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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
	      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	 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_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 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
       (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

   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_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 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
	      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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       latest version of this page, can be found at

Linux				  2016-12-12		       TIMER_CREATE(2)