al_ustr_offset - Allegro 5 API

       Return  the  byte  offset  (from the start of the string) of the code point at the specified index in the
       string.  A zero index parameter will return the first character of the string.  If index is negative,  it
       counts  backward  from  the  end  of the string, so an index of -1 will return an offset to the last code
       point.

       If the index is past the end of the string, returns the offset of the end of the string.

       al_ustr_offset - Allegro 5 API

al_ustr_length(3alleg5)

Allegro reference manual                                                                 al_ustr_offset(3alleg5)

              #include <allegro5/allegro.h>

              int al_ustr_offset(const ALLEGRO_USTR *us, int index)