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TIMER_CREATE(2)            Linux Programmer's Manual           TIMER_CREATE(2)

       timer_create - create a POSIX per-process timer

       #include <signal.h>
       #include <time.h>

       int timer_create(clockid_t clockid, struct sigevent *evp,
                        timer_t *timerid);

       Link with -lrt.

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       timer_create(): _POSIX_C_SOURCE >= 199309

       timer_create()  creates  a new per-process interval timer.  The ID of the new timer
       is returned in the buffer pointed to by timerid, which must be a non-NULL  pointer.
       This ID is unique within the process, until the timer is deleted.  The new timer is
       initially disarmed.

       The clockid argument specifies the clock that the new timer uses to  measure  time.
       It can be specified as one of the following values:

              A settable system-wide real-time clock.

              A  non-settable  monotonically increasing clock that measures time from some
              unspecified point in the past that does not change after system startup.

       CLOCK_PROCESS_CPUTIME_ID (since Linux 2.6.12)
              A clock that measures (user and system) CPU time consumed  by  (all  of  the
              threads in) the calling process.

       CLOCK_THREAD_CPUTIME_ID (since Linux 2.6.12)
              A  clock  that  measures  (user and system) CPU time consumed by the calling

       As well as the above values, clockid can be specified as the clockid returned by  a
       call to clock_getcpuclockid(3) or pthread_getcpuclockid(3).

       The  evp  argument  points  to  a  sigevent structure that specifies how the caller
       should be notified when the timer expires.  This  structure  is  defined  something
       like the following:

           union sigval {
               int   sival_int;
               void *sival_ptr;

           struct sigevent {
               int          sigev_notify;    /* Notification method */
               int          sigev_signo;     /* Timer expiration signal */
               union sigval sigev_value;     /* Value accompanying signal or
                                                passed to thread function */
               void       (*sigev_notify_function) (union sigval);
                              /* Function used for thread
                                 notifications (SIGEV_THREAD) */
               void        *sigev_notify_attributes;
                              /* Attributes for notification thread
                                 (SIGEV_THREAD) */
               pid_t        sigev_notify_thread_id;
                              /* ID of thread to signal (SIGEV_THREAD_ID) */

       Some  of  these  fields  may  be  defined as part of a union: a program should only
       employ those fields relevant to the value specified in  sigev_notify.   This  field
       can have the following values:

              Don't  asynchronously  notify when the timer expires.  Progress of the timer
              can be monitored using timer_gettime(2).

              Upon timer expiration, generate the signal sigev_signo for the process.   If
              sigev_signo  is  a real-time signal, then it will be accompanied by the data
              specified  in   sigev_value   (like   the   signal-accompanying   data   for
              sigqueue(2)).   At  any  point  in time, at most one signal is queued to the
              process for a given timer; see timer_getoverrun(2) for more details.

              Upon timer expiration, invoke sigev_notify_function as if it were the  start
              function  of a new thread.  (Among the implementation possibilities here are
              that each timer notification could result in the creation of a  new  thread,
              or that a single thread is created to receive all notifications.)  The func-
              tion   is   invoked   with   sigev_value   as   its   sole   argument.    If
              sigev_notify_attributes  is  not  NULL,  it should point to a pthread_attr_t
              structure   that   defines   attributes   for   the    new    thread    (see

       SIGEV_THREAD_ID (Linux-specific)
              As  for  SIGEV_SIGNAL,  but the signal is targeted at the thread whose ID is
              given in sigev_notify_thread_id, which must be a thread in the same  process
              as  the  caller.  The sigev_notify_thread_id field specifies a kernel thread
              ID, that is, the value returned by clone(2) or gettid(2).  This flag is only
              intended for use by threading libraries.

       Specifying  evp  as NULL is equivalent to specifying a pointer to a sigevent struc-
       ture  in  which  sigev_notify  is  SIGEV_SIGNAL,  sigev_signo   is   SIGALRM,   and
       sigev_value.sival_int is the timer ID.

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

       EAGAIN Temporary error during kernel allocation of timer structures.

       EINVAL Clock ID, sigev_notify, sigev_signo, sigev_notify_thread_id is invalid.

       ENOMEM Could not allocate memory.

       This system call is available since Linux 2.6.


       A program may create multiple interval timers using timer_create().

       Timers are not inherited by the child of a fork(2), and are  disarmed  and  deleted
       during an execve(2).

       The  kernel  preallocates  a "queued real-time signal" for each timer created using
       timer_create().   Consequently,  the  number  of   timers   is   limited   by   the
       RLIMIT_SIGPENDING resource limit (see setrlimit(2)).

       The  timers  created  by  timer_create()  are  commonly  known as "POSIX (interval)
       timers".  The POSIX timers API consists of the following interfaces:

       *  timer_create(): Create a timer.

       *  timer_settime(2): Arm (start) or disarm (stop) a timer.

       *  timer_gettime(2): Fetch the time remaining until the next expiration of a timer,
          along with the interval setting of the timer.

       *  timer_getoverrun(2): Return the overrun count for the last timer expiration.

       *  timer_delete(2): Disarm and delete a timer.

       Part  of  the implementation of the POSIX timers API is provided by glibc.  In par-

       *  The functionality for SIGEV_THREAD is implemented within glibc, rather than  the

       *  The  timer IDs presented at user level are maintained by glibc, which maps these
          IDs to the timer IDs employed by the kernel.

       The POSIX timers system calls first appeared in Linux 2.6.  Prior  to  this,  glibc
       provided  an incomplete userspace implementation (CLOCK_REALTIME timers only) using
       POSIX threads, and current glibc falls back to this implementation on systems  run-
       ning pre-2.6 Linux kernels.

       The  program below takes two arguments: a sleep period in seconds, and a timer fre-
       quency in nanoseconds.  The program establishes a handler for the  signal  it  uses
       for  the  timer, blocks that signal, creates and arms a timer that expires with the
       given frequency, sleeps for the specified number of seconds, and then unblocks  the
       timer  signal.   Assuming  that  the  timer expired at least once while the program
       slept, the signal handler will be invoked, and the handler displays  some  informa-
       tion  about the timer notification.  The program terminates after one invocation of
       the signal handler.

       In the following example run, the program sleeps for 1  second,  after  creating  a
       timer that has a frequency of 100 nanoseconds.  By the time the signal is unblocked
       and delivered, there have been around ten million overruns.

           $ ./a.out 1 10
           Establishing handler for signal 34
           Blocking signal 34
           timer ID is 0x804c008
           Sleeping for 1 seconds
           Unblocking signal 34
           Caught signal 34
               sival_ptr = 0xbfb174f4;     *sival_ptr = 0x804c008
               overrun count = 10004886

   Program Source

       #include <stdlib.h>
       #include <unistd.h>
       #include <stdio.h>
       #include <signal.h>
       #include <time.h>

       #define SIG SIGRTMIN

       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                               } while (0)

       static void
       print_siginfo(siginfo_t *si)
           timer_t *tidp;
           int or;

           tidp = si->si_value.sival_ptr;

           printf("    sival_ptr = %p; ", si->si_value.sival_ptr);
           printf("    *sival_ptr = 0x%lx\n", (long) *tidp);

           or = timer_getoverrun(*tidp);
           if (or == -1)
               printf("    overrun count = %d\n", or);

       static void
       handler(int sig, siginfo_t *si, void *uc)
           /* Note: calling printf() from a signal handler is not
              strictly correct, since printf() is not async-signal-safe;
              see signal(7) */

           printf("Caught signal %d\n", sig);
           signal(sig, SIG_IGN);

       main(int argc, char *argv[])
           timer_t timerid;
           struct sigevent sev;
           struct itimerspec its;
           long long freq_nanosecs;
           sigset_t mask;
           struct sigaction sa;

           if (argc != 3) {
               fprintf(stderr, "Usage: %s <sleep-secs> <freq-nanosecs>\n",

           /* Establish handler for timer signal */

           printf("Establishing handler for signal %d\n", SIG);
           sa.sa_flags = SA_SIGINFO;
           sa.sa_sigaction = handler;
           if (sigaction(SIG, &sa, NULL) == -1)

           /* Block timer signal temporarily */

           printf("Blocking signal %d\n", SIG);
           sigaddset(&mask, SIG);
           if (sigprocmask(SIG_SETMASK, &mask, NULL) == -1)

           /* Create the timer */

           sev.sigev_notify = SIGEV_SIGNAL;
           sev.sigev_signo = SIG;
           sev.sigev_value.sival_ptr = &timerid;
           if (timer_create(CLOCKID, &sev, &timerid) == -1)

           printf("timer ID is 0x%lx\n", (long) timerid);

           /* Start the timer */

           freq_nanosecs = atoll(argv[2]);
           its.it_value.tv_sec = freq_nanosecs / 1000000000;
           its.it_value.tv_nsec = freq_nanosecs % 1000000000;
           its.it_interval.tv_sec = its.it_value.tv_sec;
           its.it_interval.tv_nsec = its.it_value.tv_nsec;

           if (timer_settime(timerid, 0, &its, NULL) == -1)

           /* Sleep for a while; meanwhile, the timer may expire
              multiple times */

           printf("Sleeping for %d seconds\n", atoi(argv[1]));

           /* Unlock the timer signal, so that timer notification
              can be delivered */

           printf("Unblocking signal %d\n", SIG);
           if (sigprocmask(SIG_UNBLOCK, &mask, NULL) == -1)


       clock_gettime(2), setitimer(2), timer_delete(2),  timer_settime(2),  timer_getover-
       run(2),    timerfd_create(2),   clock_getcpuclockid(3),   pthread_getcpuclockid(3),
       pthreads(7), signal(7), time(7)

       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the  project, and information about reporting bugs, can be found at http://www.ker-

Linux                             2009-02-20                   TIMER_CREATE(2)

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