dll_obj_t *dll_findkey(const dll_t *restrict dll,
    f_dll_obj_handler fn_serach,
    void *restrict any_data);
dll_obj_t *dll_findkeyr(const dll_t *restrict dll,
    f_dll_obj_handler fn_serach,
    void *restrict any_data);

• dll_findkey: Find object by data(key) from start
• dll_findkeyr: Find object by data(key) from end

The third argument(any_data) provided for more flexible use. It's just will be the second argument to your fn_search handler

Pointer to desired object:

• when fn_search returns a zero.

NULL:

• when fn_serach returns a negative value or if the desired object doesn't exist in the list
• dll is NULL
• if DLL_BIT_EIGN is not specified:
  • fn_search is NULL
  • list dll hasn't any objects
  • data inside at least 1 object is pointed to NULL