Intel® Fortran Compiler 16.0 User and Reference Guide
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.
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.
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Fortran Code Example |
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! 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
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Called C Routine Example |
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#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;
}
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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.
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.
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C Code Example |
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#include <stdio.h>
char *GetFortranWords(void);
int main() {
printf ("Fortran says this: %s\n", GetFortranWords());
return 0;
}
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Called Fortran Routine Example |
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! 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 |