empty_string - Man Page

#include <allegro.h>

extern char empty_string[];

You can't just rely on "" to be a valid empty string in any encoding  format. This global buffer contains a number of consecutive zeros, so it  will be a valid empty string no matter whether the program is running in  ASCII, Unicode, or UTF-8 mode.

get_datafile_property(3), ustrsize(3), ustrsizez(3).