ustrncat man page
ustrncat — Concatenates a string to another one, specifying size. Allegro game programming library.
char *ustrncat(char *dest, const char *src, int n);
This function is like ustrcat() except that no more than `n' characters from `src' are appended to the end of `dest'. If the terminating null character in `src' is reached before `n' characters have been written, the null character is copied, but no other characters are written. If `n' characters are written before a terminating null is encountered, the function appends its own null character to `dest', so that `n+1' characters are written. You should try to avoid this function because it is very easy to overflow the destination buffer. Use ustrzncat instead.
The return value is the value of `dest'.
uconvert(3), ustrcat(3), ustrzncat(3)