Return a string array from a C++ function to Fortran

Question

I am struggling with filling a string array in a C++ code. Here is what I tried to do:

main.f90:


program main
    implicit none
    character(len=100) arr(3)
    call get_string_array(arr, 100, 3)
    write(*,'(3(a,3x))') arr
end

cpp_func.cpp:


#include <cstring>
#include <algorithm>

using namespace std;

void ConvertToFortran(char* fstring, size_t fstring_len, const char* cstring)
{
    size_t inlen = strlen(cstring);
    size_t cpylen = min(inlen, fstring_len);

    fill(fstring, fstring + fstring_len, ' ');
    copy(cstring, cstring + cpylen, fstring);
}

extern "C"
{
  void get_string_array_(char** arr, int len, int n)
  {
      const char* strings[] = {"duma1", "duma2", "duma3"};
      for (int i = 0; i < n; i++)
          ConvertToFortran(arr[i], len, strings[i]);
  }
}

After compiling the two files as

ifx -c main.f90
icx -c cpp_func.cpp
ifx main.o cpp_func.o

I receive a segmentation fault. Could you please help me how to print array arr properly within main.f90?

*** Edit ***

Any solution which is able to pass a string array from C++ to Fortran would be appreciated.

You need to establish on which side of the fence the error is occuring, whether it is in Fortran or C++. The C++ code by itself has nothing wrong with it provided that the pointers are valid and there is room to copy the characters into the buffer. — PaulMcKenzie
– PaulMcKenzie, Commented Sep 24, 2024 at 11:52
Are you certain that the array is represented as char*[3] and not as char[3][100]? In the latter case, you would receive a char* pointer pointing to a contiguous array of 300 characters. — nneonneo
– nneonneo, Commented Sep 24, 2024 at 11:59
Is there a particular constraint you have which means you can't use the language interoperability features defined by Fortran? If you are relying on implementation-specific methods then you will need to provide us substantially more detail about the implementations and systems you are using. — francescalus
– francescalus, Commented Sep 24, 2024 at 12:13
The first crucial thing is that you should be implementing the interactions between languages using C-interoperable interfaces ("bind(C)") and the second is to work out what the interoperable things are. If you can write using C interop with a clear statement of design goals, we'll be able to give more concrete advice. — francescalus
– francescalus, Commented Sep 24, 2024 at 13:07
Without trying to debug your code myself, I'd wonder whether you need pass-by-value for len and n. (Again: bind(C).) — francescalus
– francescalus, Commented Sep 24, 2024 at 13:14

PierU · Accepted Answer · 2024-09-24 20:30:55Z

6

First, you should use the C binding features of Fortran, to have a more robust code:

program main
   use iso_c_binding
   implicit none
   character(len=100) arr(3)
   
   interface get_string_array
      subroutine get_string_array(a, len, n) bind(C)
         import c_int, c_char
         integer(c_int), value :: len, n
         character(kind=c_char,len=1) :: a(*)
      end subroutine
   end interface
   
   call get_string_array(arr, 100, 3)
   write(*,'(a)') arr
end

As you can see the interface of the C routine is described. In particular, len and n have the value attribute, because C passes the scalar objects by value. Regarding the character array, it is described in the interface as a flat 1D array of individual characters, because this is how the C side sees it with the char* declaration.

On the C side you don't have a lot of changes. char* instead of char**, and &arr[i*len] instead of arr[i]: for, to address the i-th string of the Fortran array, you have to address the (len*i)-th character in C.

#include <cstring>
#include <algorithm>

using namespace std;

void ConvertToFortran(char* fstring, size_t fstring_len, const char* cstring)
{
    size_t inlen = strlen(cstring);
    size_t cpylen = min(inlen, fstring_len);
    fill(fstring, fstring + fstring_len, ' ');
    copy(cstring, cstring + cpylen, fstring);
}

extern "C"
{
    void get_string_array(char* arr, int len, int n)
    {
        const char* strings[] = {"duma1", "duma2", "duma3"};
        for (int i = 0; i < n; i++) {
            ConvertToFortran(&arr[i*len], (size_t)len, strings[i]);
        }
    }
}

answered Sep 24, 2024 at 20:30

PierU

2,7371 gold badge5 silver badges17 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

TobiR Over a year ago

Thank you very much your detailed implementation! I really appreciate what you provided, which of course works well. Hopefully this will be helpful for those who would like to see a simple example in this topic.

Collectives™ on Stack Overflow

Return a string array from a C++ function to Fortran

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related