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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 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
                      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

       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
       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 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)


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