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BSD 2.11 - man page for usleep (bsd section 3)

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USLEEP(3)										USLEEP(3)

       usleep - suspend execution for interval

       unsigned useconds;

       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(2), getitimer(2), sigpause(2), ualarm(3), sleep(3), alarm(3)

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

4.3 Berkeley Distribution		 August 26, 1988				USLEEP(3)
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