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 execut-
ing, 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 expira-
tion, 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 out-
standing 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.