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       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 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

       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; /* 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 specified 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) 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  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  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
       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)


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