bsd man page for usleep

USLEEP(3)						     Library Functions Manual							 USLEEP(3)

NAME
       usleep - suspend execution for interval

SYNOPSIS
       usleep(useconds)
       unsigned useconds;

DESCRIPTION
       The  current  process is suspended from execution for the number of microseconds specified by the argument.  The actual suspension time may
       be an arbitrary amount longer because of other activity in the system or because of the time spent in processing the call.

       The routine is implemented by setting an interval timer and pausing until it occurs.  The  previous  state  of  this  timer  is	saved  and
       restored.   If the sleep time exceeds the time to the expiration of the previous timer, the process sleeps only until the signal would have
       occurred, and the signal is sent a short time later.

       This routine is implemented using setitimer(2); it requires eight system calls each time it is invoked.	 A  similar  but  less	compatible
       function can be obtained with a single select(2); it would not restart after signals, but would not interfere with other uses of setitimer.

SEE ALSO
       setitimer(2), getitimer(2), sigpause(2), ualarm(3), sleep(3), alarm(3)

NOTES  (PDP-11)
       On  the PDP-11, setitimer(2) rounds the number of microseconds up to seconds resolution, therefore usleep doesn't give you any more resolu-
       tion than sleep(3).  Select(2) offers clock resolution (usually 60Hz in the U.S.A. and 50Hz elsewhere) and so should be used instead.

4.3 Berkeley Distribution					  August 26, 1988							 USLEEP(3)