sleep gives a simple way to make the program wait
for short periods of time. If your program doesn't use signals (except
to terminate), then you can expect
sleep to wait reliably for
the specified amount of time. Otherwise,
sleep can return sooner
if a signal arrives; if you want to wait for a given period regardless
of signals, use
select (see section Waiting for Input or Output) and don't
specify any descriptors to wait for.
sleepfunction waits for seconds or until a signal is delivered, whichever happens first.
sleep function returns because the requested time has
elapsed, it returns a value of zero. If it returns because of delivery
of a signal, its return value is the remaining time in the sleep period.
sleep function is declared in `unistd.h'.
Resist the temptation to implement a sleep for a fixed amount of time by
using the return value of
sleep, when nonzero, to call
sleep again. This will work with a certain amount of accuracy as
long as signals arrive infrequently. But each signal can cause the
eventual wakeup time to be off by an additional second or so. Suppose a
few signals happen to arrive in rapid succession by bad luck--there is
no limit on how much this could shorten or lengthen the wait.
Instead, compute the time at which the program should stop waiting, and
keep trying to wait until that time. This won't be off by more than a
second. With just a little more work, you can use
make the waiting period quite accurate. (Of course, heavy system load
can cause unavoidable additional delays--unless the machine is
dedicated to one application, there is no way you can avoid this.)
On some systems,
sleep can do strange things if your program uses
SIGALRM explicitly. Even if
SIGALRM signals are being
ignored or blocked when
sleep is called,
return prematurely on delivery of a
SIGALRM signal. If you have
established a handler for
SIGALRM signals and a
signal is delivered while the process is sleeping, the action taken
might be just to cause
sleep to return instead of invoking your
handler. And, if
sleep is interrupted by delivery of a signal
whose handler requests an alarm or alters the handling of
this handler and
sleep will interfere.
On the GNU system, it is safe to use
the same program, because
sleep does not work by means of
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