mixed FORTRAN-C programing for this question: https://stackoverflow.com/questions/53585628/minimum-modification-to-make-a-visual-studio-fortran-c-mixed-code-gfortran-gcc-c

CMAIN.C

/*     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);
}

FORSUBS.FOR

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