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