NAME
getitimer, setitimer - get or set value of an interval timer
SYNOPSIS
#include <sys/time.h>
int getitimer(int which, struct itimerval *curr_value);
int setitimer(int which, const struct itimerval *new_value,
struct itimerval *old_value);
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 */
};
The function getitimer() fills the structure pointed to by curr_value
with the current setting for the timer specified 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 specified timer to the value in
new_value. If old_value is non-NULL, 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, but may expire some
(short) time afterwards, which depends on the system timer resolution
and on the system load; see time(7). (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 appropriately.
ERRORS
EFAULT new_value, old_value, or curr_value is not valid a pointer.
EINVAL which is not one of ITIMER_REAL, ITIMER_VIRTUAL, or ITIMER_PROF;
or (since Linux 2.6.22) one of the tv_usec fields in the
structure pointed to by new_value contains a value outside the
range 0 to 999999.
CONFORMING TO
POSIX.1-2001, SVr4, 4.4BSD (this call first appeared in 4.2BSD).
POSIX.1-2008 marks getitimer() and setitimer() obsolete, recommending
the use of the POSIX timers API (timer_gettime(2), timer_settime(2),
etc.) instead.
NOTES
A child created via fork(2) does not inherit its parent’s interval
timers. Interval timers are preserved across an execve(2).
POSIX.1 leaves the interaction between setitimer() and the three
interfaces alarm(2), sleep(3), and usleep(3) unspecified.
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 loading, 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 kernels before 2.6.16, 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/i386 (where, since Linux 2.6.13, the default
jiffy is 0.004 seconds), this means that the ceiling value for a timer
is approximately 99.42 days. Since Linux 2.6.16, the kernel uses a
different internal representation for times, and this ceiling is
removed.
On certain systems (including i386), Linux kernels before version
2.6.12 have a bug which will produce premature timer expirations of up
to one jiffy under some circumstances. 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, in
kernels up to and including 2.6.21, Linux does not give an error, but
instead silently adjusts the corresponding seconds value for the timer.
From kernel 2.6.22 onwards, this nonconformance has been repaired: an
improper tv_usec value results in an EINVAL error.
SEE ALSO
gettimeofday(2), sigaction(2), signal(2), timer_create(2),
timerfd_create(2), time(7)
COLOPHON
This page is part of release 3.24 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.