Visible to Intel only — GUID: GUID-350B7218-7CEC-46D3-ACB3-5E3AC493EF14
Visible to Intel only — GUID: GUID-350B7218-7CEC-46D3-ACB3-5E3AC493EF14
Return 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 returns a pointer to a null-terminated string. A Fortran routine does not know the string’s length.
If a Fortran routine is returning a string to a C program, the Fortran program must null-terminate the string.
Return Character Types from C to Fortran
The following examples show the Fortran code that declares interfaces to a C routine, then calls the C routine. The pointer returned by the C routine can be passed along to another C routine. Fortran cannot use this method as-is, since Fortran does not know 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
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 previous example shows the C code used to call a Fortran routine. The Fortran routine returns a string that is then printed by the C program. To pass a string back to C from Fortran is simple. The string can be easily used by the C program because it is null-terminated.
In the previous examples, 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.
Return Character Types from Fortran to C
The following examples show 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(len=30),save, target :: returnval
returnval = "I like to type words!" // char(0)
GetFortranWords = C_LOC(returnval)
return
end