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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 pro-

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

       As well as the above values, clockid can be specified as the  clockid  returned  by  a  call  to  clock_getcpu-
       clockid(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

              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  cre-
              ation  of  a new thread, or that a single thread is created to receive all notifications.)  The function
              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 structure 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 particular:

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

       *  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 running pre-2.6 Linux kernels.

       The  program  below  takes two arguments: a sleep period in seconds, and a timer frequency 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 information 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 over-

           $ ./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_getoverrun(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 informa-
       tion about reporting bugs, can be found at

Linux                             2009-02-20                   TIMER_CREATE(2)