usetat man page
usetat — Replaces a character in a string. Allegro game programming library.
int usetat(char *s, int index, int c);
Replaces the character at the specified index in the string with value `c', handling any adjustments for variable width data (ie. if `c' encodes to a different width than the previous value at that location). If `index' is negative, it counts backward from the end of the string. Example:
usetat(text_string, 2, letter_a);
Returns the number of bytes by which the trailing part of the string was moved. This is of interest only with text encoding formats where characters have a variable length, like UTF-8.
uoffset(3), ugetat(3), uinsert(3), uremove(3)
set_uformat(3), ugetat(3), uinsert(3), uoffset(3), uremove(3).