gfortran: Using the Compiler
Contents
This page is meant to give a quick overview on how to use gfortran. It
is merely a substitute for a complete manual, which has not yet been
written.
Gfortran builds on GCC, and thus
shares most characteristics with it. If you know how to use GCC, there
will not be much new information for you in this document. Especially,
the options for optimization and the generation of debugging
information are not outlined here.
Gfortran is used to compile a source file, source.f90, to
an object file, object.o, or an executable,
executable. Along the way it generates module description
files for the modules it encounters, these are named
nameofmodule.mod. If a module is used, gfortran will
read from these same files.
In order to compile the source file source.f90,
one would run: gfortran -c source.f90
The output file will automatically be named
source.o. This is an object file, which cannot be executed.
Once you have compiled some source files, you can link them
together with the necessary libraries to generate an executable. This
is done as follows: gfortran -o executable object1.o object2.o
..., where the the executable will be named
executable and the objectX.o are object
files, which may have been created as above, or equally well by another
compiler from sources in a different language. If -o
executable is omitted, the executable will be named
a.out (on cygwin systems: a.exe). The
executable may then be executed like any other program.
One may also skip the separate compilation step, and enter a command
such as gfortran -o executable source1.f90 source2.f90 which
will compile the source files source1.f90 and
source2.f90, link and generate the executable
executable in one step. You can also put object files on this
command line, they will be automatically linked in during the link step.
Sometimes the basic possibilities given above do not match the user's
needs. Therefore this section outlines other stuff the user might
want to know.
When gfortran is run on a file, whose name ends in
.f90 or
.f95, gfortran assumes a free form source file. If that file
actually is a fixed form source file, the user has to give the
-ffixed-form command line option. The precise semantics
of this option, and other options relating to fixed form versus free form
input are the same as in g77, and may be found in g77's documentation.
When running gfortran one actually does not run the compiler,
but the compiler driver. This driver interprets the command
line options given, and hands the work off to the actual compiler,
the assembler, and the linker. By default, this compiler driver
decides by the extensions of the given file names what to do. A file
named foo.c is handed to the C compiler and a file named
moo.f90 is handed to the Fortran 95 compiler, etc. To
overrule this behavior, one has to precede the filename by the argument
-x lang, where lang is a string
identifying the requested language. For Fortran 95 this is
f95.
Since Fortran allows for two different kinds of input, free form
source code and fixed form source code, the compiler has to know which
kind of input it is given. The rule in place is as follows: files whose name
ends in .f or .for are assumed to be fixed
form, files whose name end in .f90 or .f95
are assumed to to be free form, for other files the source form has to
be explicitly given. This may be done by the command line options
described above, which may also be used to override these rules.
In order to efficiently implement the passing of array sections,
binary compatibility to Fortran 77 had to be abandoned. If the user
wishes to link his sources with old Fortran 77 codes, the command line
option -fg77-calls changes back to the old calling
convention used by g77.
When linking with code compiled by g77, one also has to take care, because
g77 and gfortran use different libraries. Especially I/O might get messed
up due to this. Your safest bet is to only use I/O in either the g77 compiled
parts or the gfortran compiled parts, but not both, and to use the compiler
driver of the part which uses I/O in the final link step. There might be
circumstances where doing I/O in both works, but there is nothing
guaranteed. In the final link step you should also explicitly specify
the libraries of both compilers, i.e. -lgfortran for gfortran,
-lg2c for g77.
One may use gfortran as a syntax checker (or verify that gfortran's frontend
correctly accepts or rejects a program), by specifying
-fsyntax-only on the command line. Gfortran will then not
generate object files.
When given the command line option -fdump-parse-tree,
gfortran will print a representation of the parsed program, detailing both
the data objects and the executable statements of the program in a
Lisp-inspired notation. One remark for Fortran old timers:
ASSIGN in these dumps does not refer to the
ASSIGN statement, but to the operation of assignment,
i.e. sloppily speaking, the = operator.
Please send FSF & GNU inquiries & questions to
gnu@gnu.org.
There are also other ways
to contact the FSF.
These pages are maintained by
the GCC team.
For questions related to the use of GCC, please consult these web
pages and the GCC manuals. If
that fails, the gcc-help@gcc.gnu.org
mailing list might help.
Please send comments on these web pages and the development of GCC to our
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Verbatim copying and distribution of this entire article is
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Last modified 2006-06-21
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