1 .\" Copyright (c) 2009 Linux Foundation, written by Michael Kerrisk
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26 .TH TIMER_CREATE 2 2016-12-12 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 As well as the above values,
116 can be specified as the
118 returned by a call to
119 .BR clock_getcpuclockid (3)
121 .BR pthread_getcpuclockid (3).
127 structure that specifies how the caller
128 should be notified when the timer expires.
129 For the definition and general details of this structure, see
134 field can have the following values:
137 Don't asynchronously notify when the timer expires.
138 Progress of the timer can be monitored using
139 .BR timer_gettime (2).
142 Upon timer expiration, generate the signal
152 structure will be set to
154 At any point in time,
155 at most one signal is queued to the process for a given timer; see
156 .BR timer_getoverrun (2)
160 Upon timer expiration, invoke
161 .I sigev_notify_function
162 as if it were the start function of a new thread.
167 .BR SIGEV_THREAD_ID " (Linux-specific)"
170 but the signal is targeted at the thread whose ID is given in
171 .IR sigev_notify_thread_id ,
172 which must be a thread in the same process as the caller.
174 .IR sigev_notify_thread_id
175 field specifies a kernel thread ID, that is, the value returned by
179 This flag is intended only for use by threading libraries.
183 as NULL is equivalent to specifying a pointer to a
193 .I sigev_value.sival_int
198 returns 0, and the ID of the new timer is placed in
200 On failure, \-1 is returned, and
202 is set to indicate the error.
206 Temporary error during kernel allocation of timer structures.
213 .IR sigev_notify_thread_id
217 .\" glibc layer: malloc()
218 Could not allocate memory.
220 This system call is available since Linux 2.6.
222 POSIX.1-2001, POSIX.1-2008.
224 A program may create multiple interval timers using
227 Timers are not inherited by the child of a
229 and are disarmed and deleted during an
232 The kernel preallocates a "queued real-time signal"
233 for each timer created using
235 Consequently, the number of timers is limited by the
236 .BR RLIMIT_SIGPENDING
240 The timers created by
242 are commonly known as "POSIX (interval) timers".
243 The POSIX timers API consists of the following interfaces:
248 .BR timer_settime (2):
249 Arm (start) or disarm (stop) a timer.
251 .BR timer_gettime (2):
252 Fetch the time remaining until the next expiration of a timer,
253 along with the interval setting of the timer.
255 .BR timer_getoverrun (2):
256 Return the overrun count for the last timer expiration.
258 .BR timer_delete (2):
259 Disarm and delete a timer.
261 Since Linux 3.10, the
262 .IR /proc/[pid]/timers
263 file can be used to list the POSIX timers for the process with PID
267 for further information.
270 .\" baa73d9e478ff32d62f3f9422822b59dd9a95a21
271 support for POSIX timers is a configurable option that is enabled by default.
272 Kernel support can be disabled via the
273 .BR CONFIG_POSIX_TIMERS
276 .SS C library/kernel differences
277 Part of the implementation of the POSIX timers API is provided by glibc.
278 .\" See nptl/sysdeps/unix/sysv/linux/timer_create.c
281 Much of the functionality for
283 is implemented within glibc, rather than the kernel.
284 (This is necessarily so,
285 since the thread involved in handling the notification is one
286 that must be managed by the C library POSIX threads implementation.)
287 Although the notification delivered to the process is via a thread,
288 internally the NPTL implementation uses a
292 along with a real-time signal that is reserved by the implementation (see
295 The implementation of the default case where
297 is NULL is handled inside glibc,
298 which invokes the underlying system call with a suitably populated
302 The timer IDs presented at user level are maintained by glibc,
303 which maps these IDs to the timer IDs employed by the kernel.
304 .\" See the glibc source file kernel-posix-timers.h for the structure
305 .\" that glibc uses to map user-space timer IDs to kernel timer IDs
306 .\" The kernel-level timer ID is exposed via siginfo.si_tid.
308 The POSIX timers system calls first appeared in Linux 2.6.
310 glibc provided an incomplete user-space implementation
312 timers only) using POSIX threads,
313 and in glibc versions before 2.17,
314 .\" glibc commit 93a78ac437ba44f493333d7e2a4b0249839ce460
315 the implementation falls back to this technique on systems
316 running pre-2.6 Linux kernels.
318 The program below takes two arguments: a sleep period in seconds,
319 and a timer frequency in nanoseconds.
320 The program establishes a handler for the signal it uses for the timer,
322 creates and arms a timer that expires with the given frequency,
323 sleeps for the specified number of seconds,
324 and then unblocks the timer signal.
325 Assuming that the timer expired at least once while the program slept,
326 the signal handler will be invoked,
327 and the handler displays some information about the timer notification.
328 The program terminates after one invocation of the signal handler.
330 In the following example run, the program sleeps for 1 second,
331 after creating a timer that has a frequency of 100 nanoseconds.
332 By the time the signal is unblocked and delivered,
333 there have been around ten million overruns.
337 $ \fB./a.out 1 100\fP
338 Establishing handler for signal 34
340 timer ID is 0x804c008
341 Sleeping for 1 seconds
344 sival_ptr = 0xbfb174f4; *sival_ptr = 0x804c008
345 overrun count = 10004886
357 #define CLOCKID CLOCK_REALTIME
360 #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \\
364 print_siginfo(siginfo_t *si)
369 tidp = si\->si_value.sival_ptr;
371 printf(" sival_ptr = %p; ", si\->si_value.sival_ptr);
372 printf(" *sival_ptr = 0x%lx\\n", (long) *tidp);
374 or = timer_getoverrun(*tidp);
376 errExit("timer_getoverrun");
378 printf(" overrun count = %d\\n", or);
382 handler(int sig, siginfo_t *si, void *uc)
384 /* Note: calling printf() from a signal handler is not
385 strictly correct, since printf() is not async\-signal\-safe;
388 printf("Caught signal %d\\n", sig);
390 signal(sig, SIG_IGN);
394 main(int argc, char *argv[])
398 struct itimerspec its;
399 long long freq_nanosecs;
404 fprintf(stderr, "Usage: %s <sleep\-secs> <freq\-nanosecs>\\n",
409 /* Establish handler for timer signal */
411 printf("Establishing handler for signal %d\\n", SIG);
412 sa.sa_flags = SA_SIGINFO;
413 sa.sa_sigaction = handler;
414 sigemptyset(&sa.sa_mask);
415 if (sigaction(SIG, &sa, NULL) == \-1)
416 errExit("sigaction");
418 /* Block timer signal temporarily */
420 printf("Blocking signal %d\\n", SIG);
422 sigaddset(&mask, SIG);
423 if (sigprocmask(SIG_SETMASK, &mask, NULL) == \-1)
424 errExit("sigprocmask");
426 /* Create the timer */
428 sev.sigev_notify = SIGEV_SIGNAL;
429 sev.sigev_signo = SIG;
430 sev.sigev_value.sival_ptr = &timerid;
431 if (timer_create(CLOCKID, &sev, &timerid) == \-1)
432 errExit("timer_create");
434 printf("timer ID is 0x%lx\\n", (long) timerid);
436 /* Start the timer */
438 freq_nanosecs = atoll(argv[2]);
439 its.it_value.tv_sec = freq_nanosecs / 1000000000;
440 its.it_value.tv_nsec = freq_nanosecs % 1000000000;
441 its.it_interval.tv_sec = its.it_value.tv_sec;
442 its.it_interval.tv_nsec = its.it_value.tv_nsec;
444 if (timer_settime(timerid, 0, &its, NULL) == \-1)
445 errExit("timer_settime");
447 /* Sleep for a while; meanwhile, the timer may expire
450 printf("Sleeping for %d seconds\\n", atoi(argv[1]));
451 sleep(atoi(argv[1]));
453 /* Unlock the timer signal, so that timer notification
456 printf("Unblocking signal %d\\n", SIG);
457 if (sigprocmask(SIG_UNBLOCK, &mask, NULL) == \-1)
458 errExit("sigprocmask");
466 .BR clock_gettime (2),
468 .BR timer_delete (2),
469 .BR timer_getoverrun (2),
470 .BR timer_settime (2),
471 .BR timerfd_create (2),
472 .BR clock_getcpuclockid (3),
473 .BR pthread_getcpuclockid (3),