Interoperability of assumed-type arguments (TS 29113)

To facilitate the interoperability with formal parameters of type void* in C procedures, assumed-type arguments are introduced in Fortran. Assumed-type arguments are entities that are declared with the TYPE(*) specifier. They are unlimited polymorphic, and their dynamic types and type parameters are assumed from the corresponding actual arguments. See the following example:

! example1f.f 
USE, INTRINSIC :: ISO_C_BINDING 
IMPLICIT NONE 
INTEGER(C_SHORT)      :: i_short 
INTEGER(C_INT)        :: i_int 
INTEGER(C_LONG)       :: i_long 

INTERFACE    
   SUBROUTINE c_func(a, flag) BIND(C)       
   USE, INTRINSIC :: ISO_C_BINDING       
   IMPLICIT NONE       
   TYPE(*) :: a       
   INTEGER(C_INT), VALUE :: flag    
   END SUBROUTINE c_func 
END INTERFACE  

i_short = 2 
i_int = 4 
i_long = 4 

CALL c_func(i_short, 1) 
CALL c_func(i_int, 2) 
CALL c_func(i_long, 3) 

END   

// example1c.c 
#include <stdio.h>  
void c_func(void* a, int flag) {    
   if(flag == 1)       
      printf("Type is c_short %hd\n",*(short*)a);    
   else if(flag == 2)       
      printf("Type is c_int %d\n",*(int*)a);    
   else if(flag == 3)       
      printf("Type is c_long %ld\n",*(long*)a);      
   return; 
}

The output of the example is as follows:

Type is c_short 2
Type is c_int 4
Type is c_long 4

In this example, function c_func is defined in the C program, and it is called by a Fortran procedure. Variable a of type void* is a formal parameter of function c_func in the C program. The corresponding dummy argument of a in the Fortran procedure interface is specified by the TYPE(*) specifier, so the dummy argument is of an assumed type. The dynamic type of variable a is determined by the actual argument when function c_func is called. As shown, function c_func is called three times with the actual parameters i_short, i_int, and i_long passed in, so the type of variable a is deduced to c_short, c_int, and c_long accordingly.