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       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
              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 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
              only intended 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.




       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 1 100
           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
              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_getoverrun(2),
       timer_settime(2),      timerfd_create(2),       clock_getcpuclockid(3),
       pthread_getcpuclockid(3), pthreads(7), sigevent(7), signal(7), time(7)


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