2004-08-06 Steven G. Kargl <kargls@comcast.net>

[pf3gnuchains/gcc-fork.git] / gcc / fortran / gfortran.texi

\input texinfo  @c -*-texinfo-*-
@c %**start of header
@setfilename gfortran.info
@set last-update 13 May 2004
@set copyrights-gfortran 1999-2004

@include gcc-common.texi

@settitle The GNU Fortran 95 Compiler

@c Create a separate index for command line options
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@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
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@copying
Copyright @copyright{} @value{copyrights-gfortran} Free Software Foundation, Inc.

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

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

     A GNU Manual

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

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

@ifinfo
@dircategory Programming
@direntry
* gfortran: (gfortran).                  The GNU Fortran 95 Compiler.
@end direntry
This file documents the use and the internals of
the GNU Fortran 95 compiler, (@command{gfortran}).

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

@insertcopying
@end ifinfo

Contributed by Steven Bosscher (@email{s.bosscher@@gcc.gnu.org}).

@setchapternewpage odd
@titlepage
@title Using GNU Fortran 95
@sp 2
@center Steven Bosscher
@sp 3 
@center Last updated @value{last-update}
@sp 1
@center for version @value {version-GCC}
@page
@vskip 0pt plus 1filll
For the @value{version-GCC} 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
@insertcopying
@end titlepage
@summarycontents
@contents
@page

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

This manual documents the use of @command{gfortran}, 
the GNU Fortran 95 compiler. You can find in this manual how to invoke
@command{gfortran}, as well as its features and incompatibilities.

@ifset DEVELOPMENT
@emph{Warning:} This document, and the compiler it describes, are still
under development.  While efforts are made too keep it up-to-date it might
not accurately reflect the status of the most recent @command{gfortran}.
@end ifset

@menu
* Copying::         GNU General Public License says
                    how you can copy and share GNU Fortran.
* GNU Free Documentation License::
                    How you can copy and share this manual.
* Funding::         How to help assure continued work for free software.
* Getting Started:: What you should know about @command{gfortran}.
* GFORTRAN and GCC::        You can compile Fortran, C, or other programs.
* GFORTRAN and G77::     Why we choose to start from scratch.
* Invoking GFORTRAN::    Command options supported by @command{gfortran}.
* Project Status::  Status of GFORTRAN, Roadmap, proposed extensions.
* Contributing::    Helping you can help.
* Standards::       Standards supported by GFORTRAN.
* Index::           Index of this documentation.
@end menu



@c ---------------------------------------------------------------------
@c GNU General Public License
@c ---------------------------------------------------------------------

@include gpl.texi



@c ---------------------------------------------------------------------
@c GNU Free Documentation License
@c ---------------------------------------------------------------------

@include fdl.texi



@c ---------------------------------------------------------------------
@c Funding Free Software
@c ---------------------------------------------------------------------

@include funding.texi



@c ---------------------------------------------------------------------
@c Getting Started
@c ---------------------------------------------------------------------

@node Getting Started
@chapter Getting Started

Gfortran is the GNU Fortran 95 compiler front end,
designed initially as a free replacement for,
or alternative to, the unix @command{f95} command;
@command{gfortran} is command you'll use to invoke the compiler.

Gfortran is still in an early state of development.
@command{gfortran} can generate code for most constructs and expressions,
but much work remains to be done.

When @command{gfortran} is finished,
it will do everything you expect from any decent compiler: 

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

@item
Translate the user's program into instructions a computer
can carry out more quickly than it takes to translate the
instructions in the first
place.  The result after compilation of a program is
@dfn{machine code},
code designed to be efficiently translated and processed
by a machine such as your computer.
Humans usually aren't as good writing machine code
as they are at writing Fortran (or C++, Ada, or Java),
because is easy to make tiny mistakes writing machine code.

@item
Provide the user with information about the reasons why
the compiler is unable to create a binary from the source code.
Usually this will be the case if the source code is flawed.
When writing Fortran, it is easy to make big mistakes.
The Fortran 90 requires that the compiler can point out
mistakes to the user.
An incorrect usage of the language causes an @dfn{error message}.

The compiler will also attempt to diagnose cases where the
user's program contains a correct usage of the language,
but instructs the computer to do something questionable.
This kind of diagnostics message is called a @dfn{warning message}.

@item
Provide optional information about the translation passes
from the source code to machine code.
This can help a user of the compiler to find the cause of
certain bugs which may not be obvious in the source code,
but may be more easily found at a lower level compiler output.
It also helps developers to find bugs in the compiler itself.

@item
Provide information in the generated machine code that can
make it easier to find bugs in the program (using a debugging tool,
called a @dfn{debugger}, such as the GNU Debugger @command{gdb}). 

@item
Locate and gather machine code already generated to
perform actions requested by statements in the user's program.
This machine code is organized into @dfn{modules} and is located
and @dfn{linked} to the user program. 
@end itemize

Gfortran consists of several components:

@itemize @bullet
@item
A version of the @command{gcc} command
(which also might be installed as the system's @command{cc} command)
that also understands and accepts Fortran source code.
The @command{gcc} command is the @dfn{driver} program for
all the languages in the GNU Compiler Collection (GCC);
With @command{gcc},
you can compiler the source code of any language for
which a front end is available in GCC.

@item
The @command{gfortran} command itself,
which also might be installed as the
system's @command{f95} command.
@command{gfortran} is just another driver program,
but specifically for the Fortran 95 compiler only.
The difference with @command{gcc} is that @command{gfortran}
will automatically link the correct libraries to your program.

@item
A collection of run-time libraries.
These libraries contains the machine code needed to support
capabilities of the Fortran language that are not directly
provided by the machine code generated by the
@command{gfortran} compilation phase,
such as intrinsic functions and subroutines,
and routines for interaction with files and the operating system.
@c and mechanisms to spawn,
@c unleash and pause threads in parallelized code.

@item
The Fortran compiler itself, (@command{f951}).
This is the gfortran parser and code generator,
linked to and interfaced with the GCC backend library.
@command{f951} ``translates'' the source code to
assembler code.  You would typically not use this
program directly;
instead, the @command{gcc} or @command{gfortran} driver
programs will call it for you.
@end itemize



@c ---------------------------------------------------------------------
@c GFORTRAN and GCC
@c ---------------------------------------------------------------------

@node GFORTRAN and GCC
@chapter GFORTRAN and GCC
@cindex GNU Compiler Collection

GCC used to be the GNU ``C'' Compiler,
but is now known as the @dfn{GNU Compiler Collection}.
GCC provides the GNU system with a very versatile
compiler middle end (shared optimization passes),
and with back ends (code generators) for many different
computer architectures and operating systems.
The code of the middle end and back end are shared by all
compiler front ends that are in the GNU Compiler Collection.

A GCC front end is essentially a source code parser
and a pass to generate a representation of the semantics
of the program in the source code in the GCC language
independent intermediate language,
called @dfn{GENERIC}.

The parser takes a source file written in a
particular computer language, reads and parses it,
and tries to make sure that the source code conforms to
the language rules.
Once the correctness of a program has been established,
the compiler will build a data structure known as the
@dfn{Abstract Syntax tree},
or just @dfn{AST} or ``tree'' for short.
This data structure represents the whole program
or a subroutine or a function.
The ``tree'' is passed to the GCC middle end,
which will perform optimization passes on it,
pass the optimized AST and generate assembly
for the program unit.

Different phases in this translation process can be,
and in fact @emph{are} merged in many compiler front ends.
GNU Fortran 95 has a strict separation between the
parser and code generator.

The goal of the gfortran project is to build a new front end for GCC:
A Fortran 95 front end.
In a non-gfortran installation,
@command{gcc} will not be able to compile Fortran 95 source code
(only the ``C'' front end has to be compiled if you want to build GCC,
all other languages are optional).
If you build GCC with gfortran, @command{gcc} will recognize
@file{.f/.f90/.f95} source files and accepts Fortran 95 specific
command line options.



@c ---------------------------------------------------------------------
@c GFORTRAN and G77
@c ---------------------------------------------------------------------

@node GFORTRAN and G77
@chapter GFORTRAN and G77
@cindex Fortran 77
@cindex G77

Why do we write a compiler front end from scratch? 
There's a fine Fortran 77 compiler in the
GNU Compiler Collection that accepts some features
of the Fortran 90 standard as extensions.
Why not start from there and revamp it?

One of the reasons is that Craig Burley, the author of G77,
has decided to stop working on the G77 front end.
On @uref{http://world.std.com/~burley/g77-why.html,
Craig explains the reasons for his decision to stop working on G77}
in one of the pages in his homepage.
Among the reasons is a lack of interest in improvements to
@command{g77}.
Users appear to be quite satisfied with @command{g77} as it is.
While @command{g77} is still being maintained (by Toon Moene),
it is unlikely that sufficient people will be willing
to completely rewrite the existing code. 

But there are other reasons to start from scratch.
Many people, including Craig Burley,
no longer agreed with certain design decisions in the G77 front end.
Also, the interface of @command{g77} to the back end is written in
a style which is confusing and not up to date on recommended practice.
In fact, a full rewrite had already been planned for GCC 3.0.

When Craig decided to stop,
it just seemed to be a better idea to start a new project from scratch,
because it was expected to be easier to maintain code we
develop ourselves than to do a major overhaul of @command{g77} first,
and then build a Fortran 95 compiler out of it.


@include invoke.texi

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

@node Project Status
@chapter Project Status

@quotation
As soon as gfortran can parse all of the statements correctly,
it will be in the ``larva'' state.
When we generate code, the ``puppa'' state.
When gfortran is done,
we'll see if it will be a beautiful butterfly,
or just a big bug....

--Andy Vaught, April 2000
@end quotation

The start of the GNU Fortran 95 project was announced on
the GCC homepage in March 18, 2000
(even though Andy had already been working on it for a while,
or course).

Gfortran is currently reaching the stage where is is able to compile real
world programs.  However it is still under development and has many rough
edges.

@menu
* Compiler Status::
* Library Status::
* Proposed Extensions::
@end menu

@node Compiler Status
@section Compiler Status

@table @emph
@item Front end
This is the part of gfortran which parses a source file, verifies that it
is valid Fortran 95, performs compile time replacement of constants
(PARAMETER variables) and reads and generate module files. This is
almost complete. Every Fortran 95 source should be accepted, and most
none-Fortran 95 source should be rejected. If you find a source file where
this is not true, please tell us. You can use the -fsyntax-only switch to
make gfortran quit after running the front end, effectively reducing it to
a syntax checker.

@item Middle end interface
These are the parts of gfortran that take the parse tree generated by the
front end and translate it to the GENERIC form required by the GCC back
end. Work is ongoing in these parts of gfortran, but a large part has
already been completed.
@end table

@node Library Status
@section Library Status

Some intrinsic functions map directly to library functions, and in most
cases the name of the library function used depends on the type of the
arguments.  For some intrinsics we generate inline code, and for others,
such as sin, cos and sqrt, we rely on the backend to use special
instructions in the floating point unit of the CPU if available, or to
fall back to a call to libm if these are not available.

Implementation of some non-elemental intrinsic functions (eg. DOT_PRODUCT,
AVERAGE) is not yet optimal. This is hard because we have to make decisions
whether to use inline code (good for small arrays as no function call
overhead occurs) or generate function calls (good for large arrays as it
allows use of hand-optimized assembly routines, SIMD instructions, etc.)

The IO library is still under development.  The following features should be
usable for real programs:

@itemize @minus
@item List directed
@item Unformatted sequential
@end itemize

Usable with bugs:

@itemize @minus
@item Formatted sequential ('T' edit descriptor, and others)
@item Namelist (can read a namelist that it writes, but not free-form)
@end itemize

Not recommended:

@itemize @minus
@item Unformatted direct access
@item Formatted direct access
@end itemize

Many Fortran programs only use a small subset of the available IO
capabilities, so your milage may vary.

@node Proposed Extensions
@section Proposed Extensions

Here's a list of proposed extensions for @command{gfortran}, in no particular
order.  Most of these are necessary to be fully compatible with
existing Fortran compilers, but they are not part of the official
J3 Fortran 95 standard.

@subsection Compiler extensions: 
@itemize @bullet
@item
Flag for defining the kind number for default logicals.

@item
User-specified alignment rules for structures.
@item
Flag to generate a @code{Makefile} info.

@item
Automatically extend single precision constants to double.

@item
Cray pointers (this was high on the @command{g77} wishlist).

@item
Compile code that conserves memory by dynamically allocating common and
module storage either on stack or heap.

@item
Flag to cause the compiler to distinguish between upper and lower case
names.  The Fortran 95 standard does not distinguish them.

@item
Compile switch for changing the interpretation of a backslash from a
character to ``C''-style escape characters.

@item
Compile flag to generate code for array conformance checking (suggest -CC).

@item
User control of symbol names (underscores, etc).

@item
Compile setting for maximum size of stack frame size before spilling
parts to static or heap.

@item
Flag to force local variables into static space.

@item
Flag to force local variables onto stack.

@item
Flag to compile lines beginning with ``D''.

@item
Flag to ignore lines beginning with ``D''.

@item
Flag for maximum errors before ending compile.

@item
Generate code to check for null pointer dereferences -- prints locus of
dereference instead of segfaulting.  There was some discussion about this
option in the g95 development mailing list.

@item
Allow setting default unit number.

@item
Option to initialize of otherwise uninitialized integer and floating
point variables.

@item
Support for OpenMP directives.  This also requires support from the runtime
library and the rest of the compiler.

@item
Support for Fortran 200x. This includes several new features including
floating point exceptions, extended use of allocatable arrays, C
interoperability, Parameterizer data types and function pointers.
@end itemize


@subsection Environment Options
@itemize @bullet
@item
Pluggable library modules for random numbers, linear algebra.
LA should use BLAS calling conventions.

@item
Environment variables controlling actions on arithmetic exceptions like
overflow, underflow, precision loss -- Generate NaN, abort, default.
action.

@item
Set precision for fp units that support it (i387).

@item
Variables for setting fp rounding mode.

@item
Support old style namelists ending in $end or &end.

@item
Variable to fill uninitialized variables with a user-defined bit
pattern.

@item
Environment variable controlling filename that is opened for that unit
number.

@item
Environment variable to clear/trash memory being freed.

@item
Environment variable to control tracing of allocations and frees.

@item
Environment variable to display allocated memory at normal program end.

@item
Environment variable for filename for * IO-unit.

@item
Environment variable for temporary file directory.

@item
Environment variable forcing standard output to be line buffered (unix).

@item
Variable for swapping endianness during unformatted read.

@item
Variable for swapping Endianness during unformatted write.
@end itemize



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

@node Contributing
@chapter Contributing
@cindex Contributing

Free software is only possible if people contribute to efforts
to create it.
We're always in need of more people helping out with ideas
and comments, writing documentation and contributing code.

If you want to contribute to GNU Fortran 95,
have a look at the long lists of projects you can take on.
Some of these projects are small,
some of them are large;
some are completely orthogonal to the rest of what is
happening on @command{gfortran},
but others are ``mainstream'' projects in need of enthusiastic hackers.
All of these projects are important!
We'll eventually get around to the things here,
but they are also things doable by someone who is willing and able.

@menu
* Contributors::
* Projects::
@end menu


@node Contributors
@section Contributors to GNU Fortran 95
@cindex Contributors
@cindex Credits
@cindex Authors

Most of the parser was hand-crafted by @emph{Andy Vaught}, who is
also the initiator of the whole project.  Thanks Andy!
Most of the interface with GCC was written by @emph{Paul Brook}.

The following individuals have contributed code and/or
ideas and significant help to the gfortran project
(in no particular order): 

@itemize @minus
@item Andy Vaught
@item Katherine Holcomb
@item Tobias Schlüter
@item Steven Bosscher
@item Toon Moene
@item Tim Prince
@item Niels Kristian Bech Jensen
@item Steven Johnson
@item Paul Brook
@item Feng Wang
@item Bud Davis
@end itemize

The following people have contributed bug reports,
smaller or larger patches,
and much needed feedback and encouragement for the
@command{gfortran} project: 

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

Many other individuals have helped debug,
test and improve @command{gfortran} over the past two years,
and we welcome you to do the same!
If you already have done so,
and you would like to see your name listed in the
list above, please contact us.


@node Projects
@section Projects

@table @emph

@item Help build the test suite
Solicit more code for donation to the test suite.
We can keep code private on request.

@item Bug hunting/squishing
Find bugs and write more test cases!
Test cases are especially very welcome,
because it allows us to concentrate on fixing bugs
instead of isolating them.

@item Smaller projects (``bug'' fixes):
  @itemize @minus
  @item Allow init exprs to be numbers raised to integer powers.
  @item Implement correct rounding.
  @item Implement F restrictions on Fortran 95 syntax.
  @item See about making Emacs-parsable error messages.
  @end itemize
@end table

If you wish to work on the runtime libraries,
please contact a project maintainer.
@c TODO: email!


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

@node Standards
@chapter Standards
@cindex Standards

The GNU Fortran 95 Compiler aims to be a conforming implementation of
ISO/IEC 1539:1997 (Fortran 95).

In the future it may also support other variants and extensions to the Fortran
language.  This includes ANSI Fortran 77, Fortran 90, Fortran 2000 (not yet
finalized), and OpenMP.

@node Index
@unnumbered Index

@printindex cp

@bye