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(libc.info)Sleeping

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Sleeping
========

   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' (Note: 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 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, `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 `SIGALRM'.

 - Function: int nanosleep (const struct timespec *REQUESTED_TIME,
          struct timespec *REMAINING)
     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
     sleep.

     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.

     `struct timespec' is described in Note: Elapsed Time.

     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:

    `EINTR'
          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 elapsed time.

    `EINVAL'
          The nanosecond value in the REQUESTED_TIME parameter contains
          an illegal value.  Either the value is negative or greater
          than or equal to 1000 million.

     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'.