Question

Basically, here’s my problem. I’m calling someone else’s FORTRAN functions from my C++ code, and it’s giving me headaches. Some code:

function c_error_message()
character(len = 255) :: c_error_message
errmsg(1:9) = 'ERROR MSG'
return
end

That’s the FORTRAN function. My first question is: Is there anything in there that would cause a segfault?

If not, then second: What does that return? A pointer?
I’m trying to call it with the following C statement:

char *e = c_error_message_();

That causes a segfault.

c_error_message();

That too causes a segfault.

I declared c_error_message_() earlier on with the following code:

extern"C" {
    char* c_error_message_();
}

Would declaring a function with a different return type than the actual return type cause a segfault?

I’m at a loss. Thanks for any replies.

Answer 1

Here is a method that works. When I tried to use the ISO C Binding with a function returning a string, the compiler objected. So if instead you use a subroutine argument:

subroutine fort_err_message (c_error_message) bind (C, name="fort_err_message")

use iso_c_binding, only: C_CHAR, C_NULL_CHAR

character (len=1, kind=C_CHAR), dimension (255), intent (out) :: c_error_message

character (len=255, kind=C_CHAR) :: string
integer :: i

string = 'ERROR MSG' // C_NULL_CHAR

do i=1, 255
   c_error_message (i) = string (i:i)
end do

return

end subroutine fort_err_message

The Fortran is a bit awkward because technically a C-string is an 1D array of characters.

And example C code to demo that this works:

#include <stdio.h>

void fort_err_message ( char chars [255] );

int main ( void ) {

  char chars [255];

  fort_err_message ( chars );

  printf ( "%s\n", chars );

  return 0;

}