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| <h3 class="section">21.6 Sleeping</h3> |
| |
| <p>The function <code>sleep</code> 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 <code>sleep</code> to wait reliably throughout |
| the specified interval. Otherwise, <code>sleep</code> can return sooner if a |
| signal arrives; if you want to wait for a given interval regardless of |
| signals, use <code>select</code> (see <a href="Waiting-for-I_002fO.html#Waiting-for-I_002fO">Waiting for I/O</a>) and don't specify |
| any descriptors to wait for. |
| <!-- !!! select can get EINTR; using SA_RESTART makes sleep win too. --> |
| |
| <!-- unistd.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Function: unsigned int <b>sleep</b> (<var>unsigned int seconds</var>)<var><a name="index-sleep-2697"></a></var><br> |
| <blockquote><p>The <code>sleep</code> function waits for <var>seconds</var> or until a signal |
| is delivered, whichever happens first. |
| |
| <p>If <code>sleep</code> 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. |
| |
| <p>The <code>sleep</code> function is declared in <samp><span class="file">unistd.h</span></samp>. |
| </p></blockquote></div> |
| |
| <p>Resist the temptation to implement a sleep for a fixed amount of time by |
| using the return value of <code>sleep</code>, when nonzero, to call |
| <code>sleep</code> 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. |
| |
| <p>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 |
| <code>select</code> 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.) |
| |
| <p>On some systems, <code>sleep</code> can do strange things if your program uses |
| <code>SIGALRM</code> explicitly. Even if <code>SIGALRM</code> signals are being |
| ignored or blocked when <code>sleep</code> is called, <code>sleep</code> might |
| return prematurely on delivery of a <code>SIGALRM</code> signal. If you have |
| established a handler for <code>SIGALRM</code> signals and a <code>SIGALRM</code> |
| signal is delivered while the process is sleeping, the action taken |
| might be just to cause <code>sleep</code> to return instead of invoking your |
| handler. And, if <code>sleep</code> is interrupted by delivery of a signal |
| whose handler requests an alarm or alters the handling of <code>SIGALRM</code>, |
| this handler and <code>sleep</code> will interfere. |
| |
| <p>On the GNU system, it is safe to use <code>sleep</code> and <code>SIGALRM</code> in |
| the same program, because <code>sleep</code> does not work by means of |
| <code>SIGALRM</code>. |
| |
| <!-- time.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Function: int <b>nanosleep</b> (<var>const struct timespec *requested_time, struct timespec *remaining</var>)<var><a name="index-nanosleep-2698"></a></var><br> |
| <blockquote><p>If resolution to seconds is not enough the <code>nanosleep</code> 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. |
| |
| <p>*<code>requested_time</code> is the elapsed time of the interval you want to |
| sleep. |
| |
| <p>The function returns as *<code>remaining</code> 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. |
| |
| <p><code>struct timespec</code> is described in See <a href="Elapsed-Time.html#Elapsed-Time">Elapsed Time</a>. |
| |
| <p>If the function returns because the interval is over the return value is |
| zero. If the function returns -1 the global variable <var>errno</var> |
| is set to the following values: |
| |
| <dl> |
| <dt><code>EINTR</code><dd>The call was interrupted because a signal was delivered to the thread. |
| If the <var>remaining</var> parameter is not the null pointer the structure |
| pointed to by <var>remaining</var> is updated to contain the remaining |
| elapsed time. |
| |
| <br><dt><code>EINVAL</code><dd>The nanosecond value in the <var>requested_time</var> parameter contains an |
| illegal value. Either the value is negative or greater than or equal to |
| 1000 million. |
| </dl> |
| |
| <p>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 <code>nanosleep</code> is |
| called. If the thread gets canceled these resources stay allocated |
| until the program ends. To avoid this calls to <code>nanosleep</code> should |
| be protected using cancellation handlers. |
| <!-- ref pthread_cleanup_push / pthread_cleanup_pop --> |
| |
| <p>The <code>nanosleep</code> function is declared in <samp><span class="file">time.h</span></samp>. |
| </p></blockquote></div> |
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