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233 lines
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<title>timer_create</title>
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<basefont size="3"> <a name="timer_create"></a> <a name="tag_03_776"></a><!-- timer_create -->
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<!--header start-->
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<center><font size="2">The Open Group Base Specifications Issue 6<br>
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IEEE Std 1003.1-2001<br>
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Copyright © 2001 The IEEE and The Open Group, All Rights reserved.</font></center>
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<!--header end-->
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<hr size="2" noshade>
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<h4><a name="tag_03_776_01"></a>NAME</h4>
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<blockquote>timer_create - create a per-process timer (<b>REALTIME</b>)</blockquote>
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<h4><a name="tag_03_776_02"></a>SYNOPSIS</h4>
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<blockquote class="synopsis">
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<div class="box"><code><tt><sup>[<a href="javascript:open_code('TMR')">TMR</a>]</sup> <img src="../images/opt-start.gif" alt=
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"[Option Start]" border="0"> #include <<a href="../basedefs/signal.h.html">signal.h</a>><br>
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#include <<a href="../basedefs/time.h.html">time.h</a>><br>
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<br>
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int timer_create(clockid_t</tt> <i>clockid</i><tt>, struct sigevent *restrict</tt> <i>evp</i><tt>,<br>
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timer_t *restrict</tt> <i>timerid</i><tt>); <img src="../images/opt-end.gif" alt=
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"[Option End]" border="0"></tt></code></div>
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<tt><br>
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</tt></blockquote>
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<h4><a name="tag_03_776_03"></a>DESCRIPTION</h4>
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<blockquote>
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<p>The <i>timer_create</i>() function shall create a per-process timer using the specified clock, <i>clock_id</i>, as the timing
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base. The <i>timer_create</i>() function shall return, in the location referenced by <i>timerid</i>, a timer ID of type
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<b>timer_t</b> used to identify the timer in timer requests. This timer ID shall be unique within the calling process until the
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timer is deleted. The particular clock, <i>clock_id</i>, is defined in <a href="../basedefs/time.h.html"><i><time.h></i></a>.
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The timer whose ID is returned shall be in a disarmed state upon return from <i>timer_create</i>().</p>
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<p>The <i>evp</i> argument, if non-NULL, points to a <b>sigevent</b> structure. This structure, allocated by the application,
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defines the asynchronous notification to occur as specified in <a href="xsh_chap02_04.html#tag_02_04_01"><i>Signal Generation and
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Delivery</i></a> when the timer expires. If the <i>evp</i> argument is NULL, the effect is as if the <i>evp</i> argument pointed to
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a <b>sigevent</b> structure with the <i>sigev_notify</i> member having the value SIGEV_SIGNAL, the <i>sigev_signo</i> having a
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default signal number, and the <i>sigev_value</i> member having the value of the timer ID.</p>
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<p>Each implementation shall define a set of clocks that can be used as timing bases for per-process timers. All implementations
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shall support a <i>clock_id</i> of CLOCK_REALTIME. <sup>[<a href="javascript:open_code('MON')">MON</a>]</sup> <img src=
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"../images/opt-start.gif" alt="[Option Start]" border="0"> If the Monotonic Clock option is supported, implementations shall
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support a <i>clock_id</i> of CLOCK_MONOTONIC. <img src="../images/opt-end.gif" alt="[Option End]" border="0"></p>
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<p>Per-process timers shall not be inherited by a child process across a <a href="../functions/fork.html"><i>fork</i>()</a> and
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shall be disarmed and deleted by an <i><a href="../functions/exec.html">exec</a></i>.</p>
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<p><sup>[<a href="javascript:open_code('CPT')">CPT</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
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If _POSIX_CPUTIME is defined, implementations shall support <i>clock_id</i> values representing the CPU-time clock of the calling
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process. <img src="../images/opt-end.gif" alt="[Option End]" border="0"></p>
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<p><sup>[<a href="javascript:open_code('TCT')">TCT</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
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If _POSIX_THREAD_CPUTIME is defined, implementations shall support <i>clock_id</i> values representing the CPU-time clock of the
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calling thread. <img src="../images/opt-end.gif" alt="[Option End]" border="0"></p>
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<p><sup>[<a href="javascript:open_code('CPT')">CPT|TCT</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border=
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"0"> It is implementation-defined whether a <i>timer_create</i>() function will succeed if the value defined by <i>clock_id</i>
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corresponds to the CPU-time clock of a process or thread different from the process or thread invoking the function. <img src=
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"../images/opt-end.gif" alt="[Option End]" border="0"></p>
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</blockquote>
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<h4><a name="tag_03_776_04"></a>RETURN VALUE</h4>
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<blockquote>
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<p>If the call succeeds, <i>timer_create</i>() shall return zero and update the location referenced by <i>timerid</i> to a
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<b>timer_t</b>, which can be passed to the per-process timer calls. If an error occurs, the function shall return a value of -1 and
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set <i>errno</i> to indicate the error. The value of <i>timerid</i> is undefined if an error occurs.</p>
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</blockquote>
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<h4><a name="tag_03_776_05"></a>ERRORS</h4>
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<blockquote>
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<p>The <i>timer_create</i>() function shall fail if:</p>
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<dl compact>
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<dt>[EAGAIN]</dt>
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<dd>The system lacks sufficient signal queuing resources to honor the request.</dd>
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<dt>[EAGAIN]</dt>
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<dd>The calling process has already created all of the timers it is allowed by this implementation.</dd>
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<dt>[EINVAL]</dt>
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<dd>The specified clock ID is not defined.</dd>
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<dt>[ENOTSUP]</dt>
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<dd><sup>[<a href="javascript:open_code('CPT')">CPT|TCT</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border=
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"0"> The implementation does not support the creation of a timer attached to the CPU-time clock that is specified by
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<i>clock_id</i> and associated with a process or thread different from the process or thread invoking <i>timer_create</i>(). <img
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src="../images/opt-end.gif" alt="[Option End]" border="0"></dd>
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</dl>
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</blockquote>
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<hr>
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<div class="box"><em>The following sections are informative.</em></div>
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<h4><a name="tag_03_776_06"></a>EXAMPLES</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_776_07"></a>APPLICATION USAGE</h4>
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<blockquote>
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<p>None.<br>
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</p>
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</blockquote>
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<h4><a name="tag_03_776_08"></a>RATIONALE</h4>
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<blockquote>
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<p></p>
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<h5><a name="tag_03_776_08_01"></a>Periodic Timer Overrun and Resource Allocation</h5>
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<p>The specified timer facilities may deliver realtime signals (that is, queued signals) on implementations that support this
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option. Since realtime applications cannot afford to lose notifications of asynchronous events, like timer expirations or
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asynchronous I/O completions, it must be possible to ensure that sufficient resources exist to deliver the signal when the event
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occurs. In general, this is not a difficulty because there is a one-to-one correspondence between a request and a subsequent signal
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generation. If the request cannot allocate the signal delivery resources, it can fail the call with an [EAGAIN] error.</p>
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<p>Periodic timers are a special case. A single request can generate an unspecified number of signals. This is not a problem if the
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requesting process can service the signals as fast as they are generated, thus making the signal delivery resources available for
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delivery of subsequent periodic timer expiration signals. But, in general, this cannot be assured-processing of periodic timer
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signals may "overrun''; that is, subsequent periodic timer expirations may occur before the currently pending signal has been
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delivered.</p>
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<p>Also, for signals, according to the POSIX.1-1990 standard, if subsequent occurrences of a pending signal are generated, it is
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implementation-defined whether a signal is delivered for each occurrence. This is not adequate for some realtime applications. So a
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mechanism is required to allow applications to detect how many timer expirations were delayed without requiring an indefinite
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amount of system resources to store the delayed expirations.</p>
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<p>The specified facilities provide for an overrun count. The overrun count is defined as the number of extra timer expirations
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that occurred between the time a timer expiration signal is generated and the time the signal is delivered. The signal-catching
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function, if it is concerned with overruns, can retrieve this count on entry. With this method, a periodic timer only needs one
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"signal queuing resource" that can be allocated at the time of the <i>timer_create</i>() function call.</p>
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<p>A function is defined to retrieve the overrun count so that an application need not allocate static storage to contain the
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count, and an implementation need not update this storage asynchronously on timer expirations. But, for some high-frequency
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periodic applications, the overhead of an additional system call on each timer expiration may be prohibitive. The functions, as
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defined, permit an implementation to maintain the overrun count in user space, associated with the <i>timerid</i>. The <a href=
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"../functions/timer_getoverrun.html"><i>timer_getoverrun</i>()</a> function can then be implemented as a macro that uses the
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<i>timerid</i> argument (which may just be a pointer to a user space structure containing the counter) to locate the overrun count
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with no system call overhead. Other implementations, less concerned with this class of applications, can avoid the asynchronous
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update of user space by maintaining the count in a system structure at the cost of the extra system call to obtain it.</p>
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<h5><a name="tag_03_776_08_02"></a>Timer Expiration Signal Parameters</h5>
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<p>The Realtime Signals Extension option supports an application-specific datum that is delivered to the extended signal handler.
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This value is explicitly specified by the application, along with the signal number to be delivered, in a <b>sigevent</b>
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structure. The type of the application-defined value can be either an integer constant or a pointer. This explicit specification of
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the value, as opposed to always sending the timer ID, was selected based on existing practice.</p>
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<p>It is common practice for realtime applications (on non-POSIX systems or realtime extended POSIX systems) to use the parameters
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of event handlers as the case label of a switch statement or as a pointer to an application-defined data structure. Since
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<i>timer_id</i>s are dynamically allocated by the <i>timer_create</i>() function, they can be used for neither of these functions
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without additional application overhead in the signal handler; for example, to search an array of saved timer IDs to associate the
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ID with a constant or application data structure.</p>
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</blockquote>
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<h4><a name="tag_03_776_09"></a>FUTURE DIRECTIONS</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_776_10"></a>SEE ALSO</h4>
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<blockquote>
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<p><a href="clock_getres.html"><i>clock_getres</i>()</a> , <a href="timer_delete.html"><i>timer_delete</i>()</a> , <a href=
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"timer_getoverrun.html"><i>timer_getoverrun</i>()</a> , the Base Definitions volume of IEEE Std 1003.1-2001, <a href=
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"../basedefs/time.h.html"><i><time.h></i></a></p>
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</blockquote>
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<h4><a name="tag_03_776_11"></a>CHANGE HISTORY</h4>
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<blockquote>
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<p>First released in Issue 5. Included for alignment with the POSIX Realtime Extension.</p>
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</blockquote>
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<h4><a name="tag_03_776_12"></a>Issue 6</h4>
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<blockquote>
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<p>The <i>timer_create</i>() function is marked as part of the Timers option.</p>
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<p>The [ENOSYS] error condition has been removed as stubs need not be provided if an implementation does not support the Timers
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option.</p>
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<p>CPU-time clocks are added for alignment with IEEE Std 1003.1d-1999.</p>
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<p>The DESCRIPTION is updated for alignment with IEEE Std 1003.1j-2000 by adding the requirement for the CLOCK_MONOTONIC
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clock under the Monotonic Clock option.</p>
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<p>The <b>restrict</b> keyword is added to the <i>timer_create</i>() prototype for alignment with the ISO/IEC 9899:1999
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standard.</p>
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</blockquote>
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<div class="box"><em>End of informative text.</em></div>
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<hr>
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<center><font size="2"><!--footer start-->
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UNIX ® is a registered Trademark of The Open Group.<br>
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POSIX ® is a registered Trademark of The IEEE.<br>
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[ <a href="../mindex.html">Main Index</a> | <a href="../basedefs/contents.html">XBD</a> | <a href=
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