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

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

       Link with -lrt.

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

       timer_create(): _POSIX_C_SOURCE >= 199309L

       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  nonsettable 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 thread.

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

       The sevp argument points to a sigevent structure that specifies how the
       caller should be notified when the timer expires.  For  the  definition
       and general details of this structure, see sigevent(7).

       The sevp.sigev_notify 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.   See  sigevent(7)  for  general  details.  The si_code
              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
              intended only for use by threading libraries.

       Specifying  sevp  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, or sigev_notify_thread_id
              is invalid.

       ENOMEM Could not allocate memory.

       This system call is available since Linux 2.6.

       POSIX.1-2001, POSIX.1-2008.

       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 cre-
       ated using timer_create().  Consequently, the number of timers is  lim-
       ited 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

       *  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

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

          the notification delivered to the process is via  a  thread,  inter-
          nally   the   NPTL  implementation  uses  a  sigev_notify  value  of
          SIGEV_THREAD_ID along with a real-time signal that  is  reserved  by
          the implementation (see nptl(7)).

       *  The  implementation of the default case where evp is NULL is handled
          inside glibc, which invokes the underlying system call with a  suit-
          ably populated sigevent structure.

       *  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   user-space  implementation
       (CLOCK_REALTIME timers only) using POSIX threads, and in glibc versions
       before 2.17, the implementation falls back to this technique 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 invoca-
       tion 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  mil-
       lion overruns.

           $ ./a.out 1 100
           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
           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)

           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_getoverrun(2),
       timer_settime(2), timerfd_create(2), clock_getcpuclockid(3),
       pthread_getcpuclockid(3), pthreads(7), sigevent(7), signal(7), time(7)

       This page is part of release 4.04 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                             2015-07-23                   TIMER_CREATE(2)
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