al_ustr_truncate - Allegro 5 API

       Truncate  a portion of a string from byte offset start_pos onwards.  start_pos can be past the end of the
       string (has no effect) but cannot be less than 0.

       Returns true on success, false on error.

       al_ustr_truncate - Allegro 5 API

al_ustr_remove_range(3alleg5),           al_ustr_ltrim_ws(3alleg5),            al_ustr_rtrim_ws(3alleg5),
       al_ustr_trim_ws(3alleg5)

Allegro reference manual                                                               al_ustr_truncate(3alleg5)

              #include <allegro5/allegro.h>

              bool al_ustr_truncate(ALLEGRO_USTR *us, int start_pos)