timer_create(2) — Linux manual page


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

NAME         top

       timer_create - create a POSIX per-process timer

SYNOPSIS         top

       #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

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 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 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 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

       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.

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 structures.

       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

              the caller did not have the CAP_WAKE_ALARM capability.

VERSIONS         top

       This system call is available since Linux 2.6.

CONFORMING TO         top

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

NOTES         top

       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_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 nptl(7)).

       *  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.

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 <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)


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),

COLOPHON         top

       This page is part of release 5.09 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                            2020-11-01                  TIMER_CREATE(2)

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