Calling FORTRAN from C

[Robert Kuhn]

To:gcc@gcc.gnu.org
From: Bob Kuhn
Subject: Calling FORTRAN from C

I have a FORTRAN program in which I do not know in advance how large
some of the arrays will be.   Standard FORTRAN has no "alloc"
statement to allocate arrays at execution time.

Looking at the GNU FORTRAN documentation, I have concluded that
there is no direct way to do this.  AM I CORRECT?

In ABSOFT PC FORTRAN and C, I was able to solve the problem by
allocating the arrays in C and calling my FORTRAN program as a
subroutine from C.  Is this practical to do in GNU FORTRAN and C?  I
do not want to spend a lot of time if this is very difficult to
accomplish.

In order do this, I need to do the following:

1. How to call FORTRAN from C.
2. I need to able to pass integers, arrays (both numeric and
   character) to the FORTRAN program.
3. I need to be able to use "alloc" in C with adjustable arrays in
   FORTRAN.

I was able to do all these things with the ABSOFT PC compilers.  I
have no idea how difficult this is to do with the GNU compilers.

Appendix's 1 and 2 contains two programs that worked on the ABSOFT
PC compiler. Appendix 1 contains the simplist possible program with
C calling FORTRAN. I tried it on the GNU UNIX compiler on they did
not work.  Details are in that appendix.  Appendix 2 contains very
simple working PC programs with all the features that I need.

I have two questions:
1. Why did the programs in appendix 1 not work?
2. Will it be difficult to the programs in appendix 2 to work with
   the GNU compilers?  Do you consider this a practical approach?

Any advice on this subject will be greatly appreciated.

Sincerely,
Bob Kuhn

P.S. When I ran the ABSOFT version I did not use "#include <g2c.h>"
----------------------------------------------------------
      Appendix 1 -- Simple Program for calling FORTRAN from C


1. C Program  (testbob.c)

   #include <stdio.h>
   #include <stdlib.h>
   #include <string.h>
   #include <g2c.h>

   int main(int argc, char * argv[])
   {
   void sub1(void);
   printf("Start Program\n");
   sub1();
   printf("End Program\n");
   exit(0); return(0);
   }

2. FORTRAN SUBROUTINE (sub1.for)

      subroutine sub1
      print *, 'This is SUB1'
      return
      end

3. Compile steps
   gcc -c testbob.c
   g77 -c sub1.for

4. Link edit step
   g77 testbob.o sub1.o -o testbob

5. Link edit error message:

   /data/user/rmk/mars$ g77 testbob.o sub1.o -o testbob
   Undefined
   Undifined                        first referenced
    symbol                             in file
   sub1                                testbob.o
   ld: fatal: Symbol referencing errors. No output written to
   testbob collect2: ld returned 1 exit status

The question is: WHAT DID I DO WRONG?

----------------------------------------------------------
       Appendix 2 -- Simple Example with Dynamic Allocation

1. C Program

   #include <stdio.h>
   #include <stdlib.h>
   #include <string.h>
   /*#include <g2c.h>*/

   #define false 0
   #define true 1

   int main(int argc, char * argv[])
   {
   /* Use the typedefs in <g2c.h> */
   /* real *x; integer n; */
   float *x; long n; short j;
   char c[12] ="All is well";

   void test5a(long *n,float *x,char *c);

   printf("Start Program\n");
   if ( (x = calloc(1000,4)) == NULL)  {
      printf("Allocation of 'x' failed\n");
      exit(1);
      }
   n=1000;
   printf("n=%ld\n",n);

   for (j=0; j < 1000; j++) {x[j]=2*(j+1);}

   printf("Call subroutine\n");
   test5a(&n,x,c);

   printf("End Program\n");
   exit(0); return(0);
   }

2. FORTRAN SUBROUTINE

      subroutine test5a(n,x,c)
      dimension x(n)
      character*12 c
      print *,'n=',n,' x(501)=',x(501),' c=',c(1:11)
      return
      end

This program worked with the ABSOFT PC compiler.  How difficult
would it be to get this program to work with the GNU system?