Best practices for passing C strings

19

your pointer argument is being passed by reference not by value. Your current interface is equivalent to

void open_file_c(int unit, char** filename);

which is the reason it does not work. What you want is Fortran to receive the value of that pointer:

void open_file_c(int unit, char* filename);

i.e.

! Option #1: use C pointer
subroutine open_file_c(unit, filename_cptr) bind(C)
      integer(c_int), intent(in), value :: unit
      type(c_ptr), value :: filename_cptr
end subroutine

! Option #2: use datatype directly: your pointer is the reference to it
subroutine open_file_c2(unit, filename) bind(C)
    integer(c_int), intent(in), value :: unit
    character(kind=c_char), intent(in) :: filename(*)
end subroutine
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