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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
       distinct time domain.  When any timer expires, a signal is sent to the process, and
       the timer (potentially) restarts.

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

       ITIMER_VIRTUAL decrements  only  when  the process is executing, and delivers SIGV-
                      TALRM upon expiration.

       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 usually used to profile the time spent by the applica-
                      tion  in  user  and kernel space.  SIGPROF is delivered upon expira-

       Timer values are defined by the following structures:

           struct itimerval {
               struct timeval it_interval; /* next value */
               struct timeval it_value;    /* current value */

           struct timeval {
               long tv_sec;                /* seconds */
               long tv_usec;               /* microseconds */

       The function getitimer() fills the structure pointed to by curr_value with the cur-
       rent  setting for the timer specified by which (one of ITIMER_REAL, ITIMER_VIRTUAL,
       or ITIMER_PROF).  The element it_value is set to the amount of  time  remaining  on
       the  timer, or zero if the timer is disabled.  Similarly, it_interval is set to the
       reset value.

       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 is stored there.

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

       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 afterwards, 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 gener-
       ated 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 load-

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

       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.   Inter-
       val 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 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 load-
       ing, an ITIMER_REAL timer may expire before the signal from a  previous  expiration
       has been delivered.  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 removed.

       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  cir-
       cumstances.  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 correspond-
       ing seconds value for the timer.  From kernel 2.6.22 onwards, this  non-conformance
       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),

       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.ker-

Linux                             2009-03-15                      GETITIMER(2)

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