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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 *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 SIGV-
TALRM 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 applica-
tion in user and kernel space. SIGPROF is delivered upon expira-
tion.
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 non-zero, the old value of the timer is stored there.
Timers decrement from it_value to zero, generate a signal, and reset to it_inter-
val. 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 afterwards, which depends on the system timer resolution and on the system
load. (But see BUGS below.) Upon expiration, 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 appropri-
ately.
ERRORS
EFAULT value or ovalue are not valid pointers.
EINVAL which is not one of ITIMER_REAL, ITIMER_VIRTUAL, or ITIMER_PROF.
NOTES
A child created via fork(2) does not inherit its parent's interval timers. Inter-
val timers are preserved across an execve(2).
CONFORMING TO
POSIX.1-2001, SVr4, 4.4BSD (this call first appeared in 4.2BSD).
SEE ALSO
gettimeofday(2), sigaction(2), signal(2), time(7)
BUGS
The generation and delivery of a signal are distinct, and only one instance of each
of the signals listed above may be pending for a process. Under very heavy load-
ing, an ITIMER_REAL timer may expire before the signal from a previous expiration
has been delivered. The second signal in such an event will be lost.
On Linux, 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 include/linux/jiffies.h), then the timer is silently truncated to this ceiling
value. On Linux/x86 (where, since kernel 2.6.13, the default jiffy is 0.004 sec-
onds), this means that the ceiling value for a timer is approximately 99.42 days.
On certain systems (including x86), Linux kernels before version 2.6.12 have a bug
which will produce premature timer expirations of up to one jiffy under some cir-
cumstances. 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, Linux does not give an error, but
instead silently adjusts the corresponding seconds value for the timer. In the
future (scheduled for March 2007), this non-conformance will be repaired: existing
applications should be fixed now to ensure that they supply a properly formed
tv_usec value.
Linux 2.6.16 2006-04-27 GETITIMER(2)
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