Creating a FORTRAN interface to a C function that returns a char*

Question

I\'ve been held up on this for about a week, now, and have searched forum after forum for a clear explanation of how to send a char* from C to FORTRAN. To make the matter m

Answer 1

This thread is a little old, but since I had a similar problem (and probably others will), I post a answer anyway.

The codes posted above will cause a segmentation fault if, for some reason, the C string is null. In addition, there is no need to return a 255-chars string (which will probably need to be trimmed before used), as Fortran 2003/2008 supports functions returning allocatable entities. Using all the information posted above, I ended up with the following function, which gets a C string (pointer), and returns the corresponding Fortran string; If the C string is null, it returns "NULL", similarly to C's "(null)" printed in similar cases:

function C_to_F_string(c_string_pointer) result(f_string)
use, intrinsic :: iso_c_binding, only: c_ptr,c_f_pointer,c_char,c_null_char
type(c_ptr), intent(in) :: c_string_pointer
character(len=:), allocatable :: f_string
character(kind=c_char), dimension(:), pointer :: char_array_pointer => null()
character(len=255) :: aux_string
integer :: i,length
call c_f_pointer(c_string_pointer,char_array_pointer,[255])
if (.not.associated(char_array_pointer)) then
  allocate(character(len=4)::f_string); f_string="NULL"; return
end if
aux_string=" "
do i=1,255
  if (char_array_pointer(i)==c_null_char) then
    length=i-1; exit
  end if
  aux_string(i:i)=char_array_pointer(i)
end do
allocate(character(len=length)::f_string)
f_string=aux_string(1:length)
end function C_to_F_string