retrace_count (3) Linux Manual Page
retrace_count – Retrace count simulator. Allegro game programming library.
Synopsis
#include <allegro.h>
extern volatile int retrace_count;
Description
If the retrace simulator is installed, this count is incremented on each vertical retrace; otherwise, if the refresh rate is known, the count is incremented at the same rate (ignoring retraces); otherwise, it is incremented 70 times a second. This provides a way of controlling the speed of your program without installing user timer functions.