View Source: setitimer(2) - Waikato Linux Users Group

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GETITIMER
!!!GETITIMER
----
!!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 signal is sent to the process, and the timer
(potentially) restarts.

;__ITIMER_REAL__: decrements in real time, and delivers __SIGALRM__ upon expiration.
;__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; /* next value */
  struct timeval it_value;    /* current value */
 };
 struct timeval {
  long tv_sec;                /* seconds */
  long tv_usec;               /* microseconds */
 };

getitimer(2) 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. setitimer(2) sets the indicated timer to the value in ''value''. If ''ovalue'' is nonzero, 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, instead expiring some short, constant time afterwards, dependent on the system timer resolution (currently 10ms). Upon expiration, a signal will be generated and the timer reset. If the timer expires while the process is active (always true for __ITIMER_VIRT__) 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_VIRT__, or __ITIMER_PROF__.

!!CONFORMING TO

SVr4, 4.4BSD (This call first appeared in 4.2BSD).

!!SEE ALSO

gettimeofday(2), sigaction(2), signal(2)

!!BUGS

Under Linux, the generation and delivery of a signal are distinct, and there each signal is permitted only one outstanding event. It's therefore conceivable that under pathologically heavy loading, __ITIMER_REAL__  will expire before the signal from a previous expiration has been delivered. The second signal in such an event will be lost.
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Last edited on Monday, October 28, 2002 9:16:30 pm by PerryLorier
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