.\" Copyright 7/93 by Darren Senn <sinster@scintilla.santa-clara.ca.us>
.\" Based on a similar page Copyright 1992 by Rick Faith
.\" May be freely distributed
.\" Modified Tue Oct 22 00:22:35 EDT 1996 by Eric S. Raymond <esr@thyrsus.com>
.\" 2005-04-06 mtk, Matthias Lang <matthias@corelatus.se>
.\" 	Noted MAX_SEC_IN_JIFFIES ceiling
.TH GETITIMER 2 2009-03-15 "Linux" "Linux Programmer's Manual"
.SH NAME
getitimer, setitimer \- get or set value of an interval timer
.SH SYNOPSIS
.nf
.B #include <sys/time.h>
.sp
.BI "int getitimer(int " which ", struct itimerval *" curr_value );
.br
.BI "int setitimer(int " which ", const struct itimerval *" new_value ,
.BI "              struct itimerval *" old_value );
.fi
.SH 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.
.TP 1.5i
.B ITIMER_REAL
decrements in real time, and delivers
.B SIGALRM
upon expiration.
.TP
.B ITIMER_VIRTUAL
decrements only when the process is executing, and delivers
.B SIGVTALRM
upon expiration.
.TP
.B ITIMER_PROF
decrements both when the process executes and when the system is executing
on behalf of the process.
Coupled with
.BR ITIMER_VIRTUAL ,
this timer is usually used to profile the time spent by the
application in user and kernel space.
.B SIGPROF
is delivered upon expiration.
.LP
Timer values are defined by the following structures:
.PD 0
.in +4n
.nf

struct itimerval {
    struct timeval it_interval; /* next value */
    struct timeval it_value;    /* current value */
};

struct timeval {
    long tv_sec;                /* seconds */
    long tv_usec;               /* microseconds */
};
.fi
.in
.PD
.LP
The function
.BR getitimer ()
fills the structure pointed to by
.I curr_value
with the current setting for the timer specified by
.I which
(one of
.BR ITIMER_REAL ,
.BR ITIMER_VIRTUAL ,
or
.BR ITIMER_PROF ).
The element
.I it_value
is set to the amount of time remaining on the timer, or zero if the timer
is disabled.
Similarly,
.I it_interval
is set to the reset value.

The function
.BR setitimer ()
sets the specified timer to the value in
.IR new_value .
If
.I old_value
is non-NULL, the old value of the timer is stored there.
.LP
Timers decrement from
.I it_value
to zero, generate a signal, and reset to
.IR it_interval .
A timer which is set to zero
.RI ( it_value
is zero or the timer expires and
.I it_interval
is zero) stops.
.LP
Both
.I tv_sec
and
.I tv_usec
are significant in determining the duration of a timer.
.LP
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
.BR 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
.BR 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.
.SH "RETURN VALUE"
On success, zero is returned.
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EFAULT
.IR new_value ,
.IR old_value ,
or
.I curr_value
is not valid a pointer.
.TP
.B EINVAL
.I which
is not one of
.BR ITIMER_REAL ,
.BR ITIMER_VIRTUAL ,
or
.BR ITIMER_PROF ;
or (since Linux 2.6.22) one of the
.I tv_usec
fields in the structure pointed to by
.I new_value
contains a value outside the range 0 to 999999.
.SH "CONFORMING TO"
POSIX.1-2001, SVr4, 4.4BSD (this call first appeared in 4.2BSD).
POSIX.1-2008 marks
.BR getitimer ()
and
.BR setitimer ()
obsolete, recommending the use of the POSIX timers API
.RB ( timer_gettime (2),
.BR timer_settime (2),
etc.) instead.
.SH NOTES
A child created via
.BR fork (2)
does not inherit its parent's interval timers.
Interval timers are preserved across an
.BR execve (2).

POSIX.1 leaves the
interaction between
.BR setitimer()
and the three interfaces
.BR alarm (2),
.BR sleep (3),
and
.BR usleep (3)
unspecified.
.SH 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
.B 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
.B MAX_SEC_IN_JIFFIES
(defined in
.IR 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.
.\" 4 Jul 2005: It looks like this bug may remain in 2.4.x.
.\"	http://lkml.org/lkml/2005/7/1/165

POSIX.1-2001 says that
.BR setitimer ()
should fail if a
.I 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
.I tv_usec
value results in an
.B EINVAL
error.
.\" Bugzilla report 25 Apr 2006:
.\" http://bugzilla.kernel.org/show_bug.cgi?id=6443
.\" "setitimer() should reject noncanonical arguments"
.SH "SEE ALSO"
.BR gettimeofday (2),
.BR sigaction (2),
.BR signal (2),
.BR timer_create (2),
.BR timerfd_create (2),
.BR time (7)
.SH COLOPHON
This page is part of release 3.27 of the Linux
.I man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.