Re: help with C++/Fortran linking

[Noah Misch]

On Wed, May 30, 2007 at 12:49:53PM -0700, Brian T.N. Gunney wrote:
> I'm trying to ink a C++ program with fortran objects, but I get

> /nfs/apps/gcc/3.3.6/bin/../lib/gcc-lib/i686-pc-linux-gnu/3.3.6/../../../libfrtbegin.a(frtbegin.o)(.text+0x32):
>  In function `main':
> ../../../../gcc-3.3.6/libf2c/libF77/main.c:28: undefined reference to `MAIN__'

You must mark an F77_MAIN function `extern "C"' when you define it in a C++
compilation unit.

> cprog.C
> ::::::::::::::
> #include "config.h"
> extern "C" {
> void F77_FUNC(fortsub,FORTSUB)( double &x );
> }
> 
> #ifdef F77_DUMMY_MAIN
> #  ifdef __cplusplus
>      extern "C"
> #  endif
>    int F77_DUMMY_MAIN() { return 1; }
> #endif

Use exactly one of F77_DUMMY_MAIN or F77_MAIN in a given executable.  F77_MAIN
is correct for this application; delete the above.

> int F77_MAIN( int argc, char **argv ) {

Write `extern "C" F77_MAIN( int argc, char **argv ) {' instead.  The manual was
not explicit on thise point, so I installed the following patch to (hopefully)
clarify things.

2007-06-04  Noah Misch  <address@hidden>

        * doc/autoconf.texi (AC_F77_MAIN): Give a specific usage example that
        works with both C and C++.

diff -Nurp -X dontdiff ac-clean/doc/autoconf.texi ac-fortcpp/doc/autoconf.texi
--- ac-clean/doc/autoconf.texi  2007-06-04 08:11:56.000000000 -0400
+++ ac-fortcpp/doc/autoconf.texi        2007-06-04 18:39:00.000000000 -0400
@@ -7094,8 +7094,16 @@ alternate main function name is found, @
 simply defined to @code{main}.)
 
 Thus, when calling Fortran routines from C that perform things like I/O,
-one should use this macro and name the "main" function
address@hidden or @code{FC_MAIN} instead of @code{main}.
+one should use this macro and declare the "main" function like so:
+
address@hidden
+#ifdef __cplusplus
+  extern "C"
+#endif
+int F77_MAIN(int argc, char *argv[]);
address@hidden example
+
+(Again, replace @code{F77} with @code{FC} for Fortran instead of Fortran 77.)
 @end defmac
 
 @defmac AC_F77_WRAPPERS