This is g77.info, produced by makeinfo version 4.2 from g77.texi.

INFO-DIR-SECTION Programming
START-INFO-DIR-ENTRY
* g77: (g77).                  The GNU Fortran compiler.
END-INFO-DIR-ENTRY
   This file documents the use and the internals of the GNU Fortran
(`g77') compiler.  It corresponds to the GCC-3.1 version of `g77'.

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

   Copyright (C) 1995,1996,1997,1998,1999,2000,2001,2002 Free Software
Foundation, Inc.

   Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; with the
Invariant Sections being "GNU General Public License" and "Funding Free
Software", the Front-Cover texts being (a) (see below), and with the
Back-Cover Texts being (b) (see below).  A copy of the license is
included in the section entitled "GNU Free Documentation License".

   (a) The FSF's Front-Cover Text is:

   A GNU Manual

   (b) The FSF's Back-Cover Text is:

   You have freedom to copy and modify this GNU Manual, like GNU
software.  Copies published by the Free Software Foundation raise
funds for GNU development.

   Contributed by James Craig Burley (<craig@jcb-sc.com>).  Inspired by
a first pass at translating `g77-0.5.16/f/DOC' that was contributed to
Craig by David Ronis (<ronis@onsager.chem.mcgill.ca>).


File: g77.info,  Node: Multiple Definitions of External Names,  Next: Limitation on Implicit Declarations,  Prev: Mangling of Names,  Up: Disappointments

Multiple Definitions of External Names
--------------------------------------

   `g77' doesn't allow a common block and an external procedure or
`BLOCK DATA' to have the same name.  Some systems allow this, but `g77'
does not, to be compatible with `f2c'.

   `g77' could special-case the way it handles `BLOCK DATA', since it
is not compatible with `f2c' in this particular area (necessarily,
since `g77' offers an important feature here), but it is likely that
such special-casing would be very annoying to people with programs that
use `EXTERNAL FOO', with no other mention of `FOO' in the same program
unit, to refer to external procedures, since the result would be that
`g77' would treat these references as requests to force-load BLOCK DATA
program units.

   In that case, if `g77' modified names of `BLOCK DATA' so they could
have the same names as `COMMON', users would find that their programs
wouldn't link because the `FOO' procedure didn't have its name
translated the same way.

   (Strictly speaking, `g77' could emit a
null-but-externally-satisfying definition of `FOO' with its name
transformed as if it had been a `BLOCK DATA', but that probably invites
more trouble than it's worth.)


File: g77.info,  Node: Limitation on Implicit Declarations,  Prev: Multiple Definitions of External Names,  Up: Disappointments

Limitation on Implicit Declarations
-----------------------------------

   `g77' disallows `IMPLICIT CHARACTER*(*)'.  This is not
standard-conforming.


File: g77.info,  Node: Non-bugs,  Next: Warnings and Errors,  Prev: Disappointments,  Up: Trouble

Certain Changes We Don't Want to Make
=====================================

   This section lists changes that people frequently request, but which
we do not make because we think GNU Fortran is better without them.

* Menu:

* Backslash in Constants::           Why `'\\'' is a constant that
                                       is one, not two, characters long.
* Initializing Before Specifying::   Why `DATA VAR/1/' can't precede
                                       `COMMON VAR'.
* Context-Sensitive Intrinsicness::  Why `CALL SQRT' won't work.
* Context-Sensitive Constants::      Why `9.435784839284958' is a
                                       single-precision constant,
                                       and might be interpreted as
                                       `9.435785' or similar.
* Equivalence Versus Equality::      Why `.TRUE. .EQ. .TRUE.' won't work.
* Order of Side Effects::            Why `J = IFUNC() - IFUNC()' might
                                       not behave as expected.


File: g77.info,  Node: Backslash in Constants,  Next: Initializing Before Specifying,  Up: Non-bugs

Backslash in Constants
----------------------

   In the opinion of many experienced Fortran users, `-fno-backslash'
should be the default, not `-fbackslash', as currently set by `g77'.

   First of all, you can always specify `-fno-backslash' to turn off
this processing.

   Despite not being within the spirit (though apparently within the
letter) of the ANSI FORTRAN 77 standard, `g77' defaults to
`-fbackslash' because that is what most UNIX `f77' commands default to,
and apparently lots of code depends on this feature.

   This is a particularly troubling issue.  The use of a C construct in
the midst of Fortran code is bad enough, worse when it makes existing
Fortran programs stop working (as happens when programs written for
non-UNIX systems are ported to UNIX systems with compilers that provide
the `-fbackslash' feature as the default--sometimes with no option to
turn it off).

   The author of GNU Fortran wished, for reasons of linguistic purity,
to make `-fno-backslash' the default for GNU Fortran and thus require
users of UNIX `f77' and `f2c' to specify `-fbackslash' to get the UNIX
behavior.

   However, the realization that `g77' is intended as a replacement for
_UNIX_ `f77', caused the author to choose to make `g77' as compatible
with `f77' as feasible, which meant making `-fbackslash' the default.

   The primary focus on compatibility is at the source-code level, and
the question became "What will users expect a replacement for `f77' to
do, by default?"  Although at least one UNIX `f77' does not provide
`-fbackslash' as a default, it appears that the majority of them do,
which suggests that the majority of code that is compiled by UNIX `f77'
compilers expects `-fbackslash' to be the default.

   It is probably the case that more code exists that would _not_ work
with `-fbackslash' in force than code that requires it be in force.

   However, most of _that_ code is not being compiled with `f77', and
when it is, new build procedures (shell scripts, makefiles, and so on)
must be set up anyway so that they work under UNIX.  That makes a much
more natural and safe opportunity for non-UNIX users to adapt their
build procedures for `g77''s default of `-fbackslash' than would exist
for the majority of UNIX `f77' users who would have to modify existing,
working build procedures to explicitly specify `-fbackslash' if that was
not the default.

   One suggestion has been to configure the default for `-fbackslash'
(and perhaps other options as well) based on the configuration of `g77'.

   This is technically quite straightforward, but will be avoided even
in cases where not configuring defaults to be dependent on a particular
configuration greatly inconveniences some users of legacy code.

   Many users appreciate the GNU compilers because they provide an
environment that is uniform across machines.  These users would be
inconvenienced if the compiler treated things like the format of the
source code differently on certain machines.

   Occasionally users write programs intended only for a particular
machine type.  On these occasions, the users would benefit if the GNU
Fortran compiler were to support by default the same dialect as the
other compilers on that machine.  But such applications are rare.  And
users writing a program to run on more than one type of machine cannot
possibly benefit from this kind of compatibility.  (This is consistent
with the design goals for `gcc'.  To change them for `g77', you must
first change them for `gcc'.  Do not ask the maintainers of `g77' to do
this for you, or to disassociate `g77' from the widely understood, if
not widely agreed-upon, goals for GNU compilers in general.)

   This is why GNU Fortran does and will treat backslashes in the same
fashion on all types of machines (by default).  *Note Direction of
Language Development::, for more information on this overall philosophy
guiding the development of the GNU Fortran language.

   Of course, users strongly concerned about portability should indicate
explicitly in their build procedures which options are expected by
their source code, or write source code that has as few such
expectations as possible.

   For example, avoid writing code that depends on backslash (`\')
being interpreted either way in particular, such as by starting a
program unit with:

     CHARACTER BACKSL
     PARAMETER (BACKSL = '\\')

Then, use concatenation of `BACKSL' anyplace a backslash is desired.
In this way, users can write programs which have the same meaning in
many Fortran dialects.

   (However, this technique does not work for Hollerith constants--which
is just as well, since the only generally portable uses for Hollerith
constants are in places where character constants can and should be
used instead, for readability.)


File: g77.info,  Node: Initializing Before Specifying,  Next: Context-Sensitive Intrinsicness,  Prev: Backslash in Constants,  Up: Non-bugs

Initializing Before Specifying
------------------------------

   `g77' does not allow `DATA VAR/1/' to appear in the source code
before `COMMON VAR', `DIMENSION VAR(10)', `INTEGER VAR', and so on.  In
general, `g77' requires initialization of a variable or array to be
specified _after_ all other specifications of attributes (type, size,
placement, and so on) of that variable or array are specified (though
_confirmation_ of data type is permitted).

   It is _possible_ `g77' will someday allow all of this, even though
it is not allowed by the FORTRAN 77 standard.

   Then again, maybe it is better to have `g77' always require
placement of `DATA' so that it can possibly immediately write constants
to the output file, thus saving time and space.

   That is, `DATA A/1000000*1/' should perhaps always be immediately
writable to canonical assembler, unless it's already known to be in a
`COMMON' area following as-yet-uninitialized stuff, and to do this it
cannot be followed by `COMMON A'.


File: g77.info,  Node: Context-Sensitive Intrinsicness,  Next: Context-Sensitive Constants,  Prev: Initializing Before Specifying,  Up: Non-bugs

Context-Sensitive Intrinsicness
-------------------------------

   `g77' treats procedure references to _possible_ intrinsic names as
always enabling their intrinsic nature, regardless of whether the
_form_ of the reference is valid for that intrinsic.

   For example, `CALL SQRT' is interpreted by `g77' as an invalid
reference to the `SQRT' intrinsic function, because the reference is a
subroutine invocation.

   First, `g77' recognizes the statement `CALL SQRT' as a reference to
a _procedure_ named `SQRT', not to a _variable_ with that name (as it
would for a statement such as `V = SQRT').

   Next, `g77' establishes that, in the program unit being compiled,
`SQRT' is an intrinsic--not a subroutine that happens to have the same
name as an intrinsic (as would be the case if, for example, `EXTERNAL
SQRT' was present).

   Finally, `g77' recognizes that the _form_ of the reference is
invalid for that particular intrinsic.  That is, it recognizes that it
is invalid for an intrinsic _function_, such as `SQRT', to be invoked as
a _subroutine_.

   At that point, `g77' issues a diagnostic.

   Some users claim that it is "obvious" that `CALL SQRT' references an
external subroutine of their own, not an intrinsic function.

   However, `g77' knows about intrinsic subroutines, not just
functions, and is able to support both having the same names, for
example.

   As a result of this, `g77' rejects calls to intrinsics that are not
subroutines, and function invocations of intrinsics that are not
functions, just as it (and most compilers) rejects invocations of
intrinsics with the wrong number (or types) of arguments.

   So, use the `EXTERNAL SQRT' statement in a program unit that calls a
user-written subroutine named `SQRT'.


File: g77.info,  Node: Context-Sensitive Constants,  Next: Equivalence Versus Equality,  Prev: Context-Sensitive Intrinsicness,  Up: Non-bugs

Context-Sensitive Constants
---------------------------

   `g77' does not use context to determine the types of constants or
named constants (`PARAMETER'), except for (non-standard) typeless
constants such as `'123'O'.

   For example, consider the following statement:

     PRINT *, 9.435784839284958 * 2D0

`g77' will interpret the (truncated) constant `9.435784839284958' as a
`REAL(KIND=1)', not `REAL(KIND=2)', constant, because the suffix `D0'
is not specified.

   As a result, the output of the above statement when compiled by
`g77' will appear to have "less precision" than when compiled by other
compilers.

   In these and other cases, some compilers detect the fact that a
single-precision constant is used in a double-precision context and
therefore interpret the single-precision constant as if it was
_explicitly_ specified as a double-precision constant.  (This has the
effect of appending _decimal_, not _binary_, zeros to the fractional
part of the number--producing different computational results.)

   The reason this misfeature is dangerous is that a slight, apparently
innocuous change to the source code can change the computational
results.  Consider:

     REAL ALMOST, CLOSE
     DOUBLE PRECISION FIVE
     PARAMETER (ALMOST = 5.000000000001)
     FIVE = 5
     CLOSE = 5.000000000001
     PRINT *, 5.000000000001 - FIVE
     PRINT *, ALMOST - FIVE
     PRINT *, CLOSE - FIVE
     END

Running the above program should result in the same value being printed
three times.  With `g77' as the compiler, it does.

   However, compiled by many other compilers, running the above program
would print two or three distinct values, because in two or three of
the statements, the constant `5.000000000001', which on most systems is
exactly equal to `5.' when interpreted as a single-precision constant,
is instead interpreted as a double-precision constant, preserving the
represented precision.  However, this "clever" promotion of type does
not extend to variables or, in some compilers, to named constants.

   Since programmers often are encouraged to replace manifest constants
or permanently-assigned variables with named constants (`PARAMETER' in
Fortran), and might need to replace some constants with variables
having the same values for pertinent portions of code, it is important
that compilers treat code so modified in the same way so that the
results of such programs are the same.  `g77' helps in this regard by
treating constants just the same as variables in terms of determining
their types in a context-independent way.

   Still, there is a lot of existing Fortran code that has been written
to depend on the way other compilers freely interpret constants' types
based on context, so anything `g77' can do to help flag cases of this
in such code could be very helpful.


File: g77.info,  Node: Equivalence Versus Equality,  Next: Order of Side Effects,  Prev: Context-Sensitive Constants,  Up: Non-bugs

Equivalence Versus Equality
---------------------------

   Use of `.EQ.' and `.NE.' on `LOGICAL' operands is not supported,
except via `-fugly-logint', which is not recommended except for legacy
code (where the behavior expected by the _code_ is assumed).

   Legacy code should be changed, as resources permit, to use `.EQV.'
and `.NEQV.' instead, as these are permitted by the various Fortran
standards.

   New code should never be written expecting `.EQ.' or `.NE.' to work
if either of its operands is `LOGICAL'.

   The problem with supporting this "feature" is that there is unlikely
to be consensus on how it works, as illustrated by the following sample
program:

     LOGICAL L,M,N
     DATA L,M,N /3*.FALSE./
     IF (L.AND.M.EQ.N) PRINT *,'L.AND.M.EQ.N'
     END

   The issue raised by the above sample program is: what is the
precedence of `.EQ.' (and `.NE.') when applied to `LOGICAL' operands?

   Some programmers will argue that it is the same as the precedence
for `.EQ.' when applied to numeric (such as `INTEGER') operands.  By
this interpretation, the subexpression `M.EQ.N' must be evaluated first
in the above program, resulting in a program that, when run, does not
execute the `PRINT' statement.

   Other programmers will argue that the precedence is the same as the
precedence for `.EQV.', which is restricted by the standards to
`LOGICAL' operands.  By this interpretation, the subexpression
`L.AND.M' must be evaluated first, resulting in a program that _does_
execute the `PRINT' statement.

   Assigning arbitrary semantic interpretations to syntactic expressions
that might legitimately have more than one "obvious" interpretation is
generally unwise.

   The creators of the various Fortran standards have done a good job
in this case, requiring a distinct set of operators (which have their
own distinct precedence) to compare `LOGICAL' operands.  This
requirement results in expression syntax with more certain precedence
(without requiring substantial context), making it easier for
programmers to read existing code.  `g77' will avoid muddying up
elements of the Fortran language that were well-designed in the first
place.

   (Ask C programmers about the precedence of expressions such as `(a)
& (b)' and `(a) - (b)'--they cannot even tell you, without knowing more
context, whether the `&' and `-' operators are infix (binary) or unary!)

   Most dangerous of all is the fact that, even assuming consensus on
its meaning, an expression like `L.AND.M.EQ.N', if it is the result of
a typographical error, doesn't _look_ like it has such a typo.  Even
experienced Fortran programmers would not likely notice that
`L.AND.M.EQV.N' was, in fact, intended.

   So, this is a prime example of a circumstance in which a quality
compiler diagnoses the code, instead of leaving it up to someone
debugging it to know to turn on special compiler options that might
diagnose it.


File: g77.info,  Node: Order of Side Effects,  Prev: Equivalence Versus Equality,  Up: Non-bugs

Order of Side Effects
---------------------

   `g77' does not necessarily produce code that, when run, performs
side effects (such as those performed by function invocations) in the
same order as in some other compiler--or even in the same order as
another version, port, or invocation (using different command-line
options) of `g77'.

   It is never safe to depend on the order of evaluation of side
effects.  For example, an expression like this may very well behave
differently from one compiler to another:

     J = IFUNC() - IFUNC()

There is no guarantee that `IFUNC' will be evaluated in any particular
order.  Either invocation might happen first.  If `IFUNC' returns 5 the
first time it is invoked, and returns 12 the second time, `J' might end
up with the value `7', or it might end up with `-7'.

   Generally, in Fortran, procedures with side-effects intended to be
visible to the caller are best designed as _subroutines_, not functions.
Examples of such side-effects include:

   * The generation of random numbers that are intended to influence
     return values.

   * Performing I/O (other than internal I/O to local variables).

   * Updating information in common blocks.

   An example of a side-effect that is not intended to be visible to
the caller is a function that maintains a cache of recently calculated
results, intended solely to speed repeated invocations of the function
with identical arguments.  Such a function can be safely used in
expressions, because if the compiler optimizes away one or more calls
to the function, operation of the program is unaffected (aside from
being speeded up).


File: g77.info,  Node: Warnings and Errors,  Prev: Non-bugs,  Up: Trouble

Warning Messages and Error Messages
===================================

   The GNU compiler can produce two kinds of diagnostics: errors and
warnings.  Each kind has a different purpose:

     _Errors_ report problems that make it impossible to compile your
     program.  GNU Fortran reports errors with the source file name,
     line number, and column within the line where the problem is
     apparent.

     _Warnings_ report other unusual conditions in your code that
     _might_ indicate a problem, although compilation can (and does)
     proceed.  Warning messages also report the source file name, line
     number, and column information, but include the text `warning:' to
     distinguish them from error messages.

   Warnings might indicate danger points where you should check to make
sure that your program really does what you intend; or the use of
obsolete features; or the use of nonstandard features of GNU Fortran.
Many warnings are issued only if you ask for them, with one of the `-W'
options (for instance, `-Wall' requests a variety of useful warnings).

   _Note:_ Currently, the text of the line and a pointer to the column
is printed in most `g77' diagnostics.

   *Note Options to Request or Suppress Warnings: Warning Options, for
more detail on these and related command-line options.


File: g77.info,  Node: Open Questions,  Next: Bugs,  Prev: Trouble,  Up: Top

Open Questions
**************

   Please consider offering useful answers to these questions!

   * `LOC()' and other intrinsics are probably somewhat misclassified.
     Is the a need for more precise classification of intrinsics, and
     if so, what are the appropriate groupings?  Is there a need to
     individually enable/disable/delete/hide intrinsics from the
     command line?


File: g77.info,  Node: Bugs,  Next: Service,  Prev: Open Questions,  Up: Top

Reporting Bugs
**************

   Your bug reports play an essential role in making GNU Fortran
reliable.

   When you encounter a problem, the first thing to do is to see if it
is already known.  *Note Trouble::.  If it isn't known, then you should
report the problem.

   Reporting a bug might help you by bringing a solution to your
problem, or it might not.  (If it does not, look in the service
directory; see *Note Service::.)  In any case, the principal function
of a bug report is to help the entire community by making the next
version of GNU Fortran work better.  Bug reports are your contribution
to the maintenance of GNU Fortran.

   Since the maintainers are very overloaded, we cannot respond to every
bug report.  However, if the bug has not been fixed, we are likely to
send you a patch and ask you to tell us whether it works.

   In order for a bug report to serve its purpose, you must include the
information that makes for fixing the bug.

* Menu:

* Criteria: Bug Criteria.    Have you really found a bug?
* Where: Bug Lists.          Where to send your bug report.
* Reporting: Bug Reporting.  How to report a bug effectively.

   *Note Known Causes of Trouble with GNU Fortran: Trouble, for
information on problems we already know about.

   *Note How To Get Help with GNU Fortran: Service, for information on
where to ask for help.


File: g77.info,  Node: Bug Criteria,  Next: Bug Lists,  Up: Bugs

Have You Found a Bug?
=====================

   If you are not sure whether you have found a bug, here are some
guidelines:

   * If the compiler gets a fatal signal, for any input whatever, that
     is a compiler bug.  Reliable compilers never crash--they just
     remain obsolete.

   * If the compiler produces invalid assembly code, for any input
     whatever, that is a compiler bug, unless the compiler reports
     errors (not just warnings) which would ordinarily prevent the
     assembler from being run.

   * If the compiler produces valid assembly code that does not
     correctly execute the input source code, that is a compiler bug.

     However, you must double-check to make sure, because you might
     have run into an incompatibility between GNU Fortran and
     traditional Fortran.  These incompatibilities might be considered
     bugs, but they are inescapable consequences of valuable features.

     Or you might have a program whose behavior is undefined, which
     happened by chance to give the desired results with another
     Fortran compiler.  It is best to check the relevant Fortran
     standard thoroughly if it is possible that the program indeed does
     something undefined.

     After you have localized the error to a single source line, it
     should be easy to check for these things.  If your program is
     correct and well defined, you have found a compiler bug.

     It might help if, in your submission, you identified the specific
     language in the relevant Fortran standard that specifies the
     desired behavior, if it isn't likely to be obvious and agreed-upon
     by all Fortran users.

   * If the compiler produces an error message for valid input, that is
     a compiler bug.

   * If the compiler does not produce an error message for invalid
     input, that is a compiler bug.  However, you should note that your
     idea of "invalid input" might be someone else's idea of "an
     extension" or "support for traditional practice".

   * If you are an experienced user of Fortran compilers, your
     suggestions for improvement of GNU Fortran are welcome in any case.

   Many, perhaps most, bug reports against `g77' turn out to be bugs in
the user's code.  While we find such bug reports educational, they
sometimes take a considerable amount of time to track down or at least
respond to--time we could be spending making `g77', not some user's
code, better.

   Some steps you can take to verify that the bug is not certainly in
the code you're compiling with `g77':

   * Compile your code using the `g77' options `-W -Wall -O'.  These
     options enable many useful warning; the `-O' option enables flow
     analysis that enables the uninitialized-variable warning.

     If you investigate the warnings and find evidence of possible bugs
     in your code, fix them first and retry `g77'.

   * Compile your code using the `g77' options `-finit-local-zero',
     `-fno-automatic', `-ffloat-store', and various combinations
     thereof.

     If your code works with any of these combinations, that is not
     proof that the bug isn't in `g77'--a `g77' bug exposed by your
     code might simply be avoided, or have a different, more subtle
     effect, when different options are used--but it can be a strong
     indicator that your code is making unwarranted assumptions about
     the Fortran dialect and/or underlying machine it is being compiled
     and run on.

     *Note Overly Convenient Command-Line Options: Overly Convenient
     Options, for information on the `-fno-automatic' and
     `-finit-local-zero' options and how to convert their use into
     selective changes in your own code.

   * Validate your code with `ftnchek' or a similar code-checking tool.
     `ftnchek' can be found at `ftp://ftp.netlib.org/fortran' or
     `ftp://ftp.dsm.fordham.edu'.

     Here are some sample `Makefile' rules using `ftnchek' "project"
     files to do cross-file checking and `sfmakedepend' (from
     `ftp://ahab.rutgers.edu/pub/perl/sfmakedepend') to maintain
     dependencies automatically.  These assume the use of GNU `make'.

          # Dummy suffix for ftnchek targets:
          .SUFFIXES: .chek
          .PHONY: chekall
          
          # How to compile .f files (for implicit rule):
          FC = g77
          # Assume `include' directory:
          FFLAGS = -Iinclude -g -O -Wall
          
          # Flags for ftnchek:
          CHEK1 = -array=0 -include=includes -noarray
          CHEK2 = -nonovice -usage=1 -notruncation
          CHEKFLAGS = $(CHEK1) $(CHEK2)
          
          # Run ftnchek with all the .prj files except the one corresponding
          # to the target's root:
          %.chek : %.f ; \
            ftnchek $(filter-out $*.prj,$(PRJS)) $(CHEKFLAGS) \
              -noextern -library $<
          
          # Derive a project file from a source file:
          %.prj : %.f ; \
            ftnchek $(CHEKFLAGS) -noextern -project -library $<
          
          # The list of objects is assumed to be in variable OBJS.
          # Sources corresponding to the objects:
          SRCS = $(OBJS:%.o=%.f)
          # ftnchek project files:
          PRJS = $(OBJS:%.o=%.prj)
          
          # Build the program
          prog: $(OBJS) ; \
            $(FC) -o $ $(OBJS)
          
          chekall: $(PRJS) ; \
            ftnchek $(CHEKFLAGS) $(PRJS)
          
          prjs: $(PRJS)
          
          # For Emacs M-x find-tag:
          TAGS: $(SRCS) ; \
            etags $(SRCS)
          
          # Rebuild dependencies:
          depend: ; \
            sfmakedepend -I $(PLTLIBDIR) -I includes -a prj $(SRCS1)

   * Try your code out using other Fortran compilers, such as `f2c'.
     If it does not work on at least one other compiler (assuming the
     compiler supports the features the code needs), that is a strong
     indicator of a bug in the code.

     However, even if your code works on many compilers _except_ `g77',
     that does _not_ mean the bug is in `g77'.  It might mean the bug
     is in your code, and that `g77' simply exposes it more readily
     than other compilers.


File: g77.info,  Node: Bug Lists,  Next: Bug Reporting,  Prev: Bug Criteria,  Up: Bugs

Where to Report Bugs
====================

   Send bug reports for GNU Fortran to <gcc-bugs@gcc.gnu.org or
bug-gcc@gnu.org>.

   Often people think of posting bug reports to a newsgroup instead of
mailing them.  This sometimes appears to work, but it has one problem
which can be crucial: a newsgroup posting does not contain a mail path
back to the sender.  Thus, if maintainers need more information, they
might be unable to reach you.  For this reason, you should always send
bug reports by mail to the proper mailing list.

   As a last resort, send bug reports on paper to:

     GNU Compiler Bugs
     Free Software Foundation
     59 Temple Place - Suite 330
     Boston, MA 02111-1307, USA


File: g77.info,  Node: Bug Reporting,  Prev: Bug Lists,  Up: Bugs

How to Report Bugs
==================

   The fundamental principle of reporting bugs usefully is this:
*report all the facts*.  If you are not sure whether to state a fact or
leave it out, state it!

   Often people omit facts because they think they know what causes the
problem and they conclude that some details don't matter.  Thus, you
might assume that the name of the variable you use in an example does
not matter.  Well, probably it doesn't, but one cannot be sure.
Perhaps the bug is a stray memory reference which happens to fetch from
the location where that name is stored in memory; perhaps, if the name
were different, the contents of that location would fool the compiler
into doing the right thing despite the bug.  Play it safe and give a
specific, complete example.  That is the easiest thing for you to do,
and the most helpful.

   Keep in mind that the purpose of a bug report is to enable someone to
fix the bug if it is not known.  It isn't very important what happens if
the bug is already known.  Therefore, always write your bug reports on
the assumption that the bug is not known.

   Sometimes people give a few sketchy facts and ask, "Does this ring a
bell?"  This cannot help us fix a bug, so it is rarely helpful.  We
respond by asking for enough details to enable us to investigate.  You
might as well expedite matters by sending them to begin with.
(Besides, there are enough bells ringing around here as it is.)

   Try to make your bug report self-contained.  If we have to ask you
for more information, it is best if you include all the previous
information in your response, as well as the information that was
missing.

   Please report each bug in a separate message.  This makes it easier
for us to track which bugs have been fixed and to forward your bugs
reports to the appropriate maintainer.

   Do not compress and encode any part of your bug report using programs
such as `uuencode'.  If you do so it will slow down the processing of
your bug.  If you must submit multiple large files, use `shar', which
allows us to read your message without having to run any decompression
programs.

   (As a special exception for GNU Fortran bug-reporting, at least for
now, if you are sending more than a few lines of code, if your
program's source file format contains "interesting" things like
trailing spaces or strange characters, or if you need to include binary
data files, it is acceptable to put all the files together in a `tar'
archive, and, whether you need to do that, it is acceptable to then
compress the single file (`tar' archive or source file) using `gzip'
and encode it via `uuencode'.  Do not use any MIME stuff--the current
maintainer can't decode this.  Using `compress' instead of `gzip' is
acceptable, assuming you have licensed the use of the patented
algorithm in `compress' from Unisys.)

   To enable someone to investigate the bug, you should include all
these things:

   * The version of GNU Fortran.  You can get this by running `g77'
     with the `-v' option.  (Ignore any error messages that might be
     displayed when the linker is run.)

     Without this, we won't know whether there is any point in looking
     for the bug in the current version of GNU Fortran.

   * A complete input file that will reproduce the bug.

     If your source file(s) require preprocessing (for example, their
     names have suffixes like `.F', `.fpp', `.FPP', and `.r'), and the
     bug is in the compiler proper (`f771') or in a subsequent phase of
     processing, run your source file through the C preprocessor by
     doing `g77 -E SOURCEFILE > NEWFILE'.  Then, include the contents
     of NEWFILE in the bug report.  (When you do this, use the same
     preprocessor options--such as `-I', `-D', and `-U'--that you used
     in actual compilation.)

     A single statement is not enough of an example.  In order to
     compile it, it must be embedded in a complete file of compiler
     input.  The bug might depend on the details of how this is done.

     Without a real example one can compile, all anyone can do about
     your bug report is wish you luck.  It would be futile to try to
     guess how to provoke the bug.  For example, bugs in register
     allocation and reloading can depend on every little detail of the
     source and include files that trigger them.

   * Note that you should include with your bug report any files
     included by the source file (via the `#include' or `INCLUDE'
     directive) that you send, and any files they include, and so on.

     It is not necessary to replace the `#include' and `INCLUDE'
     directives with the actual files in the version of the source file
     that you send, but it might make submitting the bug report easier
     in the end.  However, be sure to _reproduce_ the bug using the
     _exact_ version of the source material you submit, to avoid
     wild-goose chases.

   * The command arguments you gave GNU Fortran to compile that example
     and observe the bug.  For example, did you use `-O'?  To guarantee
     you won't omit something important, list all the options.

     If we were to try to guess the arguments, we would probably guess
     wrong and then we would not encounter the bug.

   * The type of machine you are using, and the operating system name
     and version number.  (Much of this information is printed by `g77
     -v'--if you include that, send along any additional info you have
     that you don't see clearly represented in that output.)

   * The operands you gave to the `configure' command when you installed
     the compiler.

   * A complete list of any modifications you have made to the compiler
     source.  (We don't promise to investigate the bug unless it
     happens in an unmodified compiler.  But if you've made
     modifications and don't tell us, then you are sending us on a
     wild-goose chase.)

     Be precise about these changes.  A description in English is not
     enough--send a context diff for them.

     Adding files of your own (such as a machine description for a
     machine we don't support) is a modification of the compiler source.

   * Details of any other deviations from the standard procedure for
     installing GNU Fortran.

   * A description of what behavior you observe that you believe is
     incorrect.  For example, "The compiler gets a fatal signal," or,
     "The assembler instruction at line 208 in the output is incorrect."

     Of course, if the bug is that the compiler gets a fatal signal,
     then one can't miss it.  But if the bug is incorrect output, the
     maintainer might not notice unless it is glaringly wrong.  None of
     us has time to study all the assembler code from a 50-line Fortran
     program just on the chance that one instruction might be wrong.
     We need _you_ to do this part!

     Even if the problem you experience is a fatal signal, you should
     still say so explicitly.  Suppose something strange is going on,
     such as, your copy of the compiler is out of synch, or you have
     encountered a bug in the C library on your system.  (This has
     happened!)  Your copy might crash and the copy here would not.  If
     you said to expect a crash, then when the compiler here fails to
     crash, we would know that the bug was not happening.  If you don't
     say to expect a crash, then we would not know whether the bug was
     happening.  We would not be able to draw any conclusion from our
     observations.

     If the problem is a diagnostic when building GNU Fortran with some
     other compiler, say whether it is a warning or an error.

     Often the observed symptom is incorrect output when your program
     is run.  Sad to say, this is not enough information unless the
     program is short and simple.  None of us has time to study a large
     program to figure out how it would work if compiled correctly,
     much less which line of it was compiled wrong.  So you will have
     to do that.  Tell us which source line it is, and what incorrect
     result happens when that line is executed.  A person who
     understands the program can find this as easily as finding a bug
     in the program itself.

   * If you send examples of assembler code output from GNU Fortran,
     please use `-g' when you make them.  The debugging information
     includes source line numbers which are essential for correlating
     the output with the input.

   * If you wish to mention something in the GNU Fortran source, refer
     to it by context, not by line number.

     The line numbers in the development sources don't match those in
     your sources.  Your line numbers would convey no convenient
     information to the maintainers.

   * Additional information from a debugger might enable someone to
     find a problem on a machine which he does not have available.
     However, you need to think when you collect this information if
     you want it to have any chance of being useful.

     For example, many people send just a backtrace, but that is never
     useful by itself.  A simple backtrace with arguments conveys little
     about GNU Fortran because the compiler is largely data-driven; the
     same functions are called over and over for different RTL insns,
     doing different things depending on the details of the insn.

     Most of the arguments listed in the backtrace are useless because
     they are pointers to RTL list structure.  The numeric values of the
     pointers, which the debugger prints in the backtrace, have no
     significance whatever; all that matters is the contents of the
     objects they point to (and most of the contents are other such
     pointers).

     In addition, most compiler passes consist of one or more loops that
     scan the RTL insn sequence.  The most vital piece of information
     about such a loop--which insn it has reached--is usually in a
     local variable, not in an argument.

     What you need to provide in addition to a backtrace are the values
     of the local variables for several stack frames up.  When a local
     variable or an argument is an RTX, first print its value and then
     use the GDB command `pr' to print the RTL expression that it points
     to.  (If GDB doesn't run on your machine, use your debugger to call
     the function `debug_rtx' with the RTX as an argument.)  In
     general, whenever a variable is a pointer, its value is no use
     without the data it points to.

   Here are some things that are not necessary:

   * A description of the envelope of the bug.

     Often people who encounter a bug spend a lot of time investigating
     which changes to the input file will make the bug go away and which
     changes will not affect it.

     This is often time consuming and not very useful, because the way
     we will find the bug is by running a single example under the
     debugger with breakpoints, not by pure deduction from a series of
     examples.  You might as well save your time for something else.

     Of course, if you can find a simpler example to report _instead_ of
     the original one, that is a convenience.  Errors in the output
     will be easier to spot, running under the debugger will take less
     time, etc.  Most GNU Fortran bugs involve just one function, so
     the most straightforward way to simplify an example is to delete
     all the function definitions except the one where the bug occurs.
     Those earlier in the file may be replaced by external declarations
     if the crucial function depends on them.  (Exception: inline
     functions might affect compilation of functions defined later in
     the file.)

     However, simplification is not vital; if you don't want to do this,
     report the bug anyway and send the entire test case you used.

   * In particular, some people insert conditionals `#ifdef BUG' around
     a statement which, if removed, makes the bug not happen.  These
     are just clutter; we won't pay any attention to them anyway.
     Besides, you should send us preprocessor output, and that can't
     have conditionals.

   * A patch for the bug.

     A patch for the bug is useful if it is a good one.  But don't omit
     the necessary information, such as the test case, on the
     assumption that a patch is all we need.  We might see problems
     with your patch and decide to fix the problem another way, or we
     might not understand it at all.

     Sometimes with a program as complicated as GNU Fortran it is very
     hard to construct an example that will make the program follow a
     certain path through the code.  If you don't send the example, we
     won't be able to construct one, so we won't be able to verify that
     the bug is fixed.

     And if we can't understand what bug you are trying to fix, or why
     your patch should be an improvement, we won't install it.  A test
     case will help us to understand.

     See `http://gcc.gnu.org/contribute.html' for guidelines on how to
     make it easy for us to understand and install your patches.

   * A guess about what the bug is or what it depends on.

     Such guesses are usually wrong.  Even the maintainer can't guess
     right about such things without first using the debugger to find
     the facts.

   * A core dump file.

     We have no way of examining a core dump for your type of machine
     unless we have an identical system--and if we do have one, we
     should be able to reproduce the crash ourselves.


File: g77.info,  Node: Service,  Next: Adding Options,  Prev: Bugs,  Up: Top

How To Get Help with GNU Fortran
********************************

   If you need help installing, using or changing GNU Fortran, there
are two ways to find it:

   * Look in the service directory for someone who might help you for a
     fee.  The service directory is found in the file named `SERVICE'
     in the GNU CC distribution.

   * Send a message to <gcc-help@gcc.gnu.org>.


File: g77.info,  Node: Adding Options,  Next: Projects,  Prev: Service,  Up: Top

Adding Options
**************

   To add a new command-line option to `g77', first decide what kind of
option you wish to add.  Search the `g77' and `gcc' documentation for
one or more options that is most closely like the one you want to add
(in terms of what kind of effect it has, and so on) to help clarify its
nature.

   * _Fortran options_ are options that apply only when compiling
     Fortran programs.  They are accepted by `g77' and `gcc', but they
     apply only when compiling Fortran programs.

   * _Compiler options_ are options that apply when compiling most any
     kind of program.

   _Fortran options_ are listed in the file `gcc/gcc/f/lang-options.h',
which is used during the build of `gcc' to build a list of all options
that are accepted by at least one language's compiler.  This list goes
into the `documented_lang_options' array in `gcc/toplev.c', which uses
this array to determine whether a particular option should be offered
to the linked-in front end for processing by calling
`lang_option_decode', which, for `g77', is in `gcc/gcc/f/com.c' and just
calls `ffe_decode_option'.

   If the linked-in front end "rejects" a particular option passed to
it, `toplev.c' just ignores the option, because _some_ language's
compiler is willing to accept it.

   This allows commands like `gcc -fno-asm foo.c bar.f' to work, even
though Fortran compilation does not currently support the `-fno-asm'
option; even though the `f771' version of `lang_decode_option' rejects
`-fno-asm', `toplev.c' doesn't produce a diagnostic because some other
language (C) does accept it.

   This also means that commands like `g77 -fno-asm foo.f' yield no
diagnostics, despite the fact that no phase of the command was able to
recognize and process `-fno-asm'--perhaps a warning about this would be
helpful if it were possible.

   Code that processes Fortran options is found in `gcc/gcc/f/top.c',
function `ffe_decode_option'.  This code needs to check positive and
negative forms of each option.

   The defaults for Fortran options are set in their global
definitions, also found in `gcc/gcc/f/top.c'.  Many of these defaults
are actually macros defined in `gcc/gcc/f/target.h', since they might be
machine-specific.  However, since, in practice, GNU compilers should
behave the same way on all configurations (especially when it comes to
language constructs), the practice of setting defaults in `target.h' is
likely to be deprecated and, ultimately, stopped in future versions of
`g77'.

   Accessor macros for Fortran options, used by code in the `g77' FFE,
are defined in `gcc/gcc/f/top.h'.

   _Compiler options_ are listed in `gcc/toplev.c' in the array
`f_options'.  An option not listed in `lang_options' is looked up in
`f_options' and handled from there.

   The defaults for compiler options are set in the global definitions
for the corresponding variables, some of which are in `gcc/toplev.c'.

   You can set different defaults for _Fortran-oriented_ or
_Fortran-reticent_ compiler options by changing the source code of
`g77' and rebuilding.  How to do this depends on the version of `g77':

`G77 0.5.24 (EGCS 1.1)'
`G77 0.5.25 (EGCS 1.2 - which became GCC 2.95)'
     Change the `lang_init_options' routine in `gcc/gcc/f/com.c'.

     (Note that these versions of `g77' perform internal consistency
     checking automatically when the `-fversion' option is specified.)

`G77 0.5.23'
`G77 0.5.24 (EGCS 1.0)'
     Change the way `f771' handles the `-fset-g77-defaults' option,
     which is always provided as the first option when called by `g77'
     or `gcc'.

     This code is in `ffe_decode_options' in `gcc/gcc/f/top.c'.  Have
     it change just the variables that you want to default to a
     different setting for Fortran compiles compared to compiles of
     other languages.

     The `-fset-g77-defaults' option is passed to `f771' automatically
     because of the specification information kept in
     `gcc/gcc/f/lang-specs.h'.  This file tells the `gcc' command how
     to recognize, in this case, Fortran source files (those to be
     preprocessed, and those that are not), and further, how to invoke
     the appropriate programs (including `f771') to process those
     source files.

     It is in `gcc/gcc/f/lang-specs.h' that `-fset-g77-defaults',
     `-fversion', and other options are passed, as appropriate, even
     when the user has not explicitly specified them.  Other "internal"
     options such as `-quiet' also are passed via this mechanism.


File: g77.info,  Node: Projects,  Next: Front End,  Prev: Adding Options,  Up: Top

Projects
********

   If you want to contribute to `g77' by doing research, design,
specification, documentation, coding, or testing, the following
information should give you some ideas.  More relevant information
might be available from `ftp://alpha.gnu.org/gnu/g77/projects/'.

* Menu:

* Efficiency::               Make `g77' itself compile code faster.
* Better Optimization::      Teach `g77' to generate faster code.
* Simplify Porting::         Make `g77' easier to configure, build,
                             and install.
* More Extensions::          Features many users won't know to ask for.
* Machine Model::            `g77' should better leverage `gcc'.
* Internals Documentation::  Make maintenance easier.
* Internals Improvements::   Make internals more robust.
* Better Diagnostics::       Make using `g77' on new code easier.

