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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 sig-
nal is sent to the process, and the timer (potentially) restarts.
ITIMER_REAL decrements in real time, and delivers SIGALRM upon expi-
ration.
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 pro-
file the time spent by the application in user and ker-
nel 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 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_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. (But see BUGS below.) Upon expiration, a sig-
nal 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 value or ovalue are not valid pointers.
EINVAL which is not one of ITIMER_REAL, ITIMER_VIRTUAL, or ITIMER_PROF.
CONFORMING TO
POSIX.1-2001, SVr4, 4.4BSD (this call first appeared in 4.2BSD).
SEE ALSO
gettimeofday(2), sigaction(2), signal(2)
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 pro-
cess. 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, 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 2.6 on x86
(where a jiffy is 0.001 seconds), this means that the ceiling value for
a timer is approximately 24.86 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 circumstances. This bug is fixed in kernel 2.6.12.
POSIX.1 says that setitimer() should fail if a tv_usec value is speci-
fied 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.
Linux 0.99.11 1993-08-05 GETITIMER(2)
Man(1) output converted with
man2html and wrapped by fishsponge
This page was generated on Tue Feb 13 02:17:40 GMT 2007
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