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NAME

       timer_create - create a POSIX per-process timer

SYNOPSIS

       #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

DESCRIPTION

       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:

       CLOCK_REALTIME
              A settable system-wide real-time clock.

       CLOCK_MONOTONIC
              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 capability 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 capability 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_getcpuclockid(3).

       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:

       SIGEV_NONE
              Don't  asynchronously  notify when the timer expires.  Progress of the timer can be
              monitored using timer_gettime(2).

       SIGEV_SIGNAL
              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.

       SIGEV_THREAD
              Upon  timer  expiration,  invoke  sigev_notify_function  as  if  it  were the start
              function of a new thread.  See sigevent(7) for details.

       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.

RETURN VALUE

       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.

ERRORS

       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.

VERSIONS

       This system call is available since Linux 2.6.

CONFORMING TO

       POSIX.1-2001, POSIX.1-2008.

NOTES

       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 interfaces:

       *  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 expiration.

       *  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 information.

       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,
          internally 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 suitably 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.

EXAMPLE

       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 CLOCKID CLOCK_REALTIME
       #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)
               errExit("timer_getoverrun");
           else
               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);
           print_siginfo(si);
           signal(sig, SIG_IGN);
       }

       int
       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",
                       argv[0]);
               exit(EXIT_FAILURE);
           }

           /* Establish handler for timer signal */

           printf("Establishing handler for signal %d\n", SIG);
           sa.sa_flags = SA_SIGINFO;
           sa.sa_sigaction = handler;
           sigemptyset(&sa.sa_mask);
           if (sigaction(SIG, &sa, NULL) == -1)
               errExit("sigaction");

           /* Block timer signal temporarily */

           printf("Blocking signal %d\n", SIG);
           sigemptyset(&mask);
           sigaddset(&mask, SIG);
           if (sigprocmask(SIG_SETMASK, &mask, NULL) == -1)
               errExit("sigprocmask");

           /* 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)
               errExit("timer_create");

           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)
                errExit("timer_settime");

           /* Sleep for a while; meanwhile, the timer may expire
              multiple times */

           printf("Sleeping for %d seconds\n", atoi(argv[1]));
           sleep(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)
               errExit("sigprocmask");

           exit(EXIT_SUCCESS);
       }

SEE ALSO

       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)

COLOPHON

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       project, information about reporting bugs, and the latest version of this page, can be
       found at https://www.kernel.org/doc/man-pages/.