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Mixed Programming Fortran and C
MODULE my_fortran
  USE iso_c_binding
  IMPLICIT NONE
  CONTAINS
  SUBROUTINE my_subroutine(a,b) BIND(C,name="my_sub")
    INTEGER(c_int),INTENT(in),VALUE :: a
    REAL(C_DOUBLE),INTENT(out) :: b
    ...
  END SUBROUTINE
END MODULE

void my_sub(int, double *);

#include "my_sub.h"

int main(){
  double b;
  int a=3;
  my_sub(a,&b);
  ...
}

double   x, y;
   ..................
   x = 2.0;
   y = gamma_(&x)

float  --- REAL     (this is typical, but not always the case)
double --- REAL*8

extern void read_(int *small, float *medium, double *large);

read_(&small, &medium, &large);

SUBROUTINE READ(small,medium,large)

  INTEGER       small
  REAL          medium
  DOUBLE        large