1 .\" Copyright (c) 2009 Linux Foundation, written by Michael Kerrisk
2 .\" <mtk.manpages@gmail.com>
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26 .TH TIMER_CREATE 2 2019-03-06 Linux "Linux Programmer's Manual"
28 timer_create \- create a POSIX per-process timer
31 .B #include <signal.h>
34 .BI "int timer_create(clockid_t " clockid ", struct sigevent *" sevp ,
35 .BI " timer_t *" timerid );
38 Link with \fI\-lrt\fP.
41 Feature Test Macro Requirements for glibc (see
42 .BR feature_test_macros (7)):
46 _POSIX_C_SOURCE\ >=\ 199309L
49 creates a new per-process interval timer.
50 The ID of the new timer is returned in the buffer pointed to by
52 which must be a non-null pointer.
53 This ID is unique within the process, until the timer is deleted.
54 The new timer is initially disarmed.
58 argument specifies the clock that the new timer uses to measure time.
59 It can be specified as one of the following values:
62 A settable system-wide real-time clock.
65 A nonsettable monotonically increasing clock that measures time
66 from some unspecified point in the past that does not change
68 .\" Note: the CLOCK_MONOTONIC_RAW clock added for clock_gettime()
69 .\" in 2.6.28 is not supported for POSIX timers -- mtk, Feb 2009
71 .BR CLOCK_PROCESS_CPUTIME_ID " (since Linux 2.6.12)"
72 A clock that measures (user and system) CPU time consumed by
73 (all of the threads in) the calling process.
75 .BR CLOCK_THREAD_CPUTIME_ID " (since Linux 2.6.12)"
76 A clock that measures (user and system) CPU time consumed by
78 .\" The CLOCK_MONOTONIC_RAW that was added in 2.6.28 can't be used
79 .\" to create a timer -- mtk, Feb 2009
81 .BR CLOCK_BOOTTIME " (Since Linux 2.6.39)"
82 .\" commit 70a08cca1227dc31c784ec930099a4417a06e7d0
85 this is a monotonically increasing clock.
88 clock does not measure the time while a system is suspended, the
90 clock does include the time during which the system is suspended.
91 This is useful for applications that need to be suspend-aware.
93 is not suitable for such applications, since that clock is affected
94 by discontinuous changes to the system clock.
96 .BR CLOCK_REALTIME_ALARM " (since Linux 3.0)"
97 .\" commit 9a7adcf5c6dea63d2e47e6f6d2f7a6c9f48b9337
100 but will wake the system if it is suspended.
101 The caller must have the
103 capability in order to set a timer against this clock.
105 .BR CLOCK_BOOTTIME_ALARM " (since Linux 3.0)"
106 .\" commit 9a7adcf5c6dea63d2e47e6f6d2f7a6c9f48b9337
109 but will wake the system if it is suspended.
110 The caller must have the
112 capability in order to set a timer against this clock.
114 .BR CLOCK_TAI " (since Linux 3.10)"
115 A system-wide clock derived from wall-clock time but ignoring leap seconds.
119 for further details on the above clocks.
121 As well as the above values,
123 can be specified as the
125 returned by a call to
126 .BR clock_getcpuclockid (3)
128 .BR pthread_getcpuclockid (3).
134 structure that specifies how the caller
135 should be notified when the timer expires.
136 For the definition and general details of this structure, see
141 field can have the following values:
144 Don't asynchronously notify when the timer expires.
145 Progress of the timer can be monitored using
146 .BR timer_gettime (2).
149 Upon timer expiration, generate the signal
159 structure will be set to
161 At any point in time,
162 at most one signal is queued to the process for a given timer; see
163 .BR timer_getoverrun (2)
167 Upon timer expiration, invoke
168 .I sigev_notify_function
169 as if it were the start function of a new thread.
174 .BR SIGEV_THREAD_ID " (Linux-specific)"
177 but the signal is targeted at the thread whose ID is given in
178 .IR sigev_notify_thread_id ,
179 which must be a thread in the same process as the caller.
181 .IR sigev_notify_thread_id
182 field specifies a kernel thread ID, that is, the value returned by
186 This flag is intended only for use by threading libraries.
190 as NULL is equivalent to specifying a pointer to a
200 .I sigev_value.sival_int
205 returns 0, and the ID of the new timer is placed in
207 On failure, \-1 is returned, and
209 is set to indicate the error.
213 Temporary error during kernel allocation of timer structures.
220 .IR sigev_notify_thread_id
224 .\" glibc layer: malloc()
225 Could not allocate memory.
228 The kernel does not support creating a timer against this
234 .BR CLOCK_REALTIME_ALARM
236 ,BR CLOCK_BOOTTIME_ALARM
237 but the caller did not have the
241 This system call is available since Linux 2.6.
243 POSIX.1-2001, POSIX.1-2008.
245 A program may create multiple interval timers using
248 Timers are not inherited by the child of a
250 and are disarmed and deleted during an
253 The kernel preallocates a "queued real-time signal"
254 for each timer created using
256 Consequently, the number of timers is limited by the
257 .BR RLIMIT_SIGPENDING
261 The timers created by
263 are commonly known as "POSIX (interval) timers".
264 The POSIX timers API consists of the following interfaces:
269 .BR timer_settime (2):
270 Arm (start) or disarm (stop) a timer.
272 .BR timer_gettime (2):
273 Fetch the time remaining until the next expiration of a timer,
274 along with the interval setting of the timer.
276 .BR timer_getoverrun (2):
277 Return the overrun count for the last timer expiration.
279 .BR timer_delete (2):
280 Disarm and delete a timer.
282 Since Linux 3.10, the
283 .IR /proc/[pid]/timers
284 file can be used to list the POSIX timers for the process with PID
288 for further information.
291 .\" baa73d9e478ff32d62f3f9422822b59dd9a95a21
292 support for POSIX timers is a configurable option that is enabled by default.
293 Kernel support can be disabled via the
294 .BR CONFIG_POSIX_TIMERS
297 .SS C library/kernel differences
298 Part of the implementation of the POSIX timers API is provided by glibc.
299 .\" See nptl/sysdeps/unix/sysv/linux/timer_create.c
302 Much of the functionality for
304 is implemented within glibc, rather than the kernel.
305 (This is necessarily so,
306 since the thread involved in handling the notification is one
307 that must be managed by the C library POSIX threads implementation.)
308 Although the notification delivered to the process is via a thread,
309 internally the NPTL implementation uses a
313 along with a real-time signal that is reserved by the implementation (see
316 The implementation of the default case where
318 is NULL is handled inside glibc,
319 which invokes the underlying system call with a suitably populated
323 The timer IDs presented at user level are maintained by glibc,
324 which maps these IDs to the timer IDs employed by the kernel.
325 .\" See the glibc source file kernel-posix-timers.h for the structure
326 .\" that glibc uses to map user-space timer IDs to kernel timer IDs
327 .\" The kernel-level timer ID is exposed via siginfo.si_tid.
329 The POSIX timers system calls first appeared in Linux 2.6.
331 glibc provided an incomplete user-space implementation
333 timers only) using POSIX threads,
334 and in glibc versions before 2.17,
335 .\" glibc commit 93a78ac437ba44f493333d7e2a4b0249839ce460
336 the implementation falls back to this technique on systems
337 running pre-2.6 Linux kernels.
339 The program below takes two arguments: a sleep period in seconds,
340 and a timer frequency in nanoseconds.
341 The program establishes a handler for the signal it uses for the timer,
343 creates and arms a timer that expires with the given frequency,
344 sleeps for the specified number of seconds,
345 and then unblocks the timer signal.
346 Assuming that the timer expired at least once while the program slept,
347 the signal handler will be invoked,
348 and the handler displays some information about the timer notification.
349 The program terminates after one invocation of the signal handler.
351 In the following example run, the program sleeps for 1 second,
352 after creating a timer that has a frequency of 100 nanoseconds.
353 By the time the signal is unblocked and delivered,
354 there have been around ten million overruns.
358 $ \fB./a.out 1 100\fP
359 Establishing handler for signal 34
361 timer ID is 0x804c008
362 Sleeping for 1 seconds
365 sival_ptr = 0xbfb174f4; *sival_ptr = 0x804c008
366 overrun count = 10004886
378 #define CLOCKID CLOCK_REALTIME
381 #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \e
385 print_siginfo(siginfo_t *si)
390 tidp = si\->si_value.sival_ptr;
392 printf(" sival_ptr = %p; ", si\->si_value.sival_ptr);
393 printf(" *sival_ptr = 0x%lx\en", (long) *tidp);
395 or = timer_getoverrun(*tidp);
397 errExit("timer_getoverrun");
399 printf(" overrun count = %d\en", or);
403 handler(int sig, siginfo_t *si, void *uc)
405 /* Note: calling printf() from a signal handler is not safe
406 (and should not be done in production programs), since
407 printf() is not async\-signal\-safe; see signal-safety(7).
408 Nevertheless, we use printf() here as a simple way of
409 showing that the handler was called. */
411 printf("Caught signal %d\en", sig);
413 signal(sig, SIG_IGN);
417 main(int argc, char *argv[])
421 struct itimerspec its;
422 long long freq_nanosecs;
427 fprintf(stderr, "Usage: %s <sleep\-secs> <freq\-nanosecs>\en",
432 /* Establish handler for timer signal */
434 printf("Establishing handler for signal %d\en", SIG);
435 sa.sa_flags = SA_SIGINFO;
436 sa.sa_sigaction = handler;
437 sigemptyset(&sa.sa_mask);
438 if (sigaction(SIG, &sa, NULL) == \-1)
439 errExit("sigaction");
441 /* Block timer signal temporarily */
443 printf("Blocking signal %d\en", SIG);
445 sigaddset(&mask, SIG);
446 if (sigprocmask(SIG_SETMASK, &mask, NULL) == \-1)
447 errExit("sigprocmask");
449 /* Create the timer */
451 sev.sigev_notify = SIGEV_SIGNAL;
452 sev.sigev_signo = SIG;
453 sev.sigev_value.sival_ptr = &timerid;
454 if (timer_create(CLOCKID, &sev, &timerid) == \-1)
455 errExit("timer_create");
457 printf("timer ID is 0x%lx\en", (long) timerid);
459 /* Start the timer */
461 freq_nanosecs = atoll(argv[2]);
462 its.it_value.tv_sec = freq_nanosecs / 1000000000;
463 its.it_value.tv_nsec = freq_nanosecs % 1000000000;
464 its.it_interval.tv_sec = its.it_value.tv_sec;
465 its.it_interval.tv_nsec = its.it_value.tv_nsec;
467 if (timer_settime(timerid, 0, &its, NULL) == \-1)
468 errExit("timer_settime");
470 /* Sleep for a while; meanwhile, the timer may expire
473 printf("Sleeping for %d seconds\en", atoi(argv[1]));
474 sleep(atoi(argv[1]));
476 /* Unlock the timer signal, so that timer notification
479 printf("Unblocking signal %d\en", SIG);
480 if (sigprocmask(SIG_UNBLOCK, &mask, NULL) == \-1)
481 errExit("sigprocmask");
489 .BR clock_gettime (2),
491 .BR timer_delete (2),
492 .BR timer_getoverrun (2),
493 .BR timer_settime (2),
494 .BR timerfd_create (2),
495 .BR clock_getcpuclockid (3),
496 .BR pthread_getcpuclockid (3),