Note: This portion of the documentation definitely needs a lot of work!
The following discussion assumes that you are running g77 in f2c
compatibility mode, i.e. not using `-fno-f2c'.
It provides some
advice about quick and simple techniques for linking Fortran and C (or
C++), the most common requirement.
For the full story consult the
description of code generation.
See section Debugging and Interfacing.
When linking Fortran and C, it's usually best to use g77 to do
the linking so that the correct libraries are included (including the
maths one).
If you're linking with C++ you will want to add
`-lstdc++', `-lg++' or whatever.
If you need to use another
driver program (or ld directly),
you can find out what linkage
options g77 passes by running `g77 -v'.
Even if you don't actually use it as a compiler, `f2c' from
@url{ftp://ftp.netlib.org/f2c/src}, can be a useful tool when you're
interfacing (linking) Fortran and C.
See section Generating Skeletons and Prototypes with f2c.
To use f2c for this purpose you only need retrieve and
build the `src' directory from the distribution, consult the
`README' instructions there for machine-specifics, and install the
f2c program on your path.
Something else that might be useful is `cfortran.h' from
@url{ftp://zebra/desy.de/cfortran}.
This is a fairly general tool which
can be used to generate interfaces for calling in both directions
between Fortran and C.
It can be used in f2c mode with
g77---consult its documentation for details.
Generally, C code written to link with
g77 code--calling and/or being
called from Fortran--should `#include <f2c.h>' to define the C
versions of the Fortran types.
Don't assume Fortran INTEGER types
correspond to C `int's, for instance; instead, declare them as
integer, a type defined by `f2c.h'.
`f2c.h' is installed where gcc will find it by
default, assuming you use a copy of gcc compatible with
g77, probably built at the same time as g77.
f2c
A simple and foolproof way to write g77-callable C routines--e.g. to
interface with an existing library--is to write a file (named, for
example, `fred.f') of dummy Fortran
skeletons comprising just the declaration of the routine(s) and dummy
arguments plus `END' statements.
Then run f2c on file `fred.f' to produce `fred.c'
into which you can edit
useful code, confident the calling sequence is correct, at least.
(There are some errors otherwise commonly made in generating C
interfaces with f2c conventions, such as not using doublereal as
the return type of a REAL FUNCTION.)
f2c also can help with calling Fortran from C, using its
`-P' option to generate C prototypes appropriate for calling the
Fortran.(1) is
probably better for this purpose.}
If the Fortran code containing any
routines to be called from C is in file `joe.f', use the command
f2c -P joe.f to generate the file `joe.P' containing
prototype information.
#include this in the C which has to call
the Fortran routines to make sure you get it right.
See section Arrays (DIMENSION), for information on the differences
between the way Fortran (including compilers like g77) and
C handle arrays.
f2c can be used to generate suitable code for compilation with a
C++ system using the `-C++' option.
The important thing about linking g77-compiled
code with C++ is that the prototypes for the g77
routines must specify C linkage to avoid name mangling.
So, use an `extern "C"' declaration.
f2c's `-C++' option will take care
of this when generating skeletons or prototype files as above, and also
avoid clashes with C++ reserved words in addition to those in C.
Unlike with some runtime systems, it shouldn't be necessary (unless there are
bugs) to use a Fortran main program to ensure the
runtime--specifically the i/o system--is initialized.
However, to use
the g77 intrinsics GETARG() and IARGC() the
main() routine from the `libf2c' library must be used, either
explicitly or implicitly by using a Fortran main program.
This
main() program calls MAIN__() (where the names are C-type
extern names, i.e. not mangled).
You need to provide this
nullary procedure as the entry point for your C code if using
`libf2c''s main.
In some cases it might be necessary to
provide a dummy version of this to avoid linkers complaining about
failure to resolve MAIN__() if linking against `libf2c' and
not using main() from it.
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