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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 *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 non-settable 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   pthread_getcpu‐

       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  sig‐
	      nal-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 implementa‐
	      tion  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,  sigev_notify_thread_id  is

       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 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.

       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 ker‐

       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 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 3.22 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2009-02-20		       TIMER_CREATE(2)

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