Returning Character Data Types

A discrepancy occurs when working with C and Fortran character strings: C strings are null-terminated while Fortran strings have known lengths. A C routine that returns a character string will be returning a pointer to a null-terminated string. A Fortran routine will not have any knowledge of the string’s length.

If a Fortran routine is returning a string to a C program, the Fortran program must null-terminate the string.

Example of Returning Character Types from C to Fortran

The following shows the Fortran code that declares interfaces to a C routine and calls the C routine. In the example, the pointer returned by the C routine can be passed along to another C routine. However, it is not usable by Fortran as-is since Fortran has no knowledge of the string's length.

Fortran Code Example
! This program declares a C program that returns a char* ! calls it, and has to pass that return value along to ! a different c program because type(c_ptr) is limited ! in a pure Fortran program. program FCallsCReturnsCptr use, intrinsic :: iso_c_binding ! declare the interfaces to the C routines interface type(C_PTR)function my_croutine1 ( input) bind(c) import integer(c_int), value :: input end function my_croutine1 subroutine my_cprint( string ) bind(c) import c_ptr type(C_PTR), value :: string end subroutine my_cprint end interface call my_cprint(my_croutine1(42)) end program

Fortran Code Example

! This program declares a C program that returns a char*
!  calls it, and has to pass that return value along to
!  a different c program because type(c_ptr) is limited
!  in a pure Fortran program.

program FCallsCReturnsCptr

use, intrinsic :: iso_c_binding

! declare the interfaces to the C routines

interface
   type(C_PTR)function my_croutine1 ( input) bind(c)
     import
     integer(c_int), value :: input
   end function my_croutine1

   subroutine my_cprint( string ) bind(c)
     import c_ptr
     type(C_PTR), value :: string
   end subroutine my_cprint
end interface

   call my_cprint(my_croutine1(42))

end program

Called C Routine Example
#include <stdio.h> char my_croutine1 (int input) { static char temp[30]; temp[0] = '\0'; sprintf(temp, "in routine, you said %d", input); return temp; } void my_cprint (char string) { printf ("cprint says %s\n", string); return; }

Called C Routine Example

#include <stdio.h>

char *my_croutine1 (int input) {
   static char temp[30];
   temp[0] = '\0';
   sprintf(temp, "in routine, you said %d", input);
  return temp;
}

void my_cprint (char *string) {
  printf ("cprint says %s\n", string);
 return;
}

The example above shows the C code used to call a Fortran routine. The Fortran routine returns a string that is then printed by the C program. As shown in the example, it is relatively straightforward to pass a string back to C from Fortran. Additionally, the string can be easily used by the C program because it is NULL-terminated.

In the above example, the following restrictions and behaviors apply:

The function's length and result do not appear in the call statement; they are added by the compiler.
The called routine must copy the result string into the location specified by result; it must not copy more than length characters.
If fewer than length characters are returned, the return location should be padded on the right with blanks; Fortran does not use zeros to terminate strings.
The called procedure is type void.

Example of Returning Character Types from Fortran to C

The following shows the C code used to call a Fortran routine; the Fortran routine returns a string that is then printed by the C program.

C Code Example
#include <stdio.h> char *GetFortranWords(void); int main() { printf ("Fortran says this: %s\n", GetFortranWords()); return 0; }

Called Fortran Routine Example
`! this routine is called from C, returning a string that ! can be printed by the caller function GetFortranWords bind(c, name="GetFortranWords") use, intrinsic :: iso_c_binding type(C_ptr) :: GetFortranWords character*30 returnval returnval = "I like to type words!" // char(0) GetFortranWords = C_LOC(returnval) return end`

Called Fortran Routine Example

! this routine is called from C, returning a string that
!  can be printed by the caller

function GetFortranWords bind(c, name="GetFortranWords")
use, intrinsic :: iso_c_binding
type(C_ptr) :: GetFortranWords
character*30 returnval

returnval = "I like to type words!" // char(0)


GetFortranWords = C_LOC(returnval)
return
end