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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 *curr_value);
       int setitimer(int which,	const struct itimerval *new_value,
		     struct itimerval *old_value);

DESCRIPTION
       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	inter-
       val (if nonzero).

       ITIMER_REAL    decrements in real time, and delivers SIGALRM upon expi-
		      ration.

       ITIMER_VIRTUAL decrements  only	when the process is executing, and de-
		      livers 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; /* 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 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
       loading.

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

ERRORS
       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	struc-
	      ture pointed to by new_value contains a value outside the	 range
	      0	to 999999.

CONFORMING TO
       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.

NOTES
       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	inter-
       faces 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.

BUGS
       The  generation and delivery of a signal	are distinct, and only one in-
       stance of each of the  signals  listed  above  may  be  pending	for  a
       process.	 Under very heavy loading, an ITIMER_REAL timer	may expire be-
       fore 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   in-
       clude/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  re-
       moved.

       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  in-
       stead  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.

SEE ALSO
       gettimeofday(2),	sigaction(2), signal(2), timer_create(2), timerfd_cre-
       ate(2), time(7)

COLOPHON
       This page is part of release 3.74 of the	Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       http://www.kernel.org/doc/man-pages/.

Linux				  2014-07-08			  GETITIMER(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

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