GETITIMER(2)		   Linux Programmers Manual		 GETITIMER(2)



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

SYNOPSIS
       #include 

       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-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.

       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; see time(7).  (But see BUGS below.)  Upon expi
       ration,	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  load
       ing.

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;
	      or  (since  Linux  2.6.22)  one of the tv_usec fields contains a
	      value outside the range 0 to 999999.

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

NOTES
       A child created via fork(2) does  not  inherit  its  parents  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.

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  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
       removed.

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

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

COLOPHON
       This page is part of release 3.05 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.



Linux				  2008-04-24			  GETITIMER(2)