Memory alignment for strings ifort vs gfortran

15

It works in practice:

! f_get_string.f90
subroutine f_get_string(str) bind(c,name="f_get_string")
    use, intrinsic :: iso_c_binding, only: c_char
    character(len=:,kind=c_char), allocatable, intent(out) :: str
    str = "hello from fortran"
end subroutine

// get_string_main.cpp

#include <iostream>
#include <string_view>
#include <ISO_Fortran_binding.h>

// str is `character(c_char,len=:), allocatable`
extern "C" void f_get_string(CFI_cdesc_t *str);

static auto make_string_view(const CFI_cdesc_t *str) {
    return std::string_view{(char *) str->base_addr, str->elem_len};
}

int main() {
    
    // Create space for the Fortran C descriptor
    CFI_CDESC_T(0) f_str_desc;
    CFI_cdesc_t *f_str = (CFI_cdesc_t *) &f_str_desc;

    // Establish that the descriptor is a deferred-length string
    CFI_establish(f_str,
        nullptr,
        CFI_attribute_allocatable,
        CFI_type_char,
        0, /* elem_len */
        0, /* rank */
        nullptr);

    // Obtain the string from the Fortran routine
    f_get_string(f_str);

    // Create a C++-17 string-view
    auto str = make_string_view(f_str);
    std::cout << str << '\n';

    // Free up memory of the Fortran string
    CFI_deallocate(f_str);
    if (f_str->base_addr) return 1;
    
    return 0;
}

$ gfortran-13 -c f_get_string.f90 
$ g++-13 -std=c++17 get_string_main.cpp f_get_string.o -lgfortran
$ ./a.out
hello from fortran