freebsd man page for vsync

vsync(3alleg4)							  Allegro manual						    vsync(3alleg4)

NAME

       vsync - Waits for a vertical retrace to begin. Allegro game programming library.

SYNOPSIS

       #include <allegro.h>

       void vsync();

DESCRIPTION

       Waits  for a vertical retrace to begin. The retrace happens when the electron beam in your monitor has reached the bottom of the screen and
       is moving back to the top ready for another scan. During this short period the graphics card isn't sending any data to the monitor, so  you
       can do things to it that aren't possible at other times, such as altering the palette without causing flickering (snow). Allegro will auto-
       matically wait for a retrace before altering the palette or doing any hardware scrolling, though, so you don't normally need to bother with
       this function.

SEE ALSO

       set_palette(3alleg4), scroll_screen(3alleg4)

Allegro 							   version 4.4.2						    vsync(3alleg4)