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GETITIMER(2) Linux Programmer's Manual GETITIMER(2)
getitimer, setitimer - get or set value of an interval timer
#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);
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 afterward, 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.
On success, zero is returned. On error, -1 is returned, and errno is set
appropriately.
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.
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.
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.
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 onward,
this nonconformance has been repaired: an improper tv_usec value results in an
EINVAL error.
gettimeofday(2), sigaction(2), signal(2), timer_create(2), timerfd_create(2),
time(7)
This page is part of release 3.32 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/.
Linux 2009-03-15 GETITIMER(2)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface