1TIMER_CREATE(2)            Linux Programmer's Manual           TIMER_CREATE(2)
2
3
4

NAME

6       timer_create - create a POSIX per-process timer
7

SYNOPSIS

9       #include <signal.h>
10       #include <time.h>
11
12       int timer_create(clockid_t clockid, struct sigevent *evp,
13                        timer_t *timerid);
14
15       Link with -lrt.
16
17   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
18
19       timer_create(): _POSIX_C_SOURCE >= 199309
20

DESCRIPTION

22       timer_create() creates a new per-process interval timer.  The ID of the
23       new timer is returned in the buffer pointed to by timerid,  which  must
24       be a non-NULL pointer.  This ID is unique within the process, until the
25       timer is deleted.  The new timer is initially disarmed.
26
27       The clockid argument specifies the clock that the  new  timer  uses  to
28       measure time.  It can be specified as one of the following values:
29
30       CLOCK_REALTIME
31              A settable system-wide real-time clock.
32
33       CLOCK_MONOTONIC
34              A non-settable monotonically increasing clock that measures time
35              from some unspecified point in the past  that  does  not  change
36              after system startup.
37
38       CLOCK_PROCESS_CPUTIME_ID (since Linux 2.6.12)
39              A  clock  that  measures  (user and system) CPU time consumed by
40              (all of the threads in) the calling process.
41
42       CLOCK_THREAD_CPUTIME_ID (since Linux 2.6.12)
43              A clock that measures (user and system) CPU time consumed by the
44              calling thread.
45
46       As  well  as  the above values, clockid can be specified as the clockid
47       returned  by  a  call  to  clock_getcpuclockid(3)  or   pthread_getcpu‐
48       clockid(3).
49
50       The  evp argument points to a sigevent structure that specifies how the
51       caller should be notified when the timer expires.   This  structure  is
52       defined something like the following:
53
54           union sigval {
55               int   sival_int;
56               void *sival_ptr;
57           };
58
59           struct sigevent {
60               int          sigev_notify;    /* Notification method */
61               int          sigev_signo;     /* Timer expiration signal */
62               union sigval sigev_value;     /* Value accompanying signal or
63                                                passed to thread function */
64               void       (*sigev_notify_function) (union sigval);
65                              /* Function used for thread
66                                 notifications (SIGEV_THREAD) */
67               void        *sigev_notify_attributes;
68                              /* Attributes for notification thread
69                                 (SIGEV_THREAD) */
70               pid_t        sigev_notify_thread_id;
71                              /* ID of thread to signal (SIGEV_THREAD_ID) */
72           };
73
74       Some  of  these  fields  may  be  defined as part of a union: a program
75       should only employ those fields relevant  to  the  value  specified  in
76       sigev_notify.  This field can have the following values:
77
78       SIGEV_NONE
79              Don't asynchronously notify when the timer expires.  Progress of
80              the timer can be monitored using timer_gettime(2).
81
82       SIGEV_SIGNAL
83              Upon timer expiration, generate the signal sigev_signo  for  the
84              process.   If sigev_signo is a real-time signal, then it will be
85              accompanied by the data specified in sigev_value (like the  sig‐
86              nal-accompanying  data  for sigqueue(2)).  At any point in time,
87              at most one signal is queued to the process for a  given  timer;
88              see timer_getoverrun(2) for more details.
89
90       SIGEV_THREAD
91              Upon  timer  expiration,  invoke  sigev_notify_function as if it
92              were the start function of a new thread.  (Among the implementa‐
93              tion  possibilities  here are that each timer notification could
94              result in the creation of a new thread, or that a single  thread
95              is  created  to  receive  all  notifications.)   The function is
96              invoked   with   sigev_value   as   its   sole   argument.    If
97              sigev_notify_attributes  is  not  NULL,  it  should  point  to a
98              pthread_attr_t structure that defines  attributes  for  the  new
99              thread (see pthread_attr_init(3).
100
101       SIGEV_THREAD_ID (Linux-specific)
102              As  for  SIGEV_SIGNAL,  but the signal is targeted at the thread
103              whose ID is given in sigev_notify_thread_id,  which  must  be  a
104              thread    in    the   same   process   as   the   caller.    The
105              sigev_notify_thread_id field specifies a kernel thread ID,  that
106              is,  the  value returned by clone(2) or gettid(2).  This flag is
107              only intended for use by threading libraries.
108
109       Specifying evp as NULL is equivalent  to  specifying  a  pointer  to  a
110       sigevent  structure  in which sigev_notify is SIGEV_SIGNAL, sigev_signo
111       is SIGALRM, and sigev_value.sival_int is the timer ID.
112

RETURN VALUE

114       On success, timer_create() returns 0, and the ID of the  new  timer  is
115       placed  in  *timerid.   On failure, -1 is returned, and errno is set to
116       indicate the error.
117

ERRORS

119       EAGAIN Temporary error during kernel allocation of timer structures.
120
121       EINVAL Clock ID, sigev_notify, sigev_signo,  sigev_notify_thread_id  is
122              invalid.
123
124       ENOMEM Could not allocate memory.
125

VERSIONS

127       This system call is available since Linux 2.6.
128

CONFORMING TO

130       POSIX.1-2001
131

NOTES

133       A program may create multiple interval timers using timer_create().
134
135       Timers  are  not  inherited by the child of a fork(2), and are disarmed
136       and deleted during an execve(2).
137
138       The kernel preallocates a "queued real-time signal" for each timer cre‐
139       ated  using timer_create().  Consequently, the number of timers is lim‐
140       ited by the RLIMIT_SIGPENDING resource limit (see setrlimit(2)).
141
142       The timers created by  timer_create()  are  commonly  known  as  "POSIX
143       (interval)  timers".   The  POSIX  timers API consists of the following
144       interfaces:
145
146       *  timer_create(): Create a timer.
147
148       *  timer_settime(2): Arm (start) or disarm (stop) a timer.
149
150       *  timer_gettime(2): Fetch the time remaining until the next expiration
151          of a timer, along with the interval setting of the timer.
152
153       *  timer_getoverrun(2):  Return  the  overrun  count for the last timer
154          expiration.
155
156       *  timer_delete(2): Disarm and delete a timer.
157
158       Part of the implementation of the  POSIX  timers  API  is  provided  by
159       glibc.  In particular:
160
161       *  The  functionality  for  SIGEV_THREAD  is  implemented within glibc,
162          rather than the kernel.
163
164       *  The timer IDs presented at user level are maintained by glibc, which
165          maps these IDs to the timer IDs employed by the kernel.
166
167       The  POSIX  timers  system calls first appeared in Linux 2.6.  Prior to
168       this,   glibc   provided   an   incomplete   userspace   implementation
169       (CLOCK_REALTIME  timers  only)  using  POSIX threads, and current glibc
170       falls back to this implementation on systems running pre-2.6 Linux ker‐
171       nels.
172

EXAMPLE

174       The program below takes two arguments: a sleep period in seconds, and a
175       timer frequency in nanoseconds.  The program establishes a handler  for
176       the  signal it uses for the timer, blocks that signal, creates and arms
177       a timer that expires with the given frequency, sleeps for the specified
178       number  of  seconds, and then unblocks the timer signal.  Assuming that
179       the timer expired at least once while the  program  slept,  the  signal
180       handler  will  be  invoked,  and  the handler displays some information
181       about the timer notification.  The program terminates after one invoca‐
182       tion of the signal handler.
183
184       In  the  following  example run, the program sleeps for 1 second, after
185       creating a timer that has a frequency of 100 nanoseconds.  By the  time
186       the  signal is unblocked and delivered, there have been around ten mil‐
187       lion overruns.
188
189           $ ./a.out 1 10
190           Establishing handler for signal 34
191           Blocking signal 34
192           timer ID is 0x804c008
193           Sleeping for 1 seconds
194           Unblocking signal 34
195           Caught signal 34
196               sival_ptr = 0xbfb174f4;     *sival_ptr = 0x804c008
197               overrun count = 10004886
198
199   Program Source
200
201       #include <stdlib.h>
202       #include <unistd.h>
203       #include <stdio.h>
204       #include <signal.h>
205       #include <time.h>
206
207       #define CLOCKID CLOCK_REALTIME
208       #define SIG SIGRTMIN
209
210       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
211                               } while (0)
212
213       static void
214       print_siginfo(siginfo_t *si)
215       {
216           timer_t *tidp;
217           int or;
218
219           tidp = si->si_value.sival_ptr;
220
221           printf("    sival_ptr = %p; ", si->si_value.sival_ptr);
222           printf("    *sival_ptr = 0x%lx\n", (long) *tidp);
223
224           or = timer_getoverrun(*tidp);
225           if (or == -1)
226               errExit("timer_getoverrun");
227           else
228               printf("    overrun count = %d\n", or);
229       }
230
231       static void
232       handler(int sig, siginfo_t *si, void *uc)
233       {
234           /* Note: calling printf() from a signal handler is not
235              strictly correct, since printf() is not async-signal-safe;
236              see signal(7) */
237
238           printf("Caught signal %d\n", sig);
239           print_siginfo(si);
240           signal(sig, SIG_IGN);
241       }
242
243       int
244       main(int argc, char *argv[])
245       {
246           timer_t timerid;
247           struct sigevent sev;
248           struct itimerspec its;
249           long long freq_nanosecs;
250           sigset_t mask;
251           struct sigaction sa;
252
253           if (argc != 3) {
254               fprintf(stderr, "Usage: %s <sleep-secs> <freq-nanosecs>\n",
255                       argv[0]);
256               exit(EXIT_FAILURE);
257           }
258
259           /* Establish handler for timer signal */
260
261           printf("Establishing handler for signal %d\n", SIG);
262           sa.sa_flags = SA_SIGINFO;
263           sa.sa_sigaction = handler;
264           sigemptyset(&sa.sa_mask);
265           if (sigaction(SIG, &sa, NULL) == -1)
266               errExit("sigaction");
267
268           /* Block timer signal temporarily */
269
270           printf("Blocking signal %d\n", SIG);
271           sigemptyset(&mask);
272           sigaddset(&mask, SIG);
273           if (sigprocmask(SIG_SETMASK, &mask, NULL) == -1)
274               errExit("sigprocmask");
275
276           /* Create the timer */
277
278           sev.sigev_notify = SIGEV_SIGNAL;
279           sev.sigev_signo = SIG;
280           sev.sigev_value.sival_ptr = &timerid;
281           if (timer_create(CLOCKID, &sev, &timerid) == -1)
282               errExit("timer_create");
283
284           printf("timer ID is 0x%lx\n", (long) timerid);
285
286           /* Start the timer */
287
288           freq_nanosecs = atoll(argv[2]);
289           its.it_value.tv_sec = freq_nanosecs / 1000000000;
290           its.it_value.tv_nsec = freq_nanosecs % 1000000000;
291           its.it_interval.tv_sec = its.it_value.tv_sec;
292           its.it_interval.tv_nsec = its.it_value.tv_nsec;
293
294           if (timer_settime(timerid, 0, &its, NULL) == -1)
295                errExit("timer_settime");
296
297           /* Sleep for a while; meanwhile, the timer may expire
298              multiple times */
299
300           printf("Sleeping for %d seconds\n", atoi(argv[1]));
301           sleep(atoi(argv[1]));
302
303           /* Unlock the timer signal, so that timer notification
304              can be delivered */
305
306           printf("Unblocking signal %d\n", SIG);
307           if (sigprocmask(SIG_UNBLOCK, &mask, NULL) == -1)
308               errExit("sigprocmask");
309
310           exit(EXIT_SUCCESS);
311       }
312

SEE ALSO

314       clock_gettime(2),  setitimer(2),   timer_delete(2),   timer_settime(2),
315       timer_getoverrun(2),     timerfd_create(2),     clock_getcpuclockid(3),
316       pthread_getcpuclockid(3), pthreads(7), signal(7), time(7)
317

COLOPHON

319       This page is part of release 3.22 of the Linux  man-pages  project.   A
320       description  of  the project, and information about reporting bugs, can
321       be found at http://www.kernel.org/doc/man-pages/.
322
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324
325Linux                             2009-02-20                   TIMER_CREATE(2)
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