| <html lang="en"> |
| <head> |
| <title>Setting an Alarm - The GNU C Library</title> |
| <meta http-equiv="Content-Type" content="text/html"> |
| <meta name="description" content="The GNU C Library"> |
| <meta name="generator" content="makeinfo 4.13"> |
| <link title="Top" rel="start" href="index.html#Top"> |
| <link rel="up" href="Date-and-Time.html#Date-and-Time" title="Date and Time"> |
| <link rel="prev" href="Calendar-Time.html#Calendar-Time" title="Calendar Time"> |
| <link rel="next" href="Sleeping.html#Sleeping" title="Sleeping"> |
| <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> |
| <!-- |
| This file documents the GNU C library. |
| |
| This is Edition 0.12, last updated 2007-10-27, |
| of `The GNU C Library Reference Manual', for version |
| 2.8 (Sourcery G++ Lite 2011.03-41). |
| |
| Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002, |
| 2003, 2007, 2008, 2010 Free Software Foundation, Inc. |
| |
| Permission is granted to copy, distribute and/or modify this document |
| under the terms of the GNU Free Documentation License, Version 1.3 or |
| any later version published by the Free Software Foundation; with the |
| Invariant Sections being ``Free Software Needs Free Documentation'' |
| and ``GNU Lesser General Public License'', the Front-Cover texts being |
| ``A GNU Manual'', and with the Back-Cover Texts as in (a) below. A |
| copy of the license is included in the section entitled "GNU Free |
| Documentation License". |
| |
| (a) The FSF's Back-Cover Text is: ``You have the freedom to |
| copy and modify this GNU manual. Buying copies from the FSF |
| supports it in developing GNU and promoting software freedom.''--> |
| <meta http-equiv="Content-Style-Type" content="text/css"> |
| <style type="text/css"><!-- |
| pre.display { font-family:inherit } |
| pre.format { font-family:inherit } |
| pre.smalldisplay { font-family:inherit; font-size:smaller } |
| pre.smallformat { font-family:inherit; font-size:smaller } |
| pre.smallexample { font-size:smaller } |
| pre.smalllisp { font-size:smaller } |
| span.sc { font-variant:small-caps } |
| span.roman { font-family:serif; font-weight:normal; } |
| span.sansserif { font-family:sans-serif; font-weight:normal; } |
| --></style> |
| <link rel="stylesheet" type="text/css" href="../cs.css"> |
| </head> |
| <body> |
| <div class="node"> |
| <a name="Setting-an-Alarm"></a> |
| <p> |
| Next: <a rel="next" accesskey="n" href="Sleeping.html#Sleeping">Sleeping</a>, |
| Previous: <a rel="previous" accesskey="p" href="Calendar-Time.html#Calendar-Time">Calendar Time</a>, |
| Up: <a rel="up" accesskey="u" href="Date-and-Time.html#Date-and-Time">Date and Time</a> |
| <hr> |
| </div> |
| |
| <h3 class="section">21.5 Setting an Alarm</h3> |
| |
| <p>The <code>alarm</code> and <code>setitimer</code> functions provide a mechanism for a |
| process to interrupt itself in the future. They do this by setting a |
| timer; when the timer expires, the process receives a signal. |
| |
| <p><a name="index-setting-an-alarm-2678"></a><a name="index-interval-timer_002c-setting-2679"></a><a name="index-alarms_002c-setting-2680"></a><a name="index-timers_002c-setting-2681"></a>Each process has three independent interval timers available: |
| |
| <ul> |
| <li>A real-time timer that counts elapsed time. This timer sends a |
| <code>SIGALRM</code> signal to the process when it expires. |
| <a name="index-real_002dtime-timer-2682"></a><a name="index-timer_002c-real_002dtime-2683"></a> |
| <li>A virtual timer that counts processor time used by the process. This timer |
| sends a <code>SIGVTALRM</code> signal to the process when it expires. |
| <a name="index-virtual-timer-2684"></a><a name="index-timer_002c-virtual-2685"></a> |
| <li>A profiling timer that counts both processor time used by the process, |
| and processor time spent in system calls on behalf of the process. This |
| timer sends a <code>SIGPROF</code> signal to the process when it expires. |
| <a name="index-profiling-timer-2686"></a><a name="index-timer_002c-profiling-2687"></a> |
| This timer is useful for profiling in interpreters. The interval timer |
| mechanism does not have the fine granularity necessary for profiling |
| native code. |
| <!-- @xref{profil} !!! --> |
| </ul> |
| |
| <p>You can only have one timer of each kind set at any given time. If you |
| set a timer that has not yet expired, that timer is simply reset to the |
| new value. |
| |
| <p>You should establish a handler for the appropriate alarm signal using |
| <code>signal</code> or <code>sigaction</code> before issuing a call to |
| <code>setitimer</code> or <code>alarm</code>. Otherwise, an unusual chain of events |
| could cause the timer to expire before your program establishes the |
| handler. In this case it would be terminated, since termination is the |
| default action for the alarm signals. See <a href="Signal-Handling.html#Signal-Handling">Signal Handling</a>. |
| |
| <p>To be able to use the alarm function to interrupt a system call which |
| might block otherwise indefinitely it is important to <em>not</em> set the |
| <code>SA_RESTART</code> flag when registering the signal handler using |
| <code>sigaction</code>. When not using <code>sigaction</code> things get even |
| uglier: the <code>signal</code> function has to fixed semantics with respect |
| to restarts. The BSD semantics for this function is to set the flag. |
| Therefore, if <code>sigaction</code> for whatever reason cannot be used, it is |
| necessary to use <code>sysv_signal</code> and not <code>signal</code>. |
| |
| <p>The <code>setitimer</code> function is the primary means for setting an alarm. |
| This facility is declared in the header file <samp><span class="file">sys/time.h</span></samp>. The |
| <code>alarm</code> function, declared in <samp><span class="file">unistd.h</span></samp>, provides a somewhat |
| simpler interface for setting the real-time timer. |
| <a name="index-unistd_002eh-2688"></a><a name="index-sys_002ftime_002eh-2689"></a> |
| <!-- sys/time.h --> |
| <!-- BSD --> |
| |
| <div class="defun"> |
| — Data Type: <b>struct itimerval</b><var><a name="index-struct-itimerval-2690"></a></var><br> |
| <blockquote><p>This structure is used to specify when a timer should expire. It contains |
| the following members: |
| <dl> |
| <dt><code>struct timeval it_interval</code><dd>This is the period between successive timer interrupts. If zero, the |
| alarm will only be sent once. |
| |
| <br><dt><code>struct timeval it_value</code><dd>This is the period between now and the first timer interrupt. If zero, |
| the alarm is disabled. |
| </dl> |
| |
| <p>The <code>struct timeval</code> data type is described in <a href="Elapsed-Time.html#Elapsed-Time">Elapsed Time</a>. |
| </p></blockquote></div> |
| |
| <!-- sys/time.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: int <b>setitimer</b> (<var>int which, struct itimerval *new, struct itimerval *old</var>)<var><a name="index-setitimer-2691"></a></var><br> |
| <blockquote><p>The <code>setitimer</code> function sets the timer specified by <var>which</var> |
| according to <var>new</var>. The <var>which</var> argument can have a value of |
| <code>ITIMER_REAL</code>, <code>ITIMER_VIRTUAL</code>, or <code>ITIMER_PROF</code>. |
| |
| <p>If <var>old</var> is not a null pointer, <code>setitimer</code> returns information |
| about any previous unexpired timer of the same kind in the structure it |
| points to. |
| |
| <p>The return value is <code>0</code> on success and <code>-1</code> on failure. The |
| following <code>errno</code> error conditions are defined for this function: |
| |
| <dl> |
| <dt><code>EINVAL</code><dd>The timer period is too large. |
| </dl> |
| </p></blockquote></div> |
| |
| <!-- sys/time.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: int <b>getitimer</b> (<var>int which, struct itimerval *old</var>)<var><a name="index-getitimer-2692"></a></var><br> |
| <blockquote><p>The <code>getitimer</code> function stores information about the timer specified |
| by <var>which</var> in the structure pointed at by <var>old</var>. |
| |
| <p>The return value and error conditions are the same as for <code>setitimer</code>. |
| </p></blockquote></div> |
| |
| <!-- sys/time.h --> |
| <!-- BSD --> |
| <dl> |
| <dt><code>ITIMER_REAL</code><a name="index-ITIMER_005fREAL-2693"></a><dd>This constant can be used as the <var>which</var> argument to the |
| <code>setitimer</code> and <code>getitimer</code> functions to specify the real-time |
| timer. |
| |
| <!-- sys/time.h --> |
| <!-- BSD --> |
| <br><dt><code>ITIMER_VIRTUAL</code><a name="index-ITIMER_005fVIRTUAL-2694"></a><dd>This constant can be used as the <var>which</var> argument to the |
| <code>setitimer</code> and <code>getitimer</code> functions to specify the virtual |
| timer. |
| |
| <!-- sys/time.h --> |
| <!-- BSD --> |
| <br><dt><code>ITIMER_PROF</code><a name="index-ITIMER_005fPROF-2695"></a><dd>This constant can be used as the <var>which</var> argument to the |
| <code>setitimer</code> and <code>getitimer</code> functions to specify the profiling |
| timer. |
| </dl> |
| |
| <!-- unistd.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Function: unsigned int <b>alarm</b> (<var>unsigned int seconds</var>)<var><a name="index-alarm-2696"></a></var><br> |
| <blockquote><p>The <code>alarm</code> function sets the real-time timer to expire in |
| <var>seconds</var> seconds. If you want to cancel any existing alarm, you |
| can do this by calling <code>alarm</code> with a <var>seconds</var> argument of |
| zero. |
| |
| <p>The return value indicates how many seconds remain before the previous |
| alarm would have been sent. If there is no previous alarm, <code>alarm</code> |
| returns zero. |
| </p></blockquote></div> |
| |
| <p>The <code>alarm</code> function could be defined in terms of <code>setitimer</code> |
| like this: |
| |
| <pre class="smallexample"> unsigned int |
| alarm (unsigned int seconds) |
| { |
| struct itimerval old, new; |
| new.it_interval.tv_usec = 0; |
| new.it_interval.tv_sec = 0; |
| new.it_value.tv_usec = 0; |
| new.it_value.tv_sec = (long int) seconds; |
| if (setitimer (ITIMER_REAL, &new, &old) < 0) |
| return 0; |
| else |
| return old.it_value.tv_sec; |
| } |
| </pre> |
| <p>There is an example showing the use of the <code>alarm</code> function in |
| <a href="Handler-Returns.html#Handler-Returns">Handler Returns</a>. |
| |
| <p>If you simply want your process to wait for a given number of seconds, |
| you should use the <code>sleep</code> function. See <a href="Sleeping.html#Sleeping">Sleeping</a>. |
| |
| <p>You shouldn't count on the signal arriving precisely when the timer |
| expires. In a multiprocessing environment there is typically some |
| amount of delay involved. |
| |
| <p><strong>Portability Note:</strong> The <code>setitimer</code> and <code>getitimer</code> |
| functions are derived from BSD Unix, while the <code>alarm</code> function is |
| specified by the POSIX.1 standard. <code>setitimer</code> is more powerful than |
| <code>alarm</code>, but <code>alarm</code> is more widely used. |
| |
| </body></html> |
| |
