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.
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.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- SEE ALSO
-
- BUGS
-
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Time: 12:24:49 GMT, March 22, 2025