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GETITIMER(2)                        Linux Programmer's Manual                        GETITIMER(2)

       getitimer, setitimer - get or set value of an interval timer

       #include <sys/time.h>

       int getitimer(int which, struct itimerval *curr_value);
       int setitimer(int which, const struct itimerval *new_value,
                     struct itimerval *old_value);

       The  system  provides each process with three interval timers, each decrementing in a dis‐
       tinct time domain.  When a timer expires, a signal is sent to the process, and  the  timer
       is reset to the specified interval (if nonzero).

       ITIMER_REAL    decrements in real time, and delivers SIGALRM upon expiration.

       ITIMER_VIRTUAL decrements  only when the process is executing, and delivers SIGVTALRM upon

       ITIMER_PROF    decrements both when the process executes and when the system is  executing
                      on  behalf of the process.  Coupled with ITIMER_VIRTUAL, this timer is usu‐
                      ally used to profile the time spent by the application in user  and  kernel
                      space.  SIGPROF is delivered upon expiration.

       Timer values are defined by the following structures:

           struct itimerval {
               struct timeval it_interval; /* Interval for periodic timer */
               struct timeval it_value;    /* Time until next expiration */

           struct timeval {
               time_t      tv_sec;         /* seconds */
               suseconds_t tv_usec;        /* microseconds */

       The  function  getitimer()  fills  the structure pointed to by curr_value with the current
       value (i.e., the amount of time remaining until the next expiration) of the  timer  speci‐
       fied  by which (one of ITIMER_REAL, ITIMER_VIRTUAL, or ITIMER_PROF).  The subfields of the
       field it_value are set to the amount of time remaining on the timer, or zero if the  timer
       is disabled.  The it_interval field is set to the timer interval (period); a value of zero
       returned in (both subfields of) this field indicates that this is a single-shot timer.

       The function setitimer() sets the specified timer to the value in new_value.  If old_value
       is  non-NULL,  the  old  value  of  the  timer  (i.e., the same information as returned by
       getitimer()) is stored there.

       Timers decrement from it_value to zero, generate a signal, and reset  to  it_interval.   A
       timer which is set to zero (it_value is zero or the timer expires and it_interval is zero)

       Both tv_sec and tv_usec are significant in determining the duration of a timer.

       Timers will never expire before the requested time,  but  may  expire  some  (short)  time
       afterward,  which  depends  on  the  system  timer  resolution and on the system load; see
       time(7).  (But see BUGS below.)  Upon expiration, a signal will be generated and the timer
       reset.  If the timer expires while the process is active (always true for ITIMER_VIRTUAL),
       the signal will be delivered immediately when generated.  Otherwise, the delivery will  be
       offset by a small time dependent on the system loading.

       On success, zero is returned.  On error, -1 is returned, and errno is set appropriately.

       EFAULT new_value, old_value, or curr_value is not valid a pointer.

       EINVAL which  is  not  one of ITIMER_REAL, ITIMER_VIRTUAL, or ITIMER_PROF; or (since Linux
              2.6.22) one of the tv_usec fields in the structure pointed to by new_value contains
              a value outside the range 0 to 999999.

       POSIX.1-2001,  SVr4,  4.4BSD  (this  call  first  appeared in 4.2BSD).  POSIX.1-2008 marks
       getitimer() and setitimer() obsolete,  recommending  the  use  of  the  POSIX  timers  API
       (timer_gettime(2), timer_settime(2), etc.) instead.

       A  child  created  via  fork(2)  does  not inherit its parent's interval timers.  Interval
       timers are preserved across an execve(2).

       POSIX.1 leaves the interaction between setitimer()  and  the  three  interfaces  alarm(2),
       sleep(3), and usleep(3) unspecified.

       The standards are silent on the meaning of the call:

           setitimer(which, NULL, &old_value);

       Many systems (Solaris, the BSDs, and perhaps others) treat this as equivalent to:

           getitimer(which, &old_value);

       In  Linux, this is treated as being equivalent to a call in which the new_value fields are
       zero; that is, the timer is disabled.  Don't use this Linux misfeature: it is  nonportable
       and unnecessary.

       The generation and delivery of a signal are distinct, and only one instance of each of the
       signals listed above may  be  pending  for  a  process.   Under  very  heavy  loading,  an
       ITIMER_REAL  timer may expire before the signal from a previous expiration has been deliv‐
       ered.  The second signal in such an event will be lost.

       On Linux kernels before 2.6.16, timer values are represented in jiffies.  If a request  is
       made  set  a  timer  with  a value whose jiffies representation exceeds MAX_SEC_IN_JIFFIES
       (defined in include/linux/jiffies.h), then the timer is silently truncated to this ceiling
       value.   On  Linux/i386  (where,  since Linux 2.6.13, the default jiffy is 0.004 seconds),
       this means that the ceiling value for a timer is approximately 99.42  days.   Since  Linux
       2.6.16, the kernel uses a different internal representation for times, and this ceiling is

       On certain systems (including i386), Linux kernels before version 2.6.12 have a bug  which
       will  produce  premature  timer  expirations  of up to one jiffy under some circumstances.
       This bug is fixed in kernel 2.6.12.

       POSIX.1-2001 says that setitimer() should fail if a tv_usec value  is  specified  that  is
       outside  of  the range 0 to 999999.  However, in kernels up to and including 2.6.21, Linux
       does not give an error, but instead silently adjusts the corresponding seconds  value  for
       the  timer.  From kernel 2.6.22 onward, this nonconformance has been repaired: an improper
       tv_usec value results in an EINVAL error.

       gettimeofday(2), sigaction(2), signal(2), timer_create(2), timerfd_create(2), time(7)

       This page is part of release 3.74 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at http://www.kernel.org/doc/man-pages/.

Linux                                       2014-07-08                               GETITIMER(2)

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