al_ustr_set_chr
size_t al_ustr_set_chr(ALLEGRO_USTR *us, int start_pos, int32_t c)
Replace the code point beginning at byte offset pos with c. pos cannot be less than 0. If pos is past the end of us1 then the space between the end of the string and pos will be padded with NUL ('\0') bytes. If 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.
See also: al_ustr_replace_range
Most helpful discussions: