1TIMER_CREATE(3P) POSIX Programmer's Manual TIMER_CREATE(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
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12 timer_create - create a per-process timer (REALTIME)
13
15 #include <signal.h>
16 #include <time.h>
17 int timer_create(clockid_t clockid, struct sigevent *restrict evp,
19 timer_t *restrict timerid);
20
23 The timer_create() function shall create a per-process timer using the
24 specified clock, clock_id, as the timing base. The timer_create() func‐
25 tion shall return, in the location referenced by timerid, a timer ID of
26 type timer_t used to identify the timer in timer requests. This timer
27 ID shall be unique within the calling process until the timer is
28 deleted. The particular clock, clock_id, is defined in <time.h>. The
29 timer whose ID is returned shall be in a disarmed state upon return
30 from timer_create().
31 The evp argument, if non-NULL, points to a sigevent structure. This
33 structure, allocated by the application, defines the asynchronous noti‐
34 fication to occur as specified in Signal Generation and Delivery when
35 the timer expires. If the evp argument is NULL, the effect is as if the
36 evp argument pointed to a sigevent structure with the sigev_notify mem‐
37 ber having the value SIGEV_SIGNAL, the sigev_signo having a default
38 signal number, and the sigev_value member having the value of the timer
39 ID.
40 Each implementation shall define a set of clocks that can be used as
42 timing bases for per-process timers. All implementations shall support
43 a clock_id of CLOCK_REALTIME. If the Monotonic Clock option is sup‐
44 ported, implementations shall support a clock_id of CLOCK_MONOTONIC.
45 Per-process timers shall not be inherited by a child process across a
47 fork() and shall be disarmed and deleted by an exec.
48 If _POSIX_CPUTIME is defined, implementations shall support clock_id
50 values representing the CPU-time clock of the calling process.
51 If _POSIX_THREAD_CPUTIME is defined, implementations shall support
53 clock_id values representing the CPU-time clock of the calling thread.
54 It is implementation-defined whether a timer_create() function will
56 succeed if the value defined by clock_id corresponds to the CPU-time
57 clock of a process or thread different from the process or thread
58 invoking the function.
59
61 If the call succeeds, timer_create() shall return zero and update the
62 location referenced by timerid to a timer_t, which can be passed to the
63 per-process timer calls. If an error occurs, the function shall return
64 a value of -1 and set errno to indicate the error. The value of timerid
65 is undefined if an error occurs.
66
68 The timer_create() function shall fail if:
69 EAGAIN The system lacks sufficient signal queuing resources to honor
71 the request.
72 EAGAIN The calling process has already created all of the timers it is
74 allowed by this implementation.
75 EINVAL The specified clock ID is not defined.
77 ENOTSUP
79 The implementation does not support the creation of a timer
80 attached to the CPU-time clock that is specified by clock_id and
81 associated with a process or thread different from the process
82 or thread invoking timer_create().
83 The following sections are informative.
86
88 None.
89
91 None.
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94 Periodic Timer Overrun and Resource Allocation
95 The specified timer facilities may deliver realtime signals (that is,
96 queued signals) on implementations that support this option. Since
97 realtime applications cannot afford to lose notifications of asynchro‐
98 nous events, like timer expirations or asynchronous I/O completions, it
99 must be possible to ensure that sufficient resources exist to deliver
100 the signal when the event occurs. In general, this is not a difficulty
101 because there is a one-to-one correspondence between a request and a
102 subsequent signal generation. If the request cannot allocate the signal
103 delivery resources, it can fail the call with an [EAGAIN] error.
104 Periodic timers are a special case. A single request can generate an
106 unspecified number of signals. This is not a problem if the requesting
107 process can service the signals as fast as they are generated, thus
108 making the signal delivery resources available for delivery of subse‐
109 quent periodic timer expiration signals. But, in general, this cannot
110 be assured-processing of periodic timer signals may "overrun''; that
111 is, subsequent periodic timer expirations may occur before the cur‐
112 rently pending signal has been delivered.
113 Also, for signals, according to the POSIX.1-1990 standard, if subse‐
115 quent occurrences of a pending signal are generated, it is implementa‐
116 tion-defined whether a signal is delivered for each occurrence. This
117 is not adequate for some realtime applications. So a mechanism is
118 required to allow applications to detect how many timer expirations
119 were delayed without requiring an indefinite amount of system resources
120 to store the delayed expirations.
121 The specified facilities provide for an overrun count. The overrun
123 count is defined as the number of extra timer expirations that occurred
124 between the time a timer expiration signal is generated and the time
125 the signal is delivered. The signal-catching function, if it is con‐
126 cerned with overruns, can retrieve this count on entry. With this
127 method, a periodic timer only needs one "signal queuing resource" that
128 can be allocated at the time of the timer_create() function call.
129 A function is defined to retrieve the overrun count so that an applica‐
131 tion need not allocate static storage to contain the count, and an
132 implementation need not update this storage asynchronously on timer
133 expirations. But, for some high-frequency periodic applications, the
134 overhead of an additional system call on each timer expiration may be
135 prohibitive. The functions, as defined, permit an implementation to
136 maintain the overrun count in user space, associated with the timerid.
137 The timer_getoverrun() function can then be implemented as a macro that
138 uses the timerid argument (which may just be a pointer to a user space
139 structure containing the counter) to locate the overrun count with no
140 system call overhead. Other implementations, less concerned with this
141 class of applications, can avoid the asynchronous update of user space
142 by maintaining the count in a system structure at the cost of the extra
143 system call to obtain it.
144 Timer Expiration Signal Parameters
146 The Realtime Signals Extension option supports an application-specific
147 datum that is delivered to the extended signal handler. This value is
148 explicitly specified by the application, along with the signal number
149 to be delivered, in a sigevent structure. The type of the application-
150 defined value can be either an integer constant or a pointer. This
151 explicit specification of the value, as opposed to always sending the
152 timer ID, was selected based on existing practice.
153 It is common practice for realtime applications (on non-POSIX systems
155 or realtime extended POSIX systems) to use the parameters of event han‐
156 dlers as the case label of a switch statement or as a pointer to an
157 application-defined data structure. Since timer_ids are dynamically
158 allocated by the timer_create() function, they can be used for neither
159 of these functions without additional application overhead in the sig‐
160 nal handler; for example, to search an array of saved timer IDs to as‐
161 sociate the ID with a constant or application data structure.
162
164 None.
165
167 clock_getres(), timer_delete(), timer_getoverrun(), the Base Defini‐
168 tions volume of IEEE Std 1003.1-2001, <time.h>
169
171 Portions of this text are reprinted and reproduced in electronic form
172 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
173 -- Portable Operating System Interface (POSIX), The Open Group Base
174 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
175 Electrical and Electronics Engineers, Inc and The Open Group. In the
176 event of any discrepancy between this version and the original IEEE and
177 The Open Group Standard, the original IEEE and The Open Group Standard
178 is the referee document. The original Standard can be obtained online
179 at http://www.opengroup.org/unix/online.html .
180IEEE/The Open Group 2003 TIMER_CREATE(3P)