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


       #include <sys/time.h>

       int getitimer(int which, struct itimerval *value);
       int setitimer(int which, const struct itimerval *value,
                     struct itimerval *ovalue);


       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

       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
                      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; /* next value */
               struct timeval it_value;    /* current value */

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

       The  function  getitimer()  fills the structure indicated by value with
       the  current  setting  for  the  timer  indicated  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 indicated timer to the  value  in  value.
       If ovalue is nonzero, 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.

       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.  (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 value or ovalue are not valid pointers.

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


       POSIX.1-2001, SVr4, 4.4BSD (this call first appeared in 4.2BSD).


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


       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, 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 kernel 2.6.13, the default jiffy is 0.004  seconds),  this  means
       that the ceiling value for a timer is approximately 99.42 days.

       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
       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 onwards, this non-conformance has been repaired:  an
       improper tv_usec value results in an EINVAL error.


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


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