Here's a quick tutorial, courtesy of Chris Ullrich,
on calling fortran from C/C++ on HP9000.
For details, refer to the fortran reference manual for 
HP9000 series computers.

1) The most important thing to remember is that fortran passes all variables
   by reference,
   - except function return values.
   - except for arrays, which have the same type of access as in C
     (pointer based).
2) the C name of a fortran subroutine is the same, with the same
   argument list (but all fortran types -> C pointers ).
3) Fortran is case insensitive.

4) Strings are an absolute nightmare, which I won't discuss.
5) multidimensional arrays are stored in reverse order in fortran (row
   major) than C (column major) -> do all numerical work in one language OR
   the other (otherwise one of them will be really slow).

consider a fortran routine:

      subroutine doStuff( k, j, y )

      implicit none

      integer k, j
      double precision y(10)

C do stuff

Could be called from C, by doing something like
// C - code

     double y[10];
     int    k,j;

     dostuff( &k, &j, y );

So to make a program:
1) compile your fortran code into object code (.o file).

   f77 -c .f

2) compile your C code, and include the fortran object in the link line.
   also include these libs in this order: -lcl -lisamstub -lc

   gcc .c -o  .o -lcl -lisamstub -lc

3) run your prog.