fortran pass character*81 array to c/c++ code

Question

I am a fresh in programming, I wanna to call a fortran function in my c++ code. the thing is I dont know how to pass a fortran character*81 array to my c++.

fortran code

Answer 1

The following code seems to work for gcc4 on Linux(x86_64), but it is not clear whether it is also valid for other platforms. (As suggested above, C-interoperability of modern Fortran may be useful.)

func01.f90

subroutine func01( a )
    character(*) :: a( 2 )
    print *
    print *, "char length = ", len(a(1)), len(a(2))
    print *, "raw a(1) : [", a(1), "]"
    print *, "raw a(2) : [", a(2), "]"
    print *, "trim     : [", trim(a(1)), "] [", trim(a(2)), "]"
end

main.cpp

extern "C" {
    void func01_( char *c, const int len );
}

#include <iostream>
#include <cstring>  // for memset()
int main()
{
    const int lenmax = 30, numstr = 3; // changed char length to 30 to fit in the terminal
    char a[ numstr ][ lenmax ];
    std::string str[ numstr ];

    str[0] = "moon"; str[1] = "mercury"; str[2] = "jupiter";

    for( int k = 0; k < numstr; k++ ) {
        memset( a[k], ' ', lenmax );  // fill space                                              
        str[k].copy( a[k], lenmax );  // copy at most lenmax char (no \0 attached)                        
    }

    func01_( a[0], lenmax );
    func01_( a[1], lenmax ); // pass from mercury
    return 0;
}

Compile

$ g++ func01.f90 main.cpp -lgfortran

Result

char length =           30          30
raw a(1) : [moon                          ]
raw a(2) : [mercury                       ]
trim     : [moon] [mercury]

char length =           30          30
raw a(1) : [mercury                       ]
raw a(2) : [jupiter                       ]
trim     : [mercury] [jupiter]

Answer 2

Here is a portable solution to pass an array of arbitrary length strings from C to Fortran.

I used a C++ file very similar to your own:

#include <iostream>

extern "C" void func01(const char **a);

int main()
{
  const char *a[2] = {"Hello World","This is a test"};
  func01(a);
  return 0;
}

The only changes above are the initialization of the character arrays and removing the not-so-portable underscoring of the Fortran function. Instead we will be using standard C interoperability provided by Fortran 2003. The Fortran implementation of func01 becomes:

subroutine func01(cstrings) bind(C,name="func01")
  use, intrinsic :: iso_c_binding, only: c_ptr, c_char, c_f_pointer
  implicit none
  type(c_ptr), dimension(2), target, intent(in) :: cstrings
  character(kind=c_char), pointer :: a1(:), a2(:)

  ! size_t strlen(char * s);
  interface
     function strlen(s) bind(C, name='strlen')
       use, intrinsic :: iso_c_binding, only: c_ptr, c_size_t
       implicit none
       type(c_ptr), intent(in), value :: s
       integer(c_size_t) :: strlen
     end function strlen
  end interface

  call c_f_pointer(cstrings(1), a1, [strlen(cstrings(1))])
  call c_f_pointer(cstrings(2), a2, [strlen(cstrings(2))])
  write (*,*) a1
  write (*,*) a2
end subroutine func01

The bind attribute is what gives us interoperability with C for the function name and we are using C types for variables. The variable cstrings will take an array of 2 pointers, or in C, *[2] or **. The bulk of the procedure is an interface block which lets us call the standard C library routine strlen to make our life easier with the following calls to c_f_pointer which translates a C pointer to a Fortran pointer.

When compiled and run, the output, as expected, is:

$ ./string-array-test
 Hello World
 This is a test

Compiled and tested with gcc 5.1.0.