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 *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 af-
              ter 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  capa-
              bility 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  capa-
              bility in order to set a timer against this clock.

       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
              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_no-
              tify_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 (in-
       terval) timers".  The POSIX timers API consists of the following inter-

       *  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 ex-

       *  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 in-

       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,  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_RE-
       ALTIME timers only) using POSIX threads, and in glibc  versions  before
       2.17,  the  implementation falls back to this technique on systems run-
       ning 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

       #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 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 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_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 5.05 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                             2019-03-06                   TIMER_CREATE(2)
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