The function sleep gives a simple way to make the program wait
for a short interval. If your program doesn't use signals (except to
terminate), then you can expect sleep to wait reliably throughout
the specified interval. Otherwise, sleep can return sooner if a
signal arrives; if you want to wait for a given interval regardless of
signals, use select (see Waiting for I/O) and don't specify
any descriptors to wait for.
— Function: unsigned int sleep (unsigned int seconds)
The sleep function waits for seconds or until a signal
is delivered, whichever happens first.
If sleep function returns because the requested interval is over,
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 interval.
The 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 calendar time at which the program should stop
waiting, and keep trying to wait until that calendar time. This won't
be off by more than a second. With just a little more work, you can use
select and make the waiting period quite accurate. (Of course,
heavy system load can cause additional unavoidable delays—unless the
machine is dedicated to one application, there is no way you can avoid
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, sleep might
return prematurely on delivery of a SIGALRM signal. If you have
established a handler for SIGALRM signals and a SIGALRM
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 SIGALRM,
this handler and sleep will interfere.
On the GNU system, it is safe to use sleep and SIGALRM in
the same program, because sleep does not work by means of
If resolution to seconds is not enough the nanosleep function can
be used. As the name suggests the sleep interval can be specified in
nanoseconds. The actual elapsed time of the sleep interval might be
longer since the system rounds the elapsed time you request up to the
next integer multiple of the actual resolution the system can deliver.
*requested_time is the elapsed time of the interval you want to
The function returns as *remaining the elapsed time left in the
interval for which you requested to sleep. If the interval completed
without getting interrupted by a signal, this is zero.
If the function returns because the interval is over the return value is
zero. If the function returns -1 the global variable errno
is set to the following values:
The call was interrupted because a signal was delivered to the thread.
If the remaining parameter is not the null pointer the structure
pointed to by remaining is updated to contain the remaining
The nanosecond value in the requested_time parameter contains an
illegal value. Either the value is negative or greater than or equal to
This function is a cancellation point in multi-threaded programs. This
is a problem if the thread allocates some resources (like memory, file
descriptors, semaphores or whatever) at the time nanosleep is
called. If the thread gets canceled these resources stay allocated
until the program ends. To avoid this calls to nanosleep should
be protected using cancellation handlers.
The nanosleep function is declared in time.h.
Published under the terms of the GNU General Public License