man-pages/man2/getitimer.2

.\" 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 2006-04-27 "Linux 2.6.16" "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 *" value );
.br
.BI "int setitimer(int " which ", const struct itimerval *" value ,
.BI "              struct itimerval *" ovalue );
.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
.RS .5i
.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
.RE
.PD
.LP
The function
.BR getitimer ()
fills the structure indicated by
.I value
with the current setting for the timer indicated 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 indicated timer to the value in
.IR value .
If
.I ovalue
is non-zero, 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.
(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
.I value
or
.I ovalue
are not valid pointers.
.TP
.B EINVAL
.I which
is not one of
.BR ITIMER_REAL ,
.BR ITIMER_VIRTUAL ,
or
.BR ITIMER_PROF .
.SH "CONFORMING TO"
POSIX.1-2001, SVr4, 4.4BSD (this call first appeared in 4.2BSD).
.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).
.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, 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
.IR 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 seconds),
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 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, 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
.I tv_usec
value.
.\" FIXME . Bugzilla report 25 Apr 2006:
.\" http://bugzilla.kernel.org/show_bug.cgi?id=6443
.\" "setitimer() should reject non-canonical arguments"
.\" The feedback on this report points to a kernel patch
.\" which apparently should result in this problem
.\" being fixed after a clean-up period of one year
.\" (i.e., March 2007).
.SH "SEE ALSO"
.BR gettimeofday (2),
.BR sigaction (2),
.BR signal (2),
.BR time (7)