Originální popis anglicky:
getitimer, setitimer - get or set value of an interval timer
Návod, kniha: Linux Programmer's Manual
int
getitimer(int which, struct itimerval
*value);
int
setitimer(int which, const struct itimerval
*value, struct itimerval
*ovalue);
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 */
};
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 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.
On success, zero is returned. On error, -1 is returned, and
errno is set
appropriately.
- EFAULT
- value or ovalue are not valid pointers.
- EINVAL
- which is not one of ITIMER_REAL,
ITIMER_VIRT, or ITIMER_PROF.
SVr4, 4.4BSD (This call first appeared in 4.2BSD).
gettimeofday(2),
sigaction(2),
signal(2)
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.
