the end of the line,
is not recognized by the preprocessor as a comment line,
so instead of being passed through ``raw'',
the line is edited according to the rules for the preprocessor.
For example, the backslash at the end of the line is removed,
along with the subsequent newline, resulting in the next
line being effectively commented out---unfortunate if that
line is a non-comment line of important code!

@emph{Note:} The @samp{-traditional} and @samp{-undef} flags are supplied
to @code{cpp} by default, to help avoid unpleasant surprises.
@xref{Preprocessor Options,,Options Controlling the Preprocessor,
gcc,Using and Porting GNU CC}.
This means that ANSI C preprocessor features (such as the @samp{#}
operator) aren't available, and only variables in the C reserved
namespace (generally, names with a leading underscore) are liable to
substitution by C predefines.
Thus, if you want to do system-specific
tests, use, for example, @samp{#ifdef __linux__} rather than @samp{#ifdef linux}.
Use the @samp{-v} option to see exactly how the preprocessor is invoked.

@cindex /*
Unfortunately, the @samp{-traditional} flag will not avoid an error from
anything that @code{cpp} sees as an unterminated C comment, such as:
@smallexample
C Some Fortran compilers accept /* as starting
C an inline comment.
@end smallexample
@xref{Trailing Comment}.

The following options that affect overall processing are recognized
by the @code{g77} and @code{gcc} commands in a GNU Fortran installation:

@table @code
@cindex -fversion option
@cindex options, -fversion
@cindex printing version information
@cindex version information, printing
@cindex consistency checks
@cindex internal consistency checks
@cindex checks, of internal consistency
@item -fversion
Ensure that the @code{g77}-specific version of the compiler phase is reported,
if run,
and, starting in @code{egcs} version 1.1,
that internal consistency checks in the @file{f771} program are run.

This option is supplied automatically when @samp{-v} or @samp{--verbose}
is specified as a command-line option for @code{g77} or @code{gcc}
and when the resulting commands compile Fortran source files.

@cindex -fset-g77-defaults option
@cindex options, -fset-g77-defaults
@item -fset-g77-defaults
@emph{Version info:}
This option was obsolete as of @code{egcs}
version 1.1.
The effect is instead achieved
by the @code{lang_init_options} routine
in @file{gcc/gcc/f/com.c}.

@cindex consistency checks
@cindex internal consistency checks
@cindex checks, of internal consistency
Set up whatever @code{gcc} options are to apply to Fortran
compilations, and avoid running internal consistency checks
that might take some time.

This option is supplied automatically when compiling Fortran code
via the @code{g77} or @code{gcc} command.
The description of this option is provided so that users seeing
it in the output of, say, @samp{g77 -v} understand why it is
there.

@cindex modifying g77
@cindex code, modifying
Also, developers who run @code{f771} directly might want to specify it
by hand to get the same defaults as they would running @code{f771}
via @code{g77} or @code{gcc}.
However, such developers should, after linking a new @code{f771}
executable, invoke it without this option once,
e.g. via @kbd{./f771 -quiet < /dev/null},
to ensure that they have not introduced any
internal inconsistencies (such as in the table of
intrinsics) before proceeding---@code{g77} will crash
with a diagnostic if it detects an inconsistency.

@cindex -fno-silent option
@cindex options, -fno-silent
@cindex f2c compatibility
@cindex compatibility, f2c
@cindex status, compilation
@cindex compilation, status
@cindex reporting compilation status
@cindex printing compilation status
@item -fno-silent
Print (to @code{stderr}) the names of the program units as
they are compiled, in a form similar to that used by popular
UNIX @code{f77} implementations and @code{f2c}.
@end table

@xref{Overall Options,,Options Controlling the Kind of Output,
gcc,Using and Porting GNU CC}, for information
on more options that control the overall operation of the @code{gcc} command
(and, by extension, the @code{g77} command).

@node Shorthand Options
@section Shorthand Options
@cindex shorthand options
@cindex options, shorthand
@cindex macro options
@cindex options, macro

The following options serve as ``shorthand''
for other options accepted by the compiler:

@table @code
@cindex -fugly option
@cindex options, -fugly
@item -fugly
@cindex ugly features
@cindex features, ugly
@emph{Note:} This option is no longer supported.
The information, below, is provided to aid
in the conversion of old scripts.

Specify that certain ``ugly'' constructs are to be quietly accepted.
Same as:

@smallexample
-fugly-args -fugly-assign -fugly-assumed
-fugly-comma -fugly-complex -fugly-init
-fugly-logint
@end smallexample

These constructs are considered inappropriate to use in new
or well-maintained portable Fortran code, but widely used
in old code.
@xref{Distensions}, for more information.

@cindex -fno-ugly option
@cindex options, -fno-ugly
@item -fno-ugly
@cindex ugly features
@cindex features, ugly
Specify that all ``ugly'' constructs are to be noisily rejected.
Same as:

@smallexample
-fno-ugly-args -fno-ugly-assign -fno-ugly-assumed
-fno-ugly-comma -fno-ugly-complex -fno-ugly-init
-fno-ugly-logint
@end smallexample

@xref{Distensions}, for more information.

@cindex -ff66 option
@cindex options, -ff66
@item -ff66
@cindex FORTRAN 66
@cindex compatibility, FORTRAN 66
Specify that the program is written in idiomatic FORTRAN 66.
Same as @samp{-fonetrip -fugly-assumed}.

The @samp{-fno-f66} option is the inverse of @samp{-ff66}.
As such, it is the same as @samp{-fno-onetrip -fno-ugly-assumed}.

The meaning of this option is likely to be refined as future
versions of @code{g77} provide more compatibility with other
existing and obsolete Fortran implementations.

@cindex -ff77 option
@cindex options, -ff77
@item -ff77
@cindex UNIX f77
@cindex f2c compatibility
@cindex compatibility, f2c
@cindex f77 compatibility
@cindex compatibility, f77
Specify that the program is written in idiomatic UNIX FORTRAN 77
and/or the dialect accepted by the @code{f2c} product.
Same as @samp{-fbackslash -fno-typeless-boz}.

The meaning of this option is likely to be refined as future
versions of @code{g77} provide more compatibility with other
existing and obsolete Fortran implementations.

@cindex -fno-f77 option
@cindex options, -fno-f77
@item -fno-f77
@cindex UNIX f77
The @samp{-fno-f77} option is @emph{not} the inverse
of @samp{-ff77}.
It specifies that the program is not written in idiomatic UNIX
FORTRAN 77 or @code{f2c}, but in a more widely portable dialect.
@samp{-fno-f77} is the same as @samp{-fno-backslash}.

The meaning of this option is likely to be refined as future
versions of @code{g77} provide more compatibility with other
existing and obsolete Fortran implementations.
@end table

@node Fortran Dialect Options
@section Options Controlling Fortran Dialect
@cindex dialect options
@cindex language, dialect options
@cindex options, dialect

The following options control the dialect of Fortran
that the compiler accepts:

@table @code
@cindex -ffree-form option
@cindex options, -ffree-form
@cindex -fno-fixed-form option
@cindex options, -fno-fixed-form
@cindex source file format
@cindex free form
@cindex fixed form
@cindex Fortran 90, features
@item -ffree-form
@item -fno-fixed-form
Specify that the source file is written in free form
(introduced in Fortran 90) instead of the more-traditional fixed form.

@cindex -ff90 option
@cindex options, -ff90
@cindex Fortran 90, features
@item -ff90
Allow certain Fortran-90 constructs.

This option controls whether certain
Fortran 90 constructs are recognized.
(Other Fortran 90 constructs
might or might not be recognized depending on other options such as
@samp{-fvxt}, @samp{-ff90-intrinsics-enable}, and the
current level of support for Fortran 90.)

@xref{Fortran 90}, for more information.

@cindex -fvxt option
@cindex options, -fvxt
@item -fvxt
@cindex Fortran 90, features
@cindex VXT extensions
Specify the treatment of certain constructs that have different
meanings depending on whether the code is written in
GNU Fortran (based on FORTRAN 77 and akin to Fortran 90)
or VXT Fortran (more like VAX FORTRAN).

The default is @samp{-fno-vxt}.
@samp{-fvxt} specifies that the VXT Fortran interpretations
for those constructs are to be chosen.

@xref{VXT Fortran}, for more information.

@cindex -fdollar-ok option
@cindex options, -fdollar-ok
@item -fdollar-ok
@cindex dollar sign
@cindex symbol names
@cindex character set
Allow @samp{$} as a valid character in a symbol name.

@cindex -fno-backslash option
@cindex options, -fno-backslash
@item -fno-backslash
@cindex backslash
@cindex character constants
@cindex Hollerith constants
Specify that @samp{\} is not to be specially interpreted in character
and Hollerith constants a la C and many UNIX Fortran compilers.

For example, with @samp{-fbackslash} in effect, @samp{A\nB} specifies
three characters, with the second one being newline.
With @samp{-fno-backslash}, it specifies four characters,
@samp{A}, @samp{\}, @samp{n}, and @samp{B}.

Note that @code{g77} implements a fairly general form of backslash
processing that is incompatible with the narrower forms supported
by some other compilers.
For example, @samp{'A\003B'} is a three-character string in @code{g77},
whereas other compilers that support backslash might not support
the three-octal-digit form, and thus treat that string as longer
than three characters.

@xref{Backslash in Constants}, for
information on why @samp{-fbackslash} is the default
instead of @samp{-fno-backslash}.

@cindex -fno-ugly-args option
@cindex options, -fno-ugly-args
@item -fno-ugly-args
Disallow passing Hollerith and typeless constants as actual
arguments (for example, @samp{CALL FOO(4HABCD)}).

@xref{Ugly Implicit Argument Conversion}, for more information.

@cindex -fugly-assign option
@cindex options, -fugly-assign
@item -fugly-assign
Use the same storage for a given variable regardless of
whether it is used to hold an assigned-statement label
(as in @samp{ASSIGN 10 TO I}) or used to hold numeric data
(as in @samp{I = 3}).

@xref{Ugly Assigned Labels}, for more information.

@cindex -fugly-assumed option
@cindex options, -fugly-assumed
@item -fugly-assumed
Assume any dummy array with a final dimension specified as @samp{1}
is really an assumed-size array, as if @samp{*} had been specified
for the final dimension instead of @samp{1}.

For example, @samp{DIMENSION X(1)} is treated as if it
had read @samp{DIMENSION X(*)}.

@xref{Ugly Assumed-Size Arrays}, for more information.

@cindex -fugly-comma option
@cindex options, -fugly-comma
@item -fugly-comma
In an external-procedure invocation,
treat a trailing comma in the argument list
as specification of a trailing null argument,
and treat an empty argument list
as specification of a single null argument.

For example, @samp{CALL FOO(,)} is treated as
@samp{CALL FOO(%VAL(0), %VAL(0))}.
That is, @emph{two} null arguments are specified
by the procedure call when @samp{-fugly-comma} is in force.
And @samp{F = FUNC()} is treated as @samp{F = FUNC(%VAL(0))}.

The default behavior, @samp{-fno-ugly-comma}, is to ignore
a single trailing comma in an argument list.
So, by default, @samp{CALL FOO(X,)} is treated
exactly the same as @samp{CALL FOO(X)}.

@xref{Ugly Null Arguments}, for more information.

@cindex -fugly-complex option
@cindex options, -fugly-complex
@item -fugly-complex
Do not complain about @samp{REAL(@var{expr})} or
@samp{AIMAG(@var{expr})} when @var{expr} is a @code{COMPLEX}
type other than @code{COMPLEX(KIND=1)}---usually
this is used to permit @code{COMPLEX(KIND=2)}
(@code{DOUBLE COMPLEX}) operands.

The @samp{-ff90} option controls the interpretation
of this construct.

@xref{Ugly Complex Part Extraction}, for more information.

@cindex -fno-ugly-init option
@cindex options, -fno-ugly-init
@item -fno-ugly-init
Disallow use of Hollerith and typeless constants as initial
values (in @code{PARAMETER} and @code{DATA} statements), and
use of character constants to
initialize numeric types and vice versa.

For example, @samp{DATA I/'F'/, CHRVAR/65/, J/4HABCD/} is disallowed by
@samp{-fno-ugly-init}.

@xref{Ugly Conversion of Initializers}, for more information.

@cindex -fugly-logint option
@cindex options, -fugly-logint
@item -fugly-logint
Treat @code{INTEGER} and @code{LOGICAL} variables and
expressions as potential stand-ins for each other.

For example, automatic conversion between @code{INTEGER} and
@code{LOGICAL} is enabled, for many contexts, via this option.

@xref{Ugly Integer Conversions}, for more information.

@cindex -fonetrip option
@cindex options, -fonetrip
@item -fonetrip
@cindex FORTRAN 66
@cindex @code{DO} loops, one-trip
@cindex one-trip @code{DO} loops
@cindex @code{DO} loops, zero-trip
@cindex zero-trip @code{DO} loops
@cindex compatibility, FORTRAN 66
Executable iterative @code{DO} loops are to be executed at
least once each time they are reached.

ANSI FORTRAN 77 and more recent versions of the Fortran standard
specify that the body of an iterative @code{DO} loop is not executed
if the number of iterations calculated from the parameters of the
loop is less than 1.
(For example, @samp{DO 10 I = 1, 0}.)
Such a loop is called a @dfn{zero-trip loop}.

Prior to ANSI FORTRAN 77, many compilers implemented @code{DO} loops
such that the body of a loop would be executed at least once, even
if the iteration count was zero.
Fortran code written assuming this behavior is said to require
@dfn{one-trip loops}.
For example, some code written to the FORTRAN 66 standard
expects this behavior from its @code{DO} loops, although that
standard did not specify this behavior.

The @samp{-fonetrip} option specifies that the source file(s) being
compiled require one-trip loops.

This option affects only those loops specified by the (iterative) @code{DO}
statement and by implied-@code{DO} lists in I/O statements.
Loops specified by implied-@code{DO} lists in @code{DATA} and
specification (non-executable) statements are not affected.

@cindex -ftypeless-boz option
@cindex options, -ftypeless-boz
@cindex prefix-radix constants
@cindex constants, prefix-radix
@cindex constants, types
@cindex types, constants
@item -ftypeless-boz
Specifies that prefix-radix non-decimal constants, such as
@samp{Z'ABCD'}, are typeless instead of @code{INTEGER(KIND=1)}.

You can test for yourself whether a particular compiler treats
the prefix form as @code{INTEGER(KIND=1)} or typeless by running the
following program:

@smallexample
EQUIVALENCE (I, R)
R = Z'ABCD1234'
J = Z'ABCD1234'
IF (J .EQ. I) PRINT *, 'Prefix form is TYPELESS'
IF (J .NE. I) PRINT *, 'Prefix form is INTEGER'
END
@end smallexample

Reports indicate that many compilers process this form as
@code{INTEGER(KIND=1)}, though a few as typeless, and at least one
based on a command-line option specifying some kind of
compatibility.

@cindex -fintrin-case-initcap option
@cindex options, -fintrin-case-initcap
@item -fintrin-case-initcap
@cindex -fintrin-case-upper option
@cindex options, -fintrin-case-upper
@item -fintrin-case-upper
@cindex -fintrin-case-lower option
@cindex options, -fintrin-case-lower
@item -fintrin-case-lower
@cindex -fintrin-case-any option
@cindex options, -fintrin-case-any
@item -fintrin-case-any
Specify expected case for intrinsic names.
@samp{-fintrin-case-lower} is the default.

@cindex -fmatch-case-initcap option
@cindex options, -fmatch-case-initcap
@item -fmatch-case-initcap
@cindex -fmatch-case-upper option
@cindex options, -fmatch-case-upper
@item -fmatch-case-upper
@cindex -fmatch-case-lower option
@cindex options, -fmatch-case-lower
@item -fmatch-case-lower
@cindex -fmatch-case-any option
@cindex options, -fmatch-case-any
@item -fmatch-case-any
Specify expected case for keywords.
@samp{-fmatch-case-lower} is the default.

@cindex -fsource-case-upper option
@cindex options, -fsource-case-upper
@item -fsource-case-upper
@cindex -fsource-case-lower option
@cindex options, -fsource-case-lower
@item -fsource-case-lower
@cindex -fsource-case-preserve option
@cindex options, -fsource-case-preserve
@item -fsource-case-preserve
Specify whether source text other than character and Hollerith constants
is to be translated to uppercase, to lowercase, or preserved as is.
@samp{-fsource-case-lower} is the default.

@cindex -fsymbol-case-initcap option
@cindex options, -fsymbol-case-initcap
@item -fsymbol-case-initcap
@cindex -fsymbol-case-upper option
@cindex options, -fsymbol-case-upper
@item -fsymbol-case-upper
@cindex -fsymbol-case-lower option
@cindex options, -fsymbol-case-lower
@item -fsymbol-case-lower
@cindex -fsymbol-case-any option
@cindex options, -fsymbol-case-any
@item -fsymbol-case-any
Specify valid cases for user-defined symbol names.
@samp{-fsymbol-case-any} is the default.

@cindex -fcase-strict-upper option
@cindex options, -fcase-strict-upper
@item -fcase-strict-upper
Same as @samp{-fintrin-case-upper -fmatch-case-upper -fsource-case-preserve
-fsymbol-case-upper}.
(Requires all pertinent source to be in uppercase.)

@cindex -fcase-strict-lower option
@cindex options, -fcase-strict-lower
@item -fcase-strict-lower
Same as @samp{-fintrin-case-lower -fmatch-case-lower -fsource-case-preserve
-fsymbol-case-lower}.
(Requires all pertinent source to be in lowercase.)

@cindex -fcase-initcap option
@cindex options, -fcase-initcap
@item -fcase-initcap
Same as @samp{-fintrin-case-initcap -fmatch-case-initcap -fsource-case-preserve
-fsymbol-case-initcap}.
(Requires all pertinent source to be in initial capitals,
as in @samp{Print *,SqRt(Value)}.)

@cindex -fcase-upper option
@cindex options, -fcase-upper
@item -fcase-upper
Same as @samp{-fintrin-case-any -fmatch-case-any -fsource-case-upper
-fsymbol-case-any}.
(Maps all pertinent source to uppercase.)

@cindex -fcase-lower option
@cindex options, -fcase-lower
@item -fcase-lower
Same as @samp{-fintrin-case-any -fmatch-case-any -fsource-case-lower
-fsymbol-case-any}.
(Maps all pertinent source to lowercase.)

@cindex -fcase-preserve option
@cindex options, -fcase-preserve
@item -fcase-preserve
Same as @samp{-fintrin-case-any -fmatch-case-any -fsource-case-preserve
-fsymbol-case-any}.
(Preserves all case in user-defined symbols,
while allowing any-case matching of intrinsics and keywords.
For example, @samp{call Foo(i,I)} would pass two @emph{different}
variables named @samp{i} and @samp{I} to a procedure named @samp{Foo}.)

@cindex -fbadu77-intrinsics-delete option
@cindex options, -fbadu77-intrinsics-delete
@item -fbadu77-intrinsics-delete
@cindex -fbadu77-intrinsics-hide option
@cindex options, -fbadu77-intrinsics-hide
@item -fbadu77-intrinsics-hide
@cindex -fbadu77-intrinsics-disable option
@cindex options, -fbadu77-intrinsics-disable
@item -fbadu77-intrinsics-disable
@cindex -fbadu77-intrinsics-enable option
@cindex options, -fbadu77-intrinsics-enable
@item -fbadu77-intrinsics-enable
@cindex @code{badu77} intrinsics
@cindex intrinsics, @code{badu77}
Specify status of UNIX intrinsics having inappropriate forms.
@samp{-fbadu77-intrinsics-enable} is the default.
@xref{Intrinsic Groups}.

@cindex -ff2c-intrinsics-delete option
@cindex options, -ff2c-intrinsics-delete
@item -ff2c-intrinsics-delete
@cindex -ff2c-intrinsics-hide option
@cindex options, -ff2c-intrinsics-hide
@item -ff2c-intrinsics-hide
@cindex -ff2c-intrinsics-disable option
@cindex options, -ff2c-intrinsics-disable
@item -ff2c-intrinsics-disable
@cindex -ff2c-intrinsics-enable option
@cindex options, -ff2c-intrinsics-enable
@item -ff2c-intrinsics-enable
@cindex @code{f2c} intrinsics
@cindex intrinsics, @code{f2c}
Specify status of f2c-specific intrinsics.
@samp{-ff2c-intrinsics-enable} is the default.
@xref{Intrinsic Groups}.

@cindex -ff90-intrinsics-delete option
@cindex options, -ff90-intrinsics-delete
@item -ff90-intrinsics-delete
@cindex -ff90-intrinsics-hide option
@cindex options, -ff90-intrinsics-hide
@item -ff90-intrinsics-hide
@cindex -ff90-intrinsics-disable option
@cindex options, -ff90-intrinsics-disable
@item -ff90-intrinsics-disable
@cindex -ff90-intrinsics-enable option
@cindex options, -ff90-intrinsics-enable
@item -ff90-intrinsics-enable
@cindex Fortran 90, intrinsics
@cindex intrinsics, Fortran 90
Specify status of F90-specific intrinsics.
@samp{-ff90-intrinsics-enable} is the default.
@xref{Intrinsic Groups}.

@cindex -fgnu-intrinsics-delete option
@cindex options, -fgnu-intrinsics-delete
@item -fgnu-intrinsics-delete
@cindex -fgnu-intrinsics-hide option
@cindex options, -fgnu-intrinsics-hide
@item -fgnu-intrinsics-hide
@cindex -fgnu-intrinsics-disable option
@cindex options, -fgnu-intrinsics-disable
@item -fgnu-intrinsics-disable
@cindex -fgnu-intrinsics-enable option
@cindex options, -fgnu-intrinsics-enable
@item -fgnu-intrinsics-enable
@cindex Digital Fortran features
@cindex @code{COMPLEX} intrinsics
@cindex intrinsics, @code{COMPLEX}
Specify status of Digital's COMPLEX-related intrinsics.
@samp{-fgnu-intrinsics-enable} is the default.
@xref{Intrinsic Groups}.

@cindex -fmil-intrinsics-delete option
@cindex options, -fmil-intrinsics-delete
@item -fmil-intrinsics-delete
@cindex -fmil-intrinsics-hide option
@cindex options, -fmil-intrinsics-hide
@item -fmil-intrinsics-hide
@cindex -fmil-intrinsics-disable option
@cindex options, -fmil-intrinsics-disable
@item -fmil-intrinsics-disable
@cindex -fmil-intrinsics-enable option
@cindex options, -fmil-intrinsics-enable
@item -fmil-intrinsics-enable
@cindex MIL-STD 1753
@cindex intrinsics, MIL-STD 1753
Specify status of MIL-STD-1753-specific intrinsics.
@samp{-fmil-intrinsics-enable} is the default.
@xref{Intrinsic Groups}.

@cindex -funix-intrinsics-delete option
@cindex options, -funix-intrinsics-delete
@item -funix-intrinsics-delete
@cindex -funix-intrinsics-hide option
@cindex options, -funix-intrinsics-hide
@item -funix-intrinsics-hide
@cindex -funix-intrinsics-disable option
@cindex options, -funix-intrinsics-disable
@item -funix-intrinsics-disable
@cindex -funix-intrinsics-enable option
@cindex options, -funix-intrinsics-enable
@item -funix-intrinsics-enable
@cindex UNIX intrinsics
@cindex intrinsics, UNIX
Specify status of UNIX intrinsics.
@samp{-funix-intrinsics-enable} is the default.
@xref{Intrinsic Groups}.

@cindex -fvxt-intrinsics-delete option
@cindex options, -fvxt-intrinsics-delete
@item -fvxt-intrinsics-delete
@cindex -fvxt-intrinsics-hide option
@cindex options, -fvxt-intrinsics-hide
@item -fvxt-intrinsics-hide
@cindex -fvxt-intrinsics-disable option
@cindex options, -fvxt-intrinsics-disable
@item -fvxt-intrinsics-disable
@cindex -fvxt-intrinsics-enable option
@cindex options, -fvxt-intrinsics-enable
@item -fvxt-intrinsics-enable
@cindex VXT intrinsics
@cindex intrinsics, VXT
Specify status of VXT intrinsics.
@samp{-fvxt-intrinsics-enable} is the default.
@xref{Intrinsic Groups}.

@cindex -ffixed-line-length-@var{n} option
@cindex options, -ffixed-line-length-@var{n}
@item -ffixed-line-length-@var{n}
@cindex source file format
@cindex lines, length
@cindex length of source lines
@cindex fixed form
@cindex limits, lengths of source lines
Set column after which characters are ignored in typical fixed-form
lines in the source file, and through which spaces are assumed (as
if padded to that length) after the ends of short fixed-form lines.

@cindex card image
@cindex extended-source option
Popular values for @var{n} include 72 (the
standard and the default), 80 (card image), and 132 (corresponds
to ``extended-source'' options in some popular compilers).
@var{n} may be @samp{none}, meaning that the entire line is meaningful
and that continued character constants never have implicit spaces appended
to them to fill out the line.
@samp{-ffixed-line-length-0} means the same thing as
@samp{-ffixed-line-length-none}.

@xref{Source Form}, for more information.
@end table

@node Warning Options
@section Options to Request or Suppress Warnings
@cindex options, warnings
@cindex warnings, suppressing
@cindex messages, warning
@cindex suppressing warnings

Warnings are diagnostic messages that report constructions which
are not inherently erroneous but which are risky or suggest there
might have been an error.

You can request many specific warnings with options beginning @samp{-W},
for example @samp{-Wimplicit} to request warnings on implicit
declarations.  Each of these specific warning options also has a
negative form beginning @samp{-Wno-} to turn off warnings;
for example, @samp{-Wno-implicit}.  This manual lists only one of the
two forms, whichever is not the default.

These options control the amount and kinds of warnings produced by GNU
Fortran:

@table @code
@cindex syntax checking
@cindex -fsyntax-only option
@cindex options, -fsyntax-only
@item -fsyntax-only
Check the code for syntax errors, but don't do anything beyond that.

@cindex -pedantic option
@cindex options, -pedantic
@item -pedantic
Issue warnings for uses of extensions to ANSI FORTRAN 77.
@samp{-pedantic} also applies to C-language constructs where they
occur in GNU Fortran source files, such as use of @samp{\e} in a
character constant within a directive like @samp{#include}.

Valid ANSI FORTRAN 77 programs should compile properly with or without
this option.
However, without this option, certain GNU extensions and traditional
Fortran features are supported as well.
With this option, many of them are rejected.

Some users try to use @samp{-pedantic} to check programs for strict ANSI
conformance.
They soon find that it does not do quite what they want---it finds some
non-ANSI practices, but not all.
However, improvements to @code{g77} in this area are welcome.

@cindex -pedantic-errors option
@cindex options, -pedantic-errors
@item -pedantic-errors
Like @samp{-pedantic}, except that errors are produced rather than
warnings.

@cindex -fpedantic option
@cindex options, -fpedantic
@item -fpedantic
Like @samp{-pedantic}, but applies only to Fortran constructs.

@cindex -w option
@cindex options, -w
@item -w
Inhibit all warning messages.

@cindex -Wno-globals option
@cindex options, -Wno-globals
@item -Wno-globals
@cindex global names, warning
@cindex warnings, global names
Inhibit warnings about use of a name as both a global name
(a subroutine, function, or block data program unit, or a
common block) and implicitly as the name of an intrinsic
in a source file.

Also inhibit warnings about inconsistent invocations and/or
definitions of global procedures (function and subroutines).
Such inconsistencies include different numbers of arguments
and different types of arguments.

@cindex -Wimplicit option
@cindex options, -Wimplicit
@item -Wimplicit
@cindex implicit declaration, warning
@cindex warnings, implicit declaration
@cindex -u option
@cindex /WARNINGS=DECLARATIONS switch
@cindex IMPLICIT NONE, similar effect
@cindex effecting IMPLICIT NONE
Warn whenever a variable, array, or function is implicitly
declared.
Has an effect similar to using the @code{IMPLICIT NONE} statement
in every program unit.
(Some Fortran compilers provide this feature by an option
named @samp{-u} or @samp{/WARNINGS=DECLARATIONS}.)

@cindex -Wunused option
@cindex options, -Wunused
@item -Wunused
@cindex unused variables
@cindex variables, unused
Warn whenever a variable is unused aside from its declaration.

@cindex -Wuninitialized option
@cindex options, -Wuninitialized
@item -Wuninitialized
@cindex uninitialized variables
@cindex variables, uninitialized
Warn whenever an automatic variable is used without first being initialized.

These warnings are possible only in optimizing compilation,
because they require data-flow information that is computed only
when optimizing.  If you don't specify @samp{-O}, you simply won't
get these warnings.

These warnings occur only for variables that are candidates for
register allocation.  Therefore, they do not occur for a variable
@c that is declared @code{VOLATILE}, or
whose address is taken, or whose size
is other than 1, 2, 4 or 8 bytes.  Also, they do not occur for
arrays, even when they are in registers.

Note that there might be no warning about a variable that is used only
to compute a value that itself is never used, because such
computations may be deleted by data-flow analysis before the warnings
are printed.

These warnings are made optional because GNU Fortran is not smart
enough to see all the reasons why the code might be correct
despite appearing to have an error.  Here is one example of how
this can happen:

@example
SUBROUTINE DISPAT(J)
IF (J.EQ.1) I=1
IF (J.EQ.2) I=4
IF (J.EQ.3) I=5
CALL FOO(I)
END
@end example

@noindent
If the value of @code{J} is always 1, 2 or 3, then @code{I} is
always initialized, but GNU Fortran doesn't know this.  Here is
another common case:

@example
SUBROUTINE MAYBE(FLAG)
LOGICAL FLAG
IF (FLAG) VALUE = 9.4
@dots{}
IF (FLAG) PRINT *, VALUE
END
@end example

@noindent
This has no bug because @code{VALUE} is used only if it is set.

@cindex -Wall option
@cindex options, -Wall
@item -Wall
@cindex all warnings
@cindex warnings, all
The @samp{-Wunused} and @samp{-Wuninitialized} options combined.
These are all the
options which pertain to usage that we recommend avoiding and that we
believe is easy to avoid.
(As more warnings are added to @code{g77}, some might
be added to the list enabled by @samp{-Wall}.)
@end table

The remaining @samp{-W@dots{}} options are not implied by @samp{-Wall}
because they warn about constructions that we consider reasonable to
use, on occasion, in clean programs.

@table @code
@c @item -W
@c Print extra warning messages for these events:
@c
@c @itemize @bullet
@c @item
@c If @samp{-Wall} or @samp{-Wunused} is also specified, warn about unused
@c arguments.
@c
@c @end itemize
@c
@cindex -Wsurprising option
@cindex options, -Wsurprising
@item -Wsurprising
Warn about ``suspicious'' constructs that are interpreted
by the compiler in a way that might well be surprising to
someone reading the code.
These differences can result in subtle, compiler-dependent
(even machine-dependent) behavioral differences.
The constructs warned about include:

@itemize @bullet
@item
Expressions having two arithmetic operators in a row, such
as @samp{X*-Y}.
Such a construct is nonstandard, and can produce
unexpected results in more complicated situations such
as @samp{X**-Y*Z}.
@code{g77}, along with many other compilers, interprets
this example differently than many programmers, and a few
other compilers.
Specifically, @code{g77} interprets @samp{X**-Y*Z} as
@samp{(X**(-Y))*Z}, while others might think it should
be interpreted as @samp{X**(-(Y*Z))}.

A revealing example is the constant expression @samp{2**-2*1.},
which @code{g77} evaluates to .25, while others might evaluate
it to 0., the difference resulting from the way precedence affects
type promotion.

(The @samp{-fpedantic} option also warns about expressions
having two arithmetic operators in a row.)

@item
Expressions with a unary minus followed by an operand and then
a binary operator other than plus or minus.
For example, @samp{-2**2} produces a warning, because
the precedence is @samp{-(2**2)}, yielding -4, not
@samp{(-2)**2}, which yields 4, and which might represent
what a programmer expects.

An example of an expression producing different results
in a surprising way is @samp{-I*S}, where @var{I} holds
the value @samp{-2147483648} and @var{S} holds @samp{0.5}.
On many systems, negating @var{I} results in the same
value, not a positive number, because it is already the
lower bound of what an @code{INTEGER(KIND=1)} variable can hold.
So, the expression evaluates to a positive number, while
the ``expected'' interpretation, @samp{(-I)*S}, would
evaluate to a negative number.

Even cases such as @samp{-I*J} produce warnings,
even though, in most configurations and situations,
there is no computational difference between the
results of the two interpretations---the purpose
of this warning is to warn about differing interpretations
and encourage a better style of coding, not to identify
only those places where bugs might exist in the user's
code.

@cindex DO statement
@cindex statements, DO
@item
@code{DO} loops with @code{DO} variables that are not
of integral type---that is, using @code{REAL}
variables as loop control variables.
Although such loops can be written to work in the
``obvious'' way, the way @code{g77} is required by the
Fortran standard to interpret such code is likely to
be quite different from the way many programmers expect.
(This is true of all @code{DO} loops, but the differences
are pronounced for non-integral loop control variables.)

@xref{Loops}, for more information.
@end itemize

@cindex -Werror option
@cindex options, -Werror
@item -Werror
Make all warnings into errors.

@cindex -W option
@cindex options, -W
@item -W
@cindex extra warnings
@cindex warnings, extra
Turns on ``extra warnings'' and, if optimization is specified
via @samp{-O}, the @samp{-Wuninitialized} option.
(This might change in future versions of @code{g77}.)

``Extra warnings'' are issued for:

@itemize @bullet
@item
@cindex unused parameters
@cindex parameters, unused
@cindex unused arguments
@cindex arguments, unused
@cindex unused dummies
@cindex dummies, unused
Unused parameters to a procedure (when @samp{-Wunused} also is
specified).

@item
@cindex overflow
Overflows involving floating-point constants (not available
for certain configurations).
@end itemize
@end table

@xref{Warning Options,,Options to Request or Suppress Warnings,
gcc,Using and Porting GNU CC}, for information on more options offered
by the GBE shared by @code{g77}, @code{gcc}, and other GNU compilers.

Some of these have no effect when compiling programs written in Fortran:

@table @code
@cindex -Wcomment option
@cindex options, -Wcomment
@item -Wcomment
@cindex -Wformat option
@cindex options, -Wformat
@item -Wformat
@cindex -Wparentheses option
@cindex options, -Wparentheses
@item -Wparentheses
@cindex -Wswitch option
@cindex options, -Wswitch
@item -Wswitch
@cindex -Wtraditional option
@cindex options, -Wtraditional
@item -Wtraditional
@cindex -Wshadow option
@cindex options, -Wshadow
@item -Wshadow
@cindex -Wid-clash-@var{len} option
@cindex options, -Wid-clash-@var{len}
@item -Wid-clash-@var{len}
@cindex -Wlarger-than-@var{len} option
@cindex options, -Wlarger-than-@var{len}
@item -Wlarger-than-@var{len}
@cindex -Wconversion option
@cindex options, -Wconversion
@item -Wconversion
@cindex -Waggregate-return option
@cindex options, -Waggregate-return
@item -Waggregate-return
@cindex -Wredundant-decls option
@cindex options, -Wredundant-decls
@item -Wredundant-decls
@cindex unsupported warnings
@cindex warnings, unsupported
These options all could have some relevant meaning for
GNU Fortran programs, but are not yet supported.
@end table

@node Debugging Options
@section Options for Debugging Your Program or GNU Fortran
@cindex options, debugging
@cindex debugging information options

GNU Fortran has various special options that are used for debugging
either your program or @code{g77}.

@table @code
@cindex -g option
@cindex options, -g
@item -g
Produce debugging information in the operating system's native format
(stabs, COFF, XCOFF, or DWARF).  GDB can work with this debugging
information.

@cindex common blocks
@cindex equivalence areas
@cindex missing debug features
Support for this option in Fortran programs was incomplete up till
version 0.5.26 of @code{g77}.
In particular, names of variables and arrays in common blocks
or that are storage-associated via @code{EQUIVALENCE} were
unavailable to the debugger.

However, version 0.5.19 of @code{g77} does provide this information
in a rudimentary way, as controlled by the
@samp{-fdebug-kludge} option.

Because version 0.5.26 of @code{g77} enables full debug information
of COMMON BLOCK and EQUIVALENCE items, this option has been disabled.

@xref{Code Gen Options,,Options for Code Generation Conventions},
for more information.
@end table

@xref{Debugging Options,,Options for Debugging Your Program or GNU CC,
gcc,Using and Porting GNU CC}, for more information on debugging options.

@node Optimize Options
@section Options That Control Optimization
@cindex optimize options
@cindex options, optimization

Most Fortran users will want to use no optimization when
developing and testing programs, and use @samp{-O} or @samp{-O2} when
compiling programs for late-cycle testing and for production use.
However, note that certain diagnostics---such as for uninitialized
variables---depend on the flow analysis done by @samp{-O}, i.e.@: you
must use @samp{-O} or @samp{-O2} to get such diagnostics.

The following flags have particular applicability when
compiling Fortran programs:

@table @code
@cindex -malign-double option
@cindex options, -malign-double
@item -malign-double
(Intel x86 architecture only.)

Noticeably improves performance of @code{g77} programs making
heavy use of @code{REAL(KIND=2)} (@code{DOUBLE PRECISION}) data
on some systems.
In particular, systems using Pentium, Pentium Pro, 586, and
686 implementations
of the i386 architecture execute programs faster when
@code{REAL(KIND=2)} (@code{DOUBLE PRECISION}) data are
aligned on 64-bit boundaries
in memory.

This option can, at least, make benchmark results more consistent
across various system configurations, versions of the program,
and data sets.

@emph{Note:} The warning in the @code{gcc} documentation about
this option does not apply, generally speaking, to Fortran
code compiled by @code{g77}.

@xref{Aligned Data}, for more information on alignment issues.

@emph{Also also note:} The negative form of @samp{-malign-double}
is @samp{-mno-align-double}, not @samp{-benign-double}.

@cindex -ffloat-store option
@cindex options, -ffloat-store
@item -ffloat-store
@cindex IEEE 754 conformance
@cindex conformance, IEEE 754
@cindex floating-point, precision
Might help a Fortran program that depends on exact IEEE conformance on
some machines, but might slow down a program that doesn't.

This option is effective when the floating-point unit is set to work in
IEEE 854 `extended precision'---as it typically is on x86 and m68k GNU
systems---rather than IEEE 754 double precision.  @samp{-ffloat-store}
tries to remove the extra precision by spilling data from floating-point
registers into memory and this typically involves a big performance
hit.  However, it doesn't affect intermediate results, so that it is
only partially effective.  `Excess precision' is avoided in code like:
@smallexample
a = b + c
d = a * e
@end smallexample
but not in code like:
@smallexample
      d = (b + c) * e
@end smallexample

For another, potentially better, way of controlling the precision,
see @ref{Floating-point precision}.

@cindex -fforce-mem option
@cindex options, -fforce-mem
@item -fforce-mem
@cindex -fforce-addr option
@cindex options, -fforce-addr
@item -fforce-addr
@cindex loops, speeding up
@cindex speed, of loops
Might improve optimization of loops.

@cindex -fno-inline option
@cindex options, -fno-inline
@item -fno-inline
@cindex in-line code
@cindex compilation, in-line
@c DL: Only relevant for -O3?
Don't compile statement functions inline.
Might reduce the size of a program unit---which might be at
expense of some speed (though it should compile faster).
Note that if you are not optimizing, no functions can be expanded inline.

@cindex -ffast-math option
@cindex options, -ffast-math
@item -ffast-math
@cindex IEEE 754 conformance
@cindex conformance, IEEE 754
Might allow some programs designed to not be too dependent
on IEEE behavior for floating-point to run faster, or die trying.

@cindex -fstrength-reduce option
@cindex options, -fstrength-reduce
@item -fstrength-reduce
@cindex loops, speeding up
@cindex speed, of loops
@c DL: normally defaulted?
Might make some loops run faster.

@cindex -frerun-cse-after-loop option
@cindex options, -frerun-cse-after-loop
@item -frerun-cse-after-loop
@cindex -fexpensive-optimizations option
@cindex options, -fexpensive-optimizations
@c DL: This is -O2?
@item -fexpensive-optimizations
@cindex -fdelayed-branch option
@cindex options, -fdelayed-branch
@item -fdelayed-branch
@cindex -fschedule-insns option
@cindex options, -fschedule-insns
@item -fschedule-insns
@cindex -fschedule-insns2 option
@cindex options, -fschedule-insns2
@item -fschedule-insns2
@cindex -fcaller-saves option
@cindex options, -fcaller-saves
@item -fcaller-saves
Might improve performance on some code.

@cindex -funroll-loops option
@cindex options, -funroll-loops
@item -funroll-loops
@cindex loops, unrolling
@cindex unrolling loops
@cindex loops, optimizing
@cindex indexed (iterative) @code{DO}
@cindex iterative @code{DO}
@c DL: fixme: Craig doesn't like `indexed' but f95 doesn't seem to
@c provide a suitable term
@c CB: I've decided on `iterative', for the time being, and changed
@c my previous, rather bizarre, use of `imperative' to that
@c (though `precomputed-trip' would be a more precise adjective)
Typically improves performance on code using iterative @code{DO} loops by
unrolling them and is probably generally appropriate for Fortran, though
it is not turned on at any optimization level.
Note that outer loop unrolling isn't done specifically; decisions about
whether to unroll a loop are made on the basis of its instruction count.

@c DL: Fixme: This should obviously go somewhere else...
Also, no `loop discovery'@footnote{@dfn{loop discovery} refers to the
process by which a compiler, or indeed any reader of a program,
determines which portions of the program are more likely to be executed
repeatedly as it is being run.  Such discovery typically is done early
when compiling using optimization techniques, so the ``discovered''
loops get more attention---and more run-time resources, such as
registers---from the compiler.  It is easy to ``discover'' loops that are
constructed out of looping constructs in the language
(such as Fortran's @code{DO}).  For some programs, ``discovering'' loops
constructed out of lower-level constructs (such as @code{IF} and
@code{GOTO}) can lead to generation of more optimal code
than otherwise.} is done, so only loops written with @code{DO}
benefit from loop optimizations, including---but not limited
to---unrolling.  Loops written with @code{IF} and @code{GOTO} are not
currently recognized as such.  This option unrolls only iterative
@code{DO} loops, not @code{DO WHILE} loops.

@cindex -funroll-all-loops option
@cindex options, -funroll-all-loops
@cindex DO WHILE
@item -funroll-all-loops
@c DL: Check my understanding of -funroll-all-loops v. -funroll-loops is correct.
Probably improves performance on code using @code{DO WHILE} loops by
unrolling them in addition to iterative @code{DO} loops.  In the absence
of @code{DO WHILE}, this option is equivalent to @samp{-funroll-loops}
but possibly slower.

@item -fno-move-all-movables
@cindex -fno-move-all-movables option
@cindex options, -fno-move-all-movables
@item -fno-reduce-all-givs
@cindex -fno-reduce-all-givs option
@cindex options, -fno-reduce-all-givs
@item -fno-rerun-loop-opt
@cindex -fno-rerun-loop-opt option
@cindex options, -fno-rerun-loop-opt
@emph{Version info:}
These options are not supported by
versions of @code{g77} based on @code{gcc} version 2.8.

Each of these might improve performance on some code.

Analysis of Fortran code optimization and the resulting
optimizations triggered by the above options were
contributed by Toon Moene (@email{toon@@moene.indiv.nluug.nl}).

These three options are intended to be removed someday, once
they have helped determine the efficacy of various
approaches to improving the performance of Fortran code.

Please let us know how use of these options affects
the performance of your production code.
We're particularly interested in code that runs faster
when these options are @emph{disabled}, and in
non-Fortran code that benefits when they are
@emph{enabled} via the above @code{gcc} command-line options.
@end table

@xref{Optimize Options,,Options That Control Optimization,
gcc,Using and Porting GNU CC}, for more information on options
to optimize the generated machine code.

@node Preprocessor Options
@section Options Controlling the Preprocessor
@cindex preprocessor options
@cindex options, preprocessor
@cindex cpp program
@cindex programs, cpp

These options control the C preprocessor, which is run on each C source
file before actual compilation.

@xref{Preprocessor Options,,Options Controlling the Preprocessor,
gcc,Using and Porting GNU CC}, for information on C preprocessor options.

@cindex INCLUDE directive
@cindex directive, INCLUDE
Some of these options also affect how @code{g77} processes the
@code{INCLUDE} directive.
Since this directive is processed even when preprocessing
is not requested, it is not described in this section.
@xref{Directory Options,,Options for Directory Search}, for
information on how @code{g77} processes the @code{INCLUDE} directive.

However, the @code{INCLUDE} directive does not apply
preprocessing to the contents of the included file itself.

Therefore, any file that contains preprocessor directives
(such as @code{#include}, @code{#define}, and @code{#if})
must be included via the @code{#include} directive, not
via the @code{INCLUDE} directive.
Therefore, any file containing preprocessor directives,
if included, is necessarily included by a file that itself
contains preprocessor directives.

@node Directory Options
@section Options for Directory Search
@cindex directory, options
@cindex options, directory search
@cindex search path

These options affect how the @code{cpp} preprocessor searches
for files specified via the @code{#include} directive.
Therefore, when compiling Fortran programs, they are meaningful
when the preprocessor is used.

@cindex INCLUDE directive
@cindex directive, INCLUDE
Some of these options also affect how @code{g77} searches
for files specified via the @code{INCLUDE} directive,
although files included by that directive are not,
themselves, preprocessed.
These options are:

@table @code
@cindex -I- option
@cindex options, -I-
@item -I-
@cindex -Idir option
@cindex options, -Idir
@item -I@var{dir}
@cindex directory, search paths for inclusion
@cindex inclusion, directory search paths for
@cindex search paths, for included files
@cindex paths, search
These affect interpretation of the @code{INCLUDE} directive
(as well as of the @code{#include} directive of the @code{cpp}
preprocessor).

Note that @samp{-I@var{dir}} must be specified @emph{without} any
spaces between @samp{-I} and the directory name---that is,
@samp{-Ifoo/bar} is valid, but @samp{-I foo/bar}
is rejected by the @code{g77} compiler (though the preprocessor supports
the latter form).
@c this is due to toplev.c's inflexible option processing
Also note that the general behavior of @samp{-I} and
@code{INCLUDE} is pretty much the same as of @samp{-I} with
@code{#include} in the @code{cpp} preprocessor, with regard to
looking for @file{header.gcc} files and other such things.

@xref{Directory Options,,Options for Directory Search,
gcc,Using and Porting GNU CC}, for information on the @samp{-I} option.
@end table

@node Code Gen Options
@section Options for Code Generation Conventions
@cindex code generation, conventions
@cindex options, code generation
@cindex run-time, options

These machine-independent options control the interface conventions
used in code generation.

Most of them have both positive and negative forms; the negative form
of @samp{-ffoo} would be @samp{-fno-foo}.  In the table below, only
one of the forms is listed---the one which is not the default.  You
can figure out the other form by either removing @samp{no-} or adding
it.

@table @code
@cindex -fno-automatic option
@cindex options, -fno-automatic
@item -fno-automatic
@cindex SAVE statement
@cindex statements, SAVE
Treat each program unit as if the @code{SAVE} statement was specified
for every local variable and array referenced in it.
Does not affect common blocks.
(Some Fortran compilers provide this option under
the name @samp{-static}.)

@cindex -finit-local-zero option
@cindex options, -finit-local-zero
@item -finit-local-zero
@cindex DATA statement
@cindex statements, DATA
@cindex initialization, of local variables
@cindex variables, initialization of
@cindex uninitialized variables
@cindex variables, uninitialized
Specify that variables and arrays that are local to a program unit
(not in a common block and not passed as an argument) are to be initialized
to binary zeros.

Since there is a run-time penalty for initialization of variables
that are not given the @code{SAVE} attribute, it might be a
good idea to also use @samp{-fno-automatic} with @samp{-finit-local-zero}.

@cindex -fno-f2c option
@cindex options, -fno-f2c
@item -fno-f2c
@cindex @code{f2c} compatibility
@cindex compatibility, @code{f2c}
Do not generate code designed to be compatible with code generated
by @code{f2c}; use the GNU calling conventions instead.

The @code{f2c} calling conventions require functions that return
type @code{REAL(KIND=1)} to actually return the C type @code{double},
and functions that return type @code{COMPLEX} to return the
values via an extra argument in the calling sequence that points
to where to store the return value.
Under the GNU calling conventions, such functions simply return
their results as they would in GNU C---@code{REAL(KIND=1)} functions
return the C type @code{float}, and @code{COMPLEX} functions
return the GNU C type @code{complex} (or its @code{struct}
equivalent).

This does not affect the generation of code that interfaces with the
@code{libg2c} library.

However, because the @code{libg2c} library uses @code{f2c}
calling conventions, @code{g77} rejects attempts to pass
intrinsics implemented by routines in this library as actual
arguments when @samp{-fno-f2c} is used, to avoid bugs when
they are actually called by code expecting the GNU calling
conventions to work.

For example, @samp{INTRINSIC ABS;CALL FOO(ABS)} is
rejected when @samp{-fno-f2c} is in force.
(Future versions of the @code{g77} run-time library might
offer routines that provide GNU-callable versions of the
routines that implement the @code{f2c}-callable intrinsics
that may be passed as actual arguments, so that
valid programs need not be rejected when @samp{-fno-f2c}
is used.)

@strong{Caution:} If @samp{-fno-f2c} is used when compiling any
source file used in a program, it must be used when compiling
@emph{all} Fortran source files used in that program.

@c seems kinda dumb to tell people about an option they can't use -- jcb
@c then again, we want users building future-compatible libraries with it.
@cindex -ff2c-library option
@cindex options, -ff2c-library
@item -ff2c-library
Specify that use of @code{libg2c} (or the original @code{libf2c})
is required.
This is the default for the current version of @code{g77}.

Currently it is not
valid to specify @samp{-fno-f2c-library}.
This option is provided so users can specify it in shell
scripts that build programs and libraries that require the
@code{libf2c} library, even when being compiled by future
versions of @code{g77} that might otherwise default to
generating code for an incompatible library.

@cindex -fno-underscoring option
@cindex options, -fno-underscoring
@item -fno-underscoring
@cindex underscore
@cindex symbol names, underscores
@cindex transforming symbol names
@cindex symbol names, transforming
Do not transform names of entities specified in the Fortran
source file by appending underscores to them.

With @samp{-funderscoring} in effect, @code{g77} appends two underscores
to names with underscores and one underscore to external names with
no underscores.  (@code{g77} also appends two underscores to internal
names with underscores to avoid naming collisions with external names.
The @samp{-fno-second-underscore} option disables appending of the
second underscore in all cases.)

This is done to ensure compatibility with code produced by many
UNIX Fortran compilers, including @code{f2c}, which perform the
same transformations.

Use of @samp{-fno-underscoring} is not recommended unless you are
experimenting with issues such as integration of (GNU) Fortran into
existing system environments (vis-a-vis existing libraries, tools, and
so on).

For example, with @samp{-funderscoring}, and assuming other defaults like
@samp{-fcase-lower} and that @samp{j()} and @samp{max_count()} are
external functions while @samp{my_var} and @samp{lvar} are local variables,
a statement like

@smallexample
I = J() + MAX_COUNT (MY_VAR, LVAR)
@end smallexample

@noindent
is implemented as something akin to:

@smallexample
i = j_() + max_count__(&my_var__, &lvar);
@end smallexample

With @samp{-fno-underscoring}, the same statement is implemented as:

@smallexample
i = j() + max_count(&my_var, &lvar);
@end smallexample

Use of @samp{-fno-underscoring} allows direct specification of
user-defined names while debugging and when interfacing @code{g77}-compiled
code with other languages.

Note that just because the names match does @emph{not} mean that the
interface implemented by @code{g77} for an external name matches the
interface implemented by some other language for that same name.
That is, getting code produced by @code{g77} to link to code produced
by some other compiler using this or any other method can be only a
small part of the overall solution---getting the code generated by
both compilers to agree on issues other than naming can require
significant effort, and, unlike naming disagreements, linkers normally
cannot detect disagreements in these other areas.

Also, note that with @samp{-fno-underscoring}, the lack of appended
underscores introduces the very real possibility that a user-defined
external name will conflict with a name in a system library, which
could make finding unresolved-reference bugs quite difficult in some
cases---they might occur at program run time, and show up only as
buggy behavior at run time.

In future versions of @code{g77}, we hope to improve naming and linking
issues so that debugging always involves using the names as they appear
in the source, even if the names as seen by the linker are mangled to
prevent accidental linking between procedures with incompatible
interfaces.

@cindex -fno-second-underscore option
@cindex options, -fno-second-underscore
@item -fno-second-underscore
@cindex underscore
@cindex symbol names, underscores
@cindex transforming symbol names
@cindex symbol names, transforming
Do not append a second underscore to names of entities specified
in the Fortran source file.

This option has no effect if @samp{-fno-underscoring} is
in effect.

Otherwise, with this option, an external name such as @samp{MAX_COUNT}
is implemented as a reference to the link-time external symbol
@samp{max_count_}, instead of @samp{max_count__}.

@cindex -fno-ident option
@cindex options, -fno-ident
@item -fno-ident
Ignore the @samp{#ident} directive.

@cindex -fzeros option
@cindex options, -fzeros
@item -fzeros
Treat initial values of zero as if they were any other value.

As of version 0.5.18, @code{g77} normally treats @code{DATA} and
other statements that are used to specify initial values of zero
for variables and arrays as if no values were actually specified,
in the sense that no diagnostics regarding multiple initializations
are produced.

This is done to speed up compiling of programs that initialize
large arrays to zeros.

Use @samp{-fzeros} to revert to the simpler, slower behavior
that can catch multiple initializations by keeping track of
all initializations, zero or otherwise.

@emph{Caution:} Future versions of @code{g77} might disregard this option
(and its negative form, the default) or interpret it somewhat
differently.
The interpretation changes will affect only non-standard
programs; standard-conforming programs should not be affected.

@cindex -fdebug-kludge option
@cindex options, -fdebug-kludge
@item -fdebug-kludge
Emit information on @code{COMMON} and @code{EQUIVALENCE} members
that might help users of debuggers work around lack of proper debugging
information on such members.

As of version 0.5.19, @code{g77} offers this option to emit
information on members of aggregate areas to help users while debugging.
This information consists of establishing the type and contents of each
such member so that, when a debugger is asked to print the contents,
the printed information provides rudimentary debugging information.
This information identifies the name of the aggregate area (either the
@code{COMMON} block name, or the @code{g77}-assigned name for the
@code{EQUIVALENCE} name) and the offset, in bytes, of the member from
the beginning of the area.

Using @code{gdb}, this information is not coherently displayed in the Fortran
language mode, so temporarily switching to the C language mode to display the
information is suggested.
Use @samp{set language c} and @samp{set language fortran} to accomplish this.

As of version 0.5.26 of @code{g77} this option has been disabled, as the
compiler is now able to emit correct and complete debug information
for COMMON BLOCK and EQUIVALENCE items.

For example:

@smallexample
      COMMON /X/A,B
      EQUIVALENCE (C,D)
      CHARACTER XX*50
      EQUIVALENCE (I,XX(20:20))
      END

GDB is free software and you are welcome to distribute copies of it
 under certain conditions; type "show copying" to see the conditions.
There is absolutely no warranty for GDB; type "show warranty" for details.
GDB 4.16 (lm-gnits-dwim), Copyright 1996 Free Software Foundation, Inc...
(gdb) b MAIN__
Breakpoint 1 at 0t1200000201120112: file cd.f, line 5.
(gdb) r
Starting program: /home/user/a.out

Breakpoint 1, MAIN__ () at cd.f:5
Current language:  auto; currently fortran
(gdb) set language c
Warning: the current language does not match this frame.
(gdb) p a
$2 = "At (COMMON) `x_' plus 0 bytes"
(gdb) p b
$3 = "At (COMMON) `x_' plus 4 bytes"
(gdb) p c
$4 = "At (EQUIVALENCE) `__g77_equiv_c' plus 0 bytes"
(gdb) p d
$5 = "At (EQUIVALENCE) `__g77_equiv_c' plus 0 bytes"
(gdb) p i
$6 = "At (EQUIVALENCE) `__g77_equiv_xx' plus 20 bytes"
(gdb) p xx
$7 = "At (EQUIVALENCE) `__g77_equiv_xx' plus 1 bytes"
(gdb) set language fortran
(gdb)
@end smallexample

@noindent
Use @samp{-fdebug-kludge} to generate this information,
which might make some programs noticeably larger.

@emph{Caution:} Future versions of @code{g77} might disregard this option
(and its negative form).
Current plans call for this to happen when published versions of @code{g77}
and @code{gdb} exist that provide proper access to debugging information on
@code{COMMON} and @code{EQUIVALENCE} members.  This is believed to have
happened as of version 0.5.26 of @code{g77}, so that this option has been
disabled starting with this release.

@cindex -femulate-complex option
@cindex options, -femulate-complex
@item -femulate-complex
Implement @code{COMPLEX} arithmetic via emulation,
instead of using the facilities of
the @code{gcc} back end that provide direct support of
@code{complex} arithmetic.

(@code{gcc} had some bugs in its back-end support
for @code{complex} arithmetic, due primarily to the support not being
completed as of version 2.8.1 and @code{egcs} 1.1.2.)

Use @samp{-femulate-complex} if you suspect code-generation bugs,
or experience compiler crashes,
that might result from @code{g77} using the @code{COMPLEX} support
in the @code{gcc} back end.
If using that option fixes the bugs or crashes you are seeing,
that indicates a likely @code{g77} bugs
(though, all compiler crashes are considered bugs),
so, please report it.
(Note that the known bugs, now believed fixed, produced compiler crashes
rather than causing the generation of incorrect code.)

Use of this option should not affect how Fortran code compiled
by @code{g77} works in terms of its interfaces to other code,
e.g. that compiled by @code{f2c}.

@emph{Caution:} Future versions of @code{g77} might ignore both forms
of this option.

@cindex -falias-check option
@cindex options, -falias-check
@cindex -fargument-alias option
@cindex options, -fargument-alias
@cindex -fargument-noalias option
@cindex options, -fargument-noalias
@cindex -fno-argument-noalias-global option
@cindex options, -fno-argument-noalias-global
@item -falias-check
@item -fargument-alias
@item -fargument-noalias
@item -fno-argument-noalias-global
@emph{Version info:}
These options are not supported by
versions of @code{g77} based on @code{gcc} version 2.8.

These options specify to what degree aliasing
(overlap)
is permitted between
arguments (passed as pointers) and @code{COMMON} (external, or
public) storage.

The default for Fortran code, as mandated by the FORTRAN 77 and
Fortran 90 standards, is @samp{-fargument-noalias-global}.
The default for code written in the C language family is
@samp{-fargument-alias}.

Note that, on some systems, compiling with @samp{-fforce-addr} in
effect can produce more optimal code when the default aliasing
options are in effect (and when optimization is enabled).

@xref{Aliasing Assumed To Work}, for detailed information on the implications
of compiling Fortran code that depends on the ability to alias dummy
arguments.

@cindex -fno-globals option
@cindex options, -fno-globals
@item -fno-globals
@cindex global names, warning
@cindex warnings, global names
@cindex in-line code
@cindex compilation, in-line
Disable diagnostics about inter-procedural
analysis problems, such as disagreements about the
type of a function or a procedure's argument,
that might cause a compiler crash when attempting
to inline a reference to a procedure within a
program unit.
(The diagnostics themselves are still produced, but
as warnings, unless @samp{-Wno-globals} is specified,
in which case no relevant diagnostics are produced.)

Further, this option disables such inlining, to
avoid compiler crashes resulting from incorrect
code that would otherwise be diagnosed.

As such, this option might be quite useful when
compiling existing, ``working'' code that happens
to have a few bugs that do not generally show themselves,
but which @code{g77} diagnoses.

Use of this option therefore has the effect of
instructing @code{g77} to behave more like it did
up through version 0.5.19.1, when it paid little or
no attention to disagreements between program units
about a procedure's type and argument information,
and when it performed no inlining of procedures
(except statement functions).

Without this option, @code{g77} defaults to performing
the potentially inlining procedures as it started doing
in version 0.5.20, but as of version 0.5.21, it also
diagnoses disagreements that might cause such inlining
to crash the compiler as (fatal) errors,
and warns about similar disagreements
that are currently believed to not
likely to result in the compiler later crashing
or producing incorrect code.

@cindex -fflatten-arrays option
@item -fflatten-arrays
@cindex array performance
@cindex arrays, flattening
Use back end's C-like constructs
(pointer plus offset)
instead of its @code{ARRAY_REF} construct
to handle all array references.

@emph{Note:} This option is not supported.
It is intended for use only by @code{g77} developers,
to evaluate code-generation issues.
It might be removed at any time.

@cindex -fbounds-check option
@cindex -ffortran-bounds-check option
@item -fbounds-check
@itemx -ffortran-bounds-check
@cindex bounds checking
@cindex range checking
@cindex array bounds checking
@cindex subscript checking
@cindex substring checking
@cindex checking subscripts
@cindex checking substrings
Enable generation of run-time checks for array subscripts
and substring start and end points
against the (locally) declared minimum and maximum values.

The current implementation uses the @code{libf2c}
library routine @code{s_rnge} to print the diagnostic.

However, whereas @code{f2c} generates a single check per
reference for a multi-dimensional array, of the computed
offset against the valid offset range (0 through the size of the array),
@code{g77} generates a single check per @emph{subscript} expression.
This catches some cases of potential bugs that @code{f2c} does not,
such as references to below the beginning of an assumed-size array.

@code{g77} also generates checks for @code{CHARACTER} substring references,
something @code{f2c} currently does not do.

Use the new @samp{-ffortran-bounds-check} option
to specify bounds-checking for only the Fortran code you are compiling,
not necessarily for code written in other languages.

@emph{Note:} To provide more detailed information on the offending subscript,
@code{g77} provides the @code{libg2c} run-time library routine @code{s_rnge}
with somewhat differently-formatted information.
Here's a sample diagnostic:

@smallexample
Subscript out of range on file line 4, procedure rnge.f/bf.
Attempt to access the -6-th element of variable b[subscript-2-of-2].
Aborted
@end smallexample

The above message indicates that the offending source line is
line 4 of the file @file{rnge.f},
within the program unit (or statement function) named @samp{bf}.
The offended array is named @samp{b}.
The offended array dimension is the second for a two-dimensional array,
and the offending, computed subscript expression was @samp{-6}.

For a @code{CHARACTER} substring reference, the second line has
this appearance:

@smallexample
Attempt to access the 11-th element of variable a[start-substring].
@end smallexample

This indicates that the offended @code{CHARACTER} variable or array
is named @samp{a},
the offended substring position is the starting (leftmost) position,
and the offending substring expression is @samp{11}.

(Though the verbage of @code{s_rnge} is not ideal
for the purpose of the @code{g77} compiler,
the above information should provide adequate diagnostic abilities
to it users.)
@end table

@xref{Code Gen Options,,Options for Code Generation Conventions,
gcc,Using and Porting GNU CC}, for information on more options
offered by the GBE
shared by @code{g77}, @code{gcc}, and other GNU compilers.

Some of these do @emph{not} work when compiling programs written in Fortran:

@table @code
@cindex -fpcc-struct-return option
@cindex options, -fpcc-struct-return
@item -fpcc-struct-return
@cindex -freg-struct-return option
@cindex options, -freg-struct-return
@item -freg-struct-return
You should not use these except strictly the same way as you
used them to build the version of @code{libg2c} with which
you will be linking all code compiled by @code{g77} with the
same option.

@cindex -fshort-double option
@cindex options, -fshort-double
@item -fshort-double
This probably either has no effect on Fortran programs, or
makes them act loopy.

@cindex -fno-common option
@cindex options, -fno-common
@item -fno-common
Do not use this when compiling Fortran programs,
or there will be Trouble.

@cindex -fpack-struct option
@cindex options, -fpack-struct
@item -fpack-struct
This probably will break any calls to the @code{libg2c} library,
at the very least, even if it is built with the same option.
@end table

@node Environment Variables
@section Environment Variables Affecting GNU Fortran
@cindex environment variables

GNU Fortran currently does not make use of any environment
variables to control its operation above and beyond those
that affect the operation of @code{gcc}.

@xref{Environment Variables,,Environment Variables Affecting GNU CC,
gcc,Using and Porting GNU CC}, for information on environment
variables.

@include news.texi

@set USERVISONLY
@include news.texi
@clear USERVISONLY

@node Language
@chapter The GNU Fortran Language

@cindex standard, ANSI FORTRAN 77
@cindex ANSI FORTRAN 77 standard
@cindex reference works
GNU Fortran supports a variety of extensions to, and dialects
of, the Fortran language.
Its primary base is the ANSI FORTRAN 77 standard, currently available on
the network at
@uref{http://www.fortran.com/fortran/F77_std/rjcnf0001.html}
or as monolithic text at
@uref{http://www.fortran.com/fortran/F77_std/f77_std.html}.
It offers some extensions that are popular among users
of UNIX @code{f77} and @code{f2c} compilers, some that
are popular among users of other compilers (such as Digital
products), some that are popular among users of the
newer Fortran 90 standard, and some that are introduced
by GNU Fortran.

@cindex textbooks
(If you need a text on Fortran,
a few freely available electronic references have pointers from
@uref{http://www.fortran.com/fortran/Books/}.  There is a `cooperative
net project', @cite{User Notes on Fortran Programming} at
@uref{ftp://vms.huji.ac.il/fortran/} and mirrors elsewhere; some of this
material might not apply specifically to @code{g77}.)

Part of what defines a particular implementation of a Fortran
system, such as @code{g77}, is the particular characteristics
of how it supports types, constants, and so on.
Much of this is left up to the implementation by the various
Fortran standards and accepted practice in the industry.

The GNU Fortran @emph{language} is described below.
Much of the material is organized along the same lines
as the ANSI FORTRAN 77 standard itself.

@xref{Other Dialects}, for information on features @code{g77} supports
that are not part of the GNU Fortran language.

@emph{Note}: This portion of the documentation definitely needs a lot
of work!

@menu
Relationship to the ANSI FORTRAN 77 standard:
* Direction of Language Development::  Where GNU Fortran is headed.
* Standard Support::  Degree of support for the standard.

Extensions to the ANSI FORTRAN 77 standard:
* Conformance::
* Notation Used::
* Terms and Concepts::
* Characters Lines Sequence::
* Data Types and Constants::
* Expressions::
* Specification Statements::
* Control Statements::
* Functions and Subroutines::
* Scope and Classes of Names::
* I/O::
* Fortran 90 Features::
@end menu

@node Direction of Language Development
@section Direction of Language Development
@cindex direction of language development
@cindex features, language
@cindex language, features

The purpose of the following description of the GNU Fortran
language is to promote wide portability of GNU Fortran programs.

GNU Fortran is an evolving language, due to the
fact that @code{g77} itself is in beta test.
Some current features of the language might later
be redefined as dialects of Fortran supported by @code{g77}
when better ways to express these features are added to @code{g77},
for example.
Such features would still be supported by
@code{g77}, but would be available only when
one or more command-line options were used.

The GNU Fortran @emph{language} is distinct from the
GNU Fortran @emph{compilation system} (@code{g77}).

For example, @code{g77} supports various dialects of
Fortran---in a sense, these are languages other than
GNU Fortran---though its primary
purpose is to support the GNU Fortran language, which also is
described in its documentation and by its implementation.

On the other hand, non-GNU compilers might offer
support for the GNU Fortran language, and are encouraged
to do so.

Currently, the GNU Fortran language is a fairly fuzzy object.
It represents something of a cross between what @code{g77} accepts
when compiling using the prevailing defaults and what this
document describes as being part of the language.

Future versions of @code{g77} are expected to clarify the
definition of the language in the documentation.
Often, this will mean adding new features to the language, in the form
of both new documentation and new support in @code{g77}.
However, it might occasionally mean removing a feature
from the language itself to ``dialect'' status.
In such a case, the documentation would be adjusted
to reflect the change, and @code{g77} itself would likely be changed
to require one or more command-line options to continue supporting
the feature.

The development of the GNU Fortran language is intended to strike
a balance between:

@itemize @bullet
@item
Serving as a mostly-upwards-compatible language from the
de facto UNIX Fortran dialect as supported by @code{f77}.

@item
Offering new, well-designed language features.
Attributes of such features include
not making existing code any harder to read
(for those who might be unaware that the new
features are not in use) and
not making state-of-the-art
compilers take longer to issue diagnostics,
among others.

@item
Supporting existing, well-written code without gratuitously
rejecting non-standard constructs, regardless of the origin
of the code (its dialect).

@item
Offering default behavior and command-line options to reduce
and, where reasonable, eliminate the need for programmers to make
any modifications to code that already works in existing
production environments.

@item
Diagnosing constructs that have different meanings in different
systems, languages, and dialects, while offering clear,
less ambiguous ways to express each of the different meanings
so programmers can change their code appropriately.
@end itemize

One of the biggest practical challenges for the developers of the
GNU Fortran language is meeting the sometimes contradictory demands
of the above items.

For example, a feature might be widely used in one popular environment,
but the exact same code that utilizes that feature might not work
as expected---perhaps it might mean something entirely different---in
another popular environment.

Traditionally, Fortran compilers---even portable ones---have solved this
problem by simply offering the appropriate feature to users of
the respective systems.
This approach treats users of various Fortran systems and dialects
as remote ``islands'', or camps, of programmers, and assume that these
camps rarely come into contact with each other (or,
especially, with each other's code).

Project GNU takes a radically different approach to software and language
design, in that it assumes that users of GNU software do not necessarily
care what kind of underlying system they are using, regardless
of whether they are using software (at the user-interface
level) or writing it (for example, writing Fortran or C code).

As such, GNU users rarely need consider just what kind of underlying
hardware (or, in many cases, operating system) they are using at any
particular time.
They can use and write software designed for a general-purpose,
widely portable, heterogenous environment---the GNU environment.

In line with this philosophy, GNU Fortran must evolve into a product
that is widely ported and portable not only in the sense that it can
be successfully built, installed, and run by users, but in the larger
sense that its users can use it in the same way, and expect largely the
same behaviors from it, regardless of the kind of system they are using
at any particular time.

This approach constrains the solutions @code{g77} can use to resolve
conflicts between various camps of Fortran users.
If these two camps disagree about what a particular construct should
mean, @code{g77} cannot simply be changed to treat that particular construct as
having one meaning without comment (such as a warning), lest the users
expecting it to have the other meaning are unpleasantly surprised that
their code misbehaves when executed.

The use of the ASCII backslash character in character constants is
an excellent (and still somewhat unresolved) example of this kind of
controversy.
@xref{Backslash in Constants}.
Other examples are likely to arise in the future, as @code{g77} developers
strive to improve its ability to accept an ever-wider variety of existing
Fortran code without requiring significant modifications to said code.

Development of GNU Fortran is further constrained by the desire
to avoid requiring programmers to change their code.
This is important because it allows programmers, administrators,
and others to more faithfully evaluate and validate @code{g77}
(as an overall product and as new versions are distributed)
without having to support multiple versions of their programs
so that they continue to work the same way on their existing
systems (non-GNU perhaps, but possibly also earlier versions
of @code{g77}).

@node Standard Support
@section ANSI FORTRAN 77 Standard Support
@cindex ANSI FORTRAN 77 support
@cindex standard, support for
@cindex support, FORTRAN 77
@cindex compatibility, FORTRAN 77
@cindex FORTRAN 77 compatibility

GNU Fortran supports ANSI FORTRAN 77 with the following caveats.