real function evapol(tu,nu,tv,nv,c,rad,x,y)

##</code><code>##

c   rad   : real function subprogram, defining the boundary of the
c           approximation domain. must be declared external in the
c           calling (sub)-program

##</code><code>##

        double precision function myfunc(x, y, func)
        double precision x, y, z, v
        double precision func
        v = 1.5
        z = func(v)
        myfunc = x * y * z
        end

##</code><code>##

$ g77 -c myfunc.f -o myfunc.o

##</code><code>##

$ h2xs -Afn P2f2p

##</code><code>##

#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
#include "ppport.h"

MODULE = P2f2p          PACKAGE = P2f2p

##</code><code>##

#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
#include "ppport.h"

static SV * userfunc;

double userfunc_wrapper(double *x) {
    int n;
    double r;
    
    dSP;
    ENTER;
    SAVETMPS;

    PUSHMARK(SP);
    XPUSHs(sv_2mortal(newSVnv(*x)));
    PUTBACK;

    n = call_sv(userfunc, G_SCALAR);

    SPAGAIN;
    if (n != 1) croak("wrong number of args returned!\n");
    r = POPn;
    
    PUTBACK;
    FREETMPS;
    LEAVE;
    return r;
}

// the Fortran function
double myfunc_(double *x, double *y, void* func);


double myfunc(double x, double y) {
    double r;
    r = myfunc_(&x, &y, &userfunc_wrapper);
    return r;
}


MODULE = P2f2p		PACKAGE = P2f2p		

double
myfunc(x, y, ufunc)
        double x
        double y
        SV * ufunc
    CODE:
        userfunc = newSVsv(ufunc);
        RETVAL = myfunc(x, y);
    OUTPUT:
        RETVAL

##</code><code>##

    LIBS              => [''], # e.g., '-lm'
    OBJECT            => '$(O_FILES)', # link all the C files too

##</code><code>##

    LIBS              => ['-lg2c -lm'], # e.g., '-lm'
    OBJECT            => '$(O_FILES) myfunc.o', # link all the C files too

##</code><code>##

    perl Makefile.PL
    make
    make test

##</code><code>##

$ mkdir -p /path/to/working/directory/auto/P2f2p
$ cp blib/arch/auto/P2f2p/P2f2p.so /path/to/working/directory/auto/P2f2p/
$ cp blib/lib/P2f2p.pm             /path/to/working/directory/

##</code><code>##

#!/usr/bin/perl

use P2f2p;

sub userfunc {
    my $v = shift;
    print "userfunc()\n  called from Fortran with arg: $v\n";
    my $r = $v + 0.5;
    print "  returning: $r\n";
    return $r;
}

my $r = P2f2p::myfunc(3, 7, \&userfunc );

printf "myfunc() returned: %.2f\n", $r;

##</code><code>##

userfunc()
  called from Fortran with arg: 1.5
  returning: 2
myfunc() returned: 42.00