#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 sig-
       nal is sent to the process, and the timer (potentially) restarts.

       ITIMER_REAL    decrements in real time, and delivers SIGALRM upon expi-

       ITIMER_VIRTUAL decrements only  when  the  process  is  executing,  and
                      delivers SIGVTALRM 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 pro-
                      file the time spent by the application in user and  ker-
                      nel space.  SIGPROF is delivered upon expiration.

       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 current 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_interval.  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.

       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 struc-
              ture 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 inter-
       faces 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 loading, 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

       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

       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  ker-
       nels  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_cre-
       ate(2), time(7)
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