Fortran accepting string(?) from C

Question

I feel like this should be an easy question, but I can't get it to work. I have some Fortran code that takes an input like:

      SUBROUTINE TRACE(X,Y,NAME,XX,YY)
      EXTERNAL NAME
      CALL NAME(X,Y,XX,YY)

and I'm trying to pass in a name from C++ in the form:

float x,y,xx,yy;
char * name="IGRF";
trace_(&x,&y,name,&xx,&yy);

It compiles, but I always get segfaults when I try to call the NAME subroutine. A subroutine called IGRF is defined in the file, and I can call the IGRF subroutine directly from C++, but need this TRACE routine. When running in gdb, it says the NAME variable comes through as a pointer to void.

I've tried passing NAME, &NAME, &NAME[0], a char NAME[4] that's stripped of its \0 to perfectly fit the name, and they all come back showing the same void pointer. Does anybody know how to get a function name from C++ into that EXTERNAL variable in Fortran?

Thank you

Answer 1

I got it working with CMake as well.

CMakeLists.txt:

cmake_minimum_required(VERSION 3.5)
project(CppFortran C CXX Fortran)

add_executable(CppFortran
    froutine.f90
    main.cpp
    )

main.cpp

#include <iostream>

extern "C" {
int getString(char *file_name);
}

int main() {
    int l;
    char *name = (char*)"IGRF";

    l = getString(name);
    std::cout << "In C++:"<< std::endl;
    std::cout << "length: " << l << std::endl;

    return 0;
}

froutine.f90

integer(kind=c_int) function getString(instr) bind(C,name='getString')
    use, intrinsic :: iso_c_binding
    character(kind=c_char), dimension(*), intent(IN) :: instr
    integer :: len
    integer :: i

    len=0
    do
        if (instr(len+1) == C_NULL_CHAR) exit
        len = len + 1
    end do

    print *, 'In Fortran:'
    print *, 'Got string: ', (instr(i),i=1,len)
    getstring = len
end function getString

Console output:

 In Fortran:
 Got string: IGRF
In C++:
length: 4

Answer 2

So one advantage of Fortran2003 and later is that C interoperability is defined into the standard; it's a bit of a PITA to use, but once it's done, it's guaranteed to work across platforms and compilers.

So here's cprogram.c, calling a Fortran routine getstring:

#include <stdio.h>

int main(int argc, char **argv) {
    int l;
    char *name="IGRF";

    l = getstring(name);

    printf("In C: l = %d\n",l);

    return 0;
}

and here's fortranroutine.f90:

integer(kind=c_int) function getstring(instr) bind(C,name='getstring') 
    use, intrinsic :: iso_c_binding
    character(kind=c_char), dimension(*), intent(IN) :: instr
    integer :: len
    integer :: i

    len=0
    do
       if (instr(len+1) == C_NULL_CHAR) exit
       len = len + 1
    end do


    print *, 'In Fortran:'
    print *, 'Got string: ', (instr(i),i=1,len)
    getstring = len
end function getstring

The makefile is simple enough:

CC=gcc
FC=gfortran

cprogram: cprogram.o fortranroutine.o
    $(CC) -o cprogram cprogram.o fortranroutine.o -lgfortran

fortranroutine.o: fortranroutine.f90
    $(FC) -c $^

clean:
    rm -f *.o cprogram *~

and running it works, under both gcc/gfortran and icc/ifort:

 In Fortran:
 Got string: IGRF
In C: l = 4

Update: Oh, I just realized that what you're doing is rather more elaborate than just passing a string; you're essentially trying to pass a function pointer pointing to a C callback routine. That's a little tricker, because you have to use Fortran interfaces to declare the C routine -- just using extern won't work (and isn't as good as explicit interfaces anyway, as there's no type checking, etc.) So this should work:

cprogram.c:

#include <stdio.h>

/* fortran routine prototype*/
int getstring(char *name, int (*)(int));

int square(int i) {
    printf("In C called from Fortran:, ");
    printf("%d squared is %d!\n",i,i*i);
    return i*i;
}


int cube(int i) {
    printf("In C called from Fortran:, ");
    printf("%d cubed is %d!\n",i,i*i*i);
    return i*i*i;
}

int main(int argc, char **argv) {
    int l;
    char *name="IGRF";

    l = getstring(name, &square);
    printf("In C: l = %d\n",l);
    l = getstring(name, &cube);
    printf("In C: l = %d\n",l);


    return 0;
}

froutine.f90:

integer(kind=c_int) function getstring(str,func) bind(C,name='getstring')
    use, intrinsic :: iso_c_binding
    implicit none
    character(kind=c_char), dimension(*), intent(in) :: str
    type(c_funptr), value :: func

    integer :: length
    integer :: i

    ! prototype for the C function; take a c_int, return a c_int
    interface
        integer (kind=c_int) function croutine(inint) bind(C)
            use, intrinsic :: iso_c_binding
            implicit none
            integer(kind=c_int), value :: inint
        end function croutine
    end interface
    procedure(croutine), pointer :: cfun

    integer(kind=c_int) :: clen

    ! convert C to fortran procedure pointer,
    ! that matches the prototype called "croutine"
    call c_f_procpointer(func, cfun)

    ! find string length
    length=0
    do
       if (str(length+1) == C_NULL_CHAR) exit
       length = length + 1
    end do

    print *, 'In Fortran, got string: ', (str(i),i=1,length), '(',length,').'

    print *, 'In Fortran, calling C function and passing length'
    clen = length
    getstring = cfun(clen)

end function getstring

And the results:

$ gcc -g -Wall   -c -o cprogram.o cprogram.c
$ gfortran -c fortranroutine.f90 -g -Wall
$ gcc -o cprogram cprogram.o fortranroutine.o -lgfortran -g -Wall
$ gpc-f103n084-$ ./cprogram 
./cprogram 
 In Fortran, got string: IGRF(           4 ).
 In Fortran, calling C function and passing length
In C called from Fortran:, 4 squared is 16!
In C: l = 16
 In Fortran, got string: IGRF(           4 ).
 In Fortran, calling C function and passing length
In C called from Fortran:, 4 cubed is 64!
In C: l = 64

Answer 3

As opposed to more dynamic languages like python that support reflection and/or runtime evaluation of expressions, Fortran, C, and C++ do not. That is, there is no built-in way to convert a string containing a name of a procedure into a procedure reference and call it.

That is, in your example, NAME needs to be a pointer to a function, not a string. By using the ISO_C_BINDING feature, you can pass function pointers between C and Fortran.

Answer 4

It appears that FORTRAN 77 requires both a pointer to the characters and the length of the string be passed from C++ (or C) into FORTRAN.

See this utah.edu document on using C and C++ with FORTRAN and specifically search for the section of the document that starts with "CHARACTER*n arguments".