al_ustr_set_chr - Allegro 5 API

       Replace  the  code point beginning at byte offset start_pos with c.  start_pos cannot be less than 0.  If
       start_pos is past the end of us then the space between the end of the string and start_pos will be padded
       with NUL ('\0') bytes.  If start_pos is not the start of a valid code point, that is  an  error  and  the
       string will be unmodified.

       On  success, returns the number of bytes written, i.e. the offset to the following code point.  On error,
       returns 0.

       al_ustr_set_chr - Allegro 5 API

al_ustr_replace_range(3alleg5)

Allegro reference manual                                                                al_ustr_set_chr(3alleg5)

              #include <allegro5/allegro.h>

              size_t al_ustr_set_chr(ALLEGRO_USTR *us, int start_pos, int32_t c)