retrace_count (3) - Linux Man Pages
retrace_count: Retrace count simulator. Allegro game programming library.
retrace_count - Retrace count simulator. Allegro game programming library.
extern volatile int retrace_count;
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.