d_text_list_proc man page
d_text_list_proc — Dialog procedure implementing a list box object with type ahead. Allegro game programming library.
int d_text_list_proc(int msg, DIALOG *d, int c);
Like d_list_proc, but allows the user to type in the first few characters of a listbox entry in order to select it. Uses dp3 internally, so you mustn't store anything important there yourself.