al_wait_for_vsync man page
al_wait_for_vsync — Allegro 5 API
#include <allegro5/allegro.h> bool al_wait_for_vsync(void)
Wait for the beginning of a vertical retrace. Some driver/card/monitor combinations may not be capable of this.
Note how al_flip_display(3) usually already waits for the vertical retrace, so unless you are doing something special, there is no reason to call this function.
Returns false if not possible, true if successful.