Man Linux: Main Page and Category List


       timer_create - create a POSIX per-process timer


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

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

       Link with -lrt.

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

       timer_create(): _POSIX_C_SOURCE >= 199309


       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

       The  evp argument points to a sigevent structure that specifies how the
       caller should be notified when the timer expires.   This  structure  is
       defined something like the following:

           union sigval {
               int   sival_int;
               void *sival_ptr;

           struct sigevent {
               int          sigev_notify;    /* Notification method */
               int          sigev_signo;     /* Timer expiration signal */
               union sigval sigev_value;     /* Value accompanying signal or
                                                passed to thread function */
               void       (*sigev_notify_function) (union sigval);
                              /* Function used for thread
                                 notifications (SIGEV_THREAD) */
               void        *sigev_notify_attributes;
                              /* Attributes for notification thread
                                 (SIGEV_THREAD) */
               pid_t        sigev_notify_thread_id;
                              /* ID of thread to signal (SIGEV_THREAD_ID) */

       Some  of  these  fields  may  be  defined as part of a union: a program
       should only employ those fields relevant  to  the  value  specified  in
       sigev_notify.  This 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.   If sigev_signo is a real-time signal, then it will be
              accompanied by the  data  specified  in  sigev_value  (like  the
              signal-accompanying  data  for  sigqueue(2)).   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.    (Among   the
              implementation   possibilities   here   are   that   each  timer
              notification could result in the creation of a  new  thread,  or
              that  a  single thread is created to receive all notifications.)
              The function is invoked with sigev_value as its  sole  argument.
              If  sigev_notify_attributes  is  not  NULL, it should point to a
              pthread_attr_t structure that defines  attributes  for  the  new
              thread (see pthread_attr_init(3)).

       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
              only intended for use by threading libraries.

       Specifying evp 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.




       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.

       Part of the implementation of the  POSIX  timers  API  is  provided  by
       glibc.  In particular:

       *  The  functionality  for  SIGEV_THREAD  is  implemented within glibc,
          rather than the kernel.

       *  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   userspace   implementation
       (CLOCK_REALTIME  timers  only)  using  POSIX threads, and current glibc
       falls back to this implementation  on  systems  running  pre-2.6  Linux


       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 10
           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
              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)

           /* 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_settime(2),
       timer_getoverrun(2),     timerfd_create(2),     clock_getcpuclockid(3),
       pthread_getcpuclockid(3), pthreads(7), signal(7), time(7)


       This page is part of release 3.24 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at