MIXED LANGUAGE PROGRAMMING

Part 4, Mixing ANSI-C with Fortran 77 or Fortran 90

"Portability or Transportability?"

Presented at the IFIP WG 2.5 International Workshop on "Current Directions in Numerical Software and High Performance Computing", 19 - 20 October 1995, Kyoto, Japan.

CONTENTS

1. INTRODUCTION
2. USE OF A SUBROUTINE AND A FUNCTION WRITTEN IN FORTRAN FROM A PROGRAM WRITTEN IN C
3. USE OF A MATRIX FROM FORTRAN AND C
4. USE OF A COMMON BLOCK FROM FORTRAN IN A C-PROGRAM
5. SUMMARY
6. SUPERCOMPUTERS
   • CRAY
   • MASPAR
   • PARSYTEC
   • FUJITSU
7. ANSI-C AND CONVENTIONAL C
8. COMMENTS

1. INTRODUCTION.

The tests have been done on Silicon Graphics (IRIX 4.0.1 and 5.3), DEC MIPS ULTRIX, DEC Alpha (Digital UNIX), and Sun OS 4.1.3.

The supercomputers Cray Y-MP (UNICOS 8.0.3.1), MasPar, and Parsytec are also discussed.

The results are rather conflicting, some combinations are permitted on some systems but not on others. Unfortunately, not all three compilers have been available for me on all the tested systems.

2. USE OF A SUBROUTINE AND A FUNCTION WRITTEN IN FORTRAN FROM A PROGRAM WRITTEN IN C.

The main program in Fortran is in the file "f2sam.f" and is conventional. The subroutine and the function are in the file "sam.f". Running in Fortran is quite normal.

% cat f2sam.f
        program f2sam
        external f
        integer f       
        character*7 s
        integer b(3)
        call sam(f, b(2), s)
        write(6,10) b(2), f(real(b(2))), s
   10   format(i5,i5,10x,a7)
        stop
        end

% cat sam.f
        subroutine sam(f, b, s) 
        external f
        character*7 s
        integer b, f
        x = 1.3
        s = 'Bo G E '
        b = f(x)
        end
        integer function f(x)
        f=3*x**3
        return
        end

1. The indices in C have to be decreased with 1.
2. Both the subroutine "sam" and the function "f" have to be given an underlining _ at every occurrence in the C-routine, even inside the call of the Fortran-routine.
3. Since Fortran transfers parameters by reference, pointers have to be used in C.
4. As follows from the listing I had a problem with the call of the function inside the print-statement. I solved this in the ugly way by introducing the auxiliary variable x.

% cat c2sam.c
#include <stdio.h>
#include <math.h>

/*     C Call to SAM in Fortran */

       int f_(float *);
main()
{
       char s[7];
       int b[3];
       float x;

       sam_(f_, &b[1], s, 7);
       x =  b[1];
       printf(" %d %d %s \n ", b[1], f_(&x), s); 
}

Linking is normal if the main program is in Fortran, if it is in C some extra libraries have to be mentioned explicitly.

Compilation and linking in the Fortran case

% f77 -c f2sam.f
% f77 -c sam.f
% f77 -o all f2sam.o sam.o
% all
    6  648          Bo G E 

% cc -c c2sam.c
% f77 -c sam.f
% cc -o all c2sam.o sam.o -lF77
% all
 6 648 Bo G E 

Compilation and linking in the C case on Sun OS 4.1.3, using the GNU compiler, gcc version 2.3.3, and Fortran 77:

% gcc -c c2sam.c
% f77 -c sam.f
% gcc -o all c2sam.o sam.o -lF77 -lm
% all
 6 648 Bo G E 

% gcc -c c2sam.c
% f90 -c sam.f
% gcc -o all c2sam.o sam.o /usr/local/lib/f90/libf90.a
% all
 6 648 Bo G E 

On Sun the compilation failed with the default C compiler (not ANSI-C).

On DEC MIPS ULTRIX no specification of extra libraries were required in most cases. I used all eight combinations of f77, f90, cc and gcc (version 2.5.8). With Fortran 77 or Fortran 90 and the GNU compiler the Fortran linking failed "Undefined: __main". Using the Fortran 90 compiler and linking with any C linker required explicit mentioning of the Fortran 90 library

	cc  sam.o c2sam.o /usr/local/NAGWare_f90/lib/libf90.a 
	gcc sam.o c2sam.o /usr/local/NAGWare_f90/lib/libf90.a 

On DEC Alpha (Digital UNIX) I used f90 and cc. It worked without extra libraries with C as the linking language, but failed with Fortran 90 ("c2sam.o: main: " multiply defined and "MAIN__" unresolved).

3. USE OF A MATRIX FROM FORTRAN AND C.

The main program in Fortran is in the file "mlp3.f" and is completely conventional.

% cat mlp3.f
      integer a(2,3)
      call p(a,1,3)
      write (6,10) a(1,3)
 10   format (1x,I9)
      stop
      end

1. The indices in C and Fortran have to be reversed.
2. The indices in C have to be decreased with 1.
3. Since Fortran transfers parameters by reference, pointers have to be used in C.
4. The subroutine "p" has to be given an "underlining".

% cat mlp4.c
#include <stdio.h>
#include <math.h>

/*      Array Handling */

void
p_(a,i,j)
int *i, *j, a[3][2];
{
a[*j-1][*i-1] = 99;
}

% cc -c mlp4.c
% f77 -c mlp3.f

% f77 -o alla mlp3.o mlp4.o
% alla
        99

% cc -o allac mlp3.o mlp4.o -lF77 -lI77 -lisam 
% allac
        99

On the Sun OS 4.1.3 the standard method worked with f77 and with gcc the libraries "-lF77 -lm" were required, while with the default cc only "-lF77" was required.

Switching to Fortran 90 there were still no problems if Fortran was the linking language, but linking with cc required explicit mentioning of "/usr/local/lib/f90/f90lib.a" and linking with gcc in addition required the "-lm" switch.

On DEC MIPS ULTRIX no specification of extra libraries were required when Fortran was used as the linker. I used all eight combinations of f77, f90, cc and gcc (version 2.5.8). Using the Fortran 77 compiler and linking with any C linker required an impressive command

% cc  mlp3.o mlp4.o -lfor -lutil -li -lots -lUfor
% gcc mlp3.o mlp4.o -lfor -lutil -li -lots -lUfor

% cc  mlp3.o mlp4.o /usr/local/NAGWare_f90/lib/libf90.a 
% gcc mlp3.o mlp4.o /usr/local/NAGWare_f90/lib/libf90.a 

% cc -o all mlp3.o mlp4.o -lfor -lutil -lots

4. USE OF A COMMON BLOCK FROM FORTRAN IN A C-PROGRAM.

We put the COMMON block into the subroutine "sam" in the file "mlp2.f". It is completely conventional.

% cat mlp2.f
      subroutine sam()
      common /namn/ i, r
      i = 786
      r = 3.2
      return
      end

1. The COMMON block is declared as a structure with the occurring variables defined in the ordinary C-style. The name "namn" of the COMMON block is given an "underlining".
2. The subroutine "sam" with the COMMON block is called, with an underlining _ .
3. When the C-program requires the values of the COMMON block, the variables are qualified with the name of the block.

% cat mlp1.c
#include <stdio.h>
#include <math.h>

/*      Accessing Common Blocks of Data */

struct S {int i; float r;} namn_;
main()
{
       sam_();
       printf(" %d %f\n",namn_.i,namn_.r); 
}

% cc -c mlp1.c
% f77 -c mlp2.f

% f77 -o all mlp1.o mlp2.o
% all
 786 3.200000

% cc -o allc mlp1.o mlp2.o
% allc
 786 3.200000

% lls
 331 -rwxr-xr-x   1 boein      169464 Aug 17 13:50 all*
  81 -rwxr-xr-x   1 boein       40988 Aug 17 13:51 allc*
%

On the Sun OS 4.1.3 the standard method worked both with f77 and f90 and the default cc.

Compiling with the GNU compiler gcc, the f77 or f90 linking did not work, resulting in an error message "Undefined symbol __main". Linking with gcc worked fine.

On DEC MIPS ULTRIX I used all eight combinations of f77, f90, cc and gcc (version 2.5.8). Everything worked with the exception of using a Fortran linker with a procedure compiled with the GNU C compiler. In these cases "Undefined: __main" was obtained.

On DEC Alpha (Digital UNIX) I used f90 and cc. It worked fine with C as the linking language, but failed with Fortran 90, complaining about multiply defined "main" and unresolved "MAIN__".

5. SUMMARY

	*	Worked without any explicit libraries
	+	Worked with explicit libraries
	-	Failed
	E	Compilation error

Example	Languages		DEC				SGI				Sun
(section)			Unix		Ultrix		4.0.1		5.3		4.1.3
2	f77	cc			*	*	*	+	*	+	E	E
	f77	gcc			-	*			-	+	-	+
	f90	cc	-	*	*	+					E	E
	f90	gcc			-	+					-	+
3	f77	cc			*	+	*	+	*	+	*	+
	f77	gcc			*	+			*	+	*	+
	f90	cc	*	-	*	+					*	+
	f90	gcc			*	+					*	+
4	f77	cc			*	*	*	*	*	*	*	*
	f77	gcc			-	*			-	*	-	*
	f90	cc	-	*	*	*					*	*
	f90	gcc			-	*					-	*

6. SUPERCOMPUTERS

CRAY

% cat tpk.c
#include <stdio.h>
#include <math.h>

/*      Function */
double fun(t)
double t;
{
        return t*t;
}
/*      Program in C */

main()
{
       int i;
       double t;
       double y;
       double a[11];
       for ( i = 0; i <= 10; i++ )
           {
           t = i;
           y = fun(t);
           printf(" %lf \n",y);
           }
}

% cat tpk-ansi.c
#include <stdio.h>
#include <math.h>

/*      Function declared */
double fun(double t);
/*      Program in C */

main()
{      /* No changes */
}
/*      Function defined */
double fun(double t)
{
        return t*t;
}