GETITIMER(2)               Linux Programmer's Manual              GETITIMER(2)
NAME
       getitimer, setitimer - get or set value of an interval timer

SYNOPSIS
       #include <sys/time.h>

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

DESCRIPTION
       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-
                      ration.

       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 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 non-zero, 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 sig-
       nal  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.

RETURN VALUE
       On success, zero is returned.  On error, -1 is returned, and  errno  is
       set appropriately.

ERRORS
       EFAULT value or ovalue are not valid pointers.

       EINVAL which is not one of ITIMER_REAL, ITIMER_VIRTUAL, or ITIMER_PROF.

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

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

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

BUGS
       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  pro-
       cess.   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/x86  (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 x86), 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, Linux
       does not give an error, but instead silently adjusts the  corresponding
       seconds value for the timer.  In the future (scheduled for March 2007),
       this non-conformance will be repaired: existing applications should  be
       fixed now to ensure that they supply a properly formed tv_usec value.

Linux 2.6.16                      2006-04-27                      GETITIMER(2)