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

       timer_create - create a POSIX per-process timer

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

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

       Link with -lrt.

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

           _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.

       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

       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 pthread_getcpuclockid(3).

       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 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 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

       ENOMEM Could not allocate memory.

              The kernel does not support creating a timer against this clockid.

              caller did not have the CAP_WAKE_ALARM capability.

       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
       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

       *  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.

       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

       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.

   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.

       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 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

           $ ./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 <stdint.h>
       #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 = %#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;
           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 %#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)


       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 5.13 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                              2021-03-22                    TIMER_CREATE(2)