C Calls to Fortran

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This article applies the principles in Mixed-Language Issues to a typical case involving one function call and one subroutine call from C to Fortran. Default conventions are assumed for Fortran, so adjustments are made to the C code.

The C main program uses the __stdcall keyword to call the Fortran routines with the correct calling convention. The C source must use all-uppercase names for the routines, because this Fortran code does not use the C, STDCALL, or ALIAS attributes. Finally, pass by value and pass by reference are specified explicitly, though pass by reference would have been assumed by default for Fortran.

/*     File CMAIN.C   */

#include <stdio.h>

extern int __stdcall FACT (int n);
extern void __stdcall PYTHAGORAS (float a, float b, float *c);

main()
{
    float c;
    printf("Factorial of 7 is: %d\n", FACT(7));
    PYTHAGORAS (30, 40, &c);
    printf("Hypotenuse if sides 30, 40 is: %f\n", c);
}

C    File FORSUBS.FOR
C
      INTEGER*4 FUNCTION Fact (n)
      INTEGER*4 n [VALUE]
      INTEGER*4 i, amt
      amt = 1
      DO i = 1, n
        amt = amt * i
      END DO
      Fact = amt
      END

      SUBROUTINE Pythagoras (a, b, c)
      REAL*4 a [VALUE]
      REAL*4 b [VALUE]
      REAL*4 c [REFERENCE]
      c = SQRT (a * a + b * b)
      END