al_ustr_next - Allegro 5 API

       Find  the  byte  offset of the next code point in string, beginning at *pos.  *pos does not have to be at
       the beginning of a code point.

       Returns true on success, and the value pointed to by pos will be updated to the found offset.   Otherwise
       returns false if *pos was already at the end of the string, and *pos is unmodified.

       This  function just looks for an appropriate byte; it doesn’t check if found offset is the beginning of a
       valid code point.  If you are working with possibly invalid UTF-8 strings then it could  skip  over  some
       invalid bytes.

       al_ustr_next - Allegro 5 API

al_ustr_prev(3alleg5)

Allegro reference manual                                                                   al_ustr_next(3alleg5)

              #include <allegro5/allegro.h>

              bool al_ustr_next(const ALLEGRO_USTR *us, int *pos)