timer_create(2) — Linux manual page


timer_create(2)            System Calls Manual           timer_create(2)

NAME         top

       timer_create - create a POSIX per-process timer

LIBRARY         top

       Real-time library (librt, -lrt)

SYNOPSIS         top

       #include <signal.h>           /* Definition of SIGEV_* constants */
       #include <time.h>

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

   Feature Test Macro Requirements for glibc (see

           _POSIX_C_SOURCE >= 199309L

DESCRIPTION         top

       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

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

              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.

       CLOCK_BOOTTIME (Since Linux 2.6.39)
              Like CLOCK_MONOTONIC, this is a monotonically increasing
              clock.  However, whereas the CLOCK_MONOTONIC clock does
              not measure the time while a system is suspended, the
              CLOCK_BOOTTIME clock does include the time during which
              the system is suspended.  This is useful for applications
              that need to be suspend-aware.  CLOCK_REALTIME is not
              suitable for such applications, since that clock is
              affected by discontinuous changes to the system clock.

       CLOCK_REALTIME_ALARM (since Linux 3.0)
              This clock is like CLOCK_REALTIME, but will wake the
              system if it is suspended.  The caller must have the
              CAP_WAKE_ALARM capability in order to set a timer against
              this clock.

       CLOCK_BOOTTIME_ALARM (since Linux 3.0)
              This clock is like CLOCK_BOOTTIME, but will wake the
              system if it is suspended.  The caller must have the
              CAP_WAKE_ALARM capability in order to set a timer against
              this clock.

       CLOCK_TAI (since Linux 3.10)
              A system-wide clock derived from wall-clock time but
              ignoring leap seconds.

       See clock_getres(2) for some further details on the above clocks.

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

       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

       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

              Upon timer expiration, generate the signal sigev_signo for
              the process.  See sigevent(7) for general details.  The
              si_code field of the siginfo_t structure will be set to
              SI_TIMER.  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.  See
              sigevent(7) for details.

       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 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

RETURN VALUE         top

       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.

ERRORS         top

       EAGAIN Temporary error during kernel allocation of timer

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

       ENOMEM Could not allocate memory.

              The kernel does not support creating a timer against this

              but the caller did not have the CAP_WAKE_ALARM capability.

VERSIONS         top

   C library/kernel differences
       Part of the implementation of the POSIX timers API is provided by
       glibc.  In particular:

       •  Much of the functionality for SIGEV_THREAD is implemented
          within glibc, rather than the kernel.  (This is necessarily
          so, since the thread involved in handling the notification is
          one that must be managed by the C library POSIX threads
          implementation.)  Although the notification delivered to the
          process is via a thread, internally 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

       •  The implementation of the default case where evp is NULL is
          handled inside glibc, which invokes the underlying system call
          with a suitably 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.

STANDARDS         top


HISTORY         top

       Linux 2.6.  POSIX.1-2001.

       Prior to Linux 2.6, glibc provided an incomplete user-space
       implementation (CLOCK_REALTIME timers only) using POSIX threads,
       and before glibc 2.17, the implementation falls back to this
       technique on systems running kernels older than Linux 2.6.

NOTES         top

       A program may create multiple interval timers using

       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

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

              Create a timer.

              Arm (start) or disarm (stop) a timer.

              Fetch the time remaining until the next expiration of a
              timer, along with the interval setting of the timer.

              Return the overrun count for the last timer expiration.

              Disarm and delete a timer.

       Since Linux 3.10, the /proc/pid/timers file can be used to list
       the POSIX timers for the process with PID pid.  See proc(5) for
       further information.

       Since Linux 4.10, support for POSIX timers is a configurable
       option that is enabled by default.  Kernel support can be
       disabled via the CONFIG_POSIX_TIMERS option.

EXAMPLES         top

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

       #define SIG SIGRTMIN

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

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

           tidp = si->si_value.sival_ptr;

           printf("    sival_ptr = %p; ", si->si_value.sival_ptr);
           printf("    *sival_ptr = %#jx\n", (uintmax_t) *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 safe
              (and should not be done in production programs), since
              printf() is not async-signal-safe; see signal-safety(7).
              Nevertheless, we use printf() here as a simple way of
              showing that the handler was called. */

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

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

           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 %#jx\n", (uintmax_t) 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)


SEE ALSO         top

       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)

Linux man-pages (unreleased)     (date)                  timer_create(2)

Pages that refer to this page: alarm(2)clock_getres(2)clock_nanosleep(2)execve(2)fork(2)getitimer(2)gettid(2)nanosleep(2)seccomp(2)sigaction(2)syscalls(2)timer_delete(2)timerfd_create(2)timer_getoverrun(2)timer_settime(2)clock_getcpuclockid(3)clockid_t(3type)pthread_getcpuclockid(3)timer_t(3type)ualarm(3)usleep(3)proc(5)systemd.exec(5)nptl(7)pthreads(7)sigevent(7)system_data_types(7)time(7)