setitimer (2) - Linux Manuals
setitimer: get or set value of an interval timer
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
These system calls provide access to interval timers, that is, timers that initially expire at some point in the future, and (optionally) at regular intervals after that. When a timer expires, a signal is generated for the calling process, and the timer is reset to the specified interval (if the interval is nonzero).Three types of timers---specified via the which argument---are provided, each of which counts against a different clock and generates a different signal on timer expiration:
- ITIMER_REAL
- This timer counts down in real (i.e., wall clock) time. At each expiration, a SIGALRM signal is generated.
- ITIMER_VIRTUAL
- This timer counts down against the user-mode CPU time consumed by the process. (The measurement includes CPU time consumed by all threads in the process.) At each expiration, a SIGVTALRM signal is generated.
- ITIMER_PROF
- This timer counts down against the total (i.e., both user and system) CPU time consumed by the process. (The measurement includes CPU time consumed by all threads in the process.) At each expiration, a SIGPROF signal is generated.
- In conjunction with ITIMER_VIRTUAL, this timer can be used to profile user and system CPU time consumed by the process.
A process has only one of each of the three types of timers.
Timer values are defined by the following structures:
struct itimerval {
struct timeval {
The
it_value
substructure is populated with the amount of time remaining until
the next expiration of the specified timer.
This value changes as the timer counts down, and will be reset to
it_interval
when the timer expires.
If both fields of
it_value
are zero, then this timer is currently disarmed (inactive).
The
it_interval
substructure is populated with the timer interval.
If both fields of
it_interval
are zero, then this is a single-shot timer (i.e., it expires just once).
If either field in
new_value.it_value
is nonzero,
then the timer is armed to initially expire at the specified time.
If both fields in
new_value.it_value
are zero, then the timer is disarmed.
The
new_value.it_interval
field specifies the new interval for the timer;
if both of its subfields are zero, the timer is single-shot.
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 standards are silent on the meaning of the call:
Many systems (Solaris, the BSDs, and perhaps others)
treat this as equivalent to:
In Linux, this is treated as being equivalent to a call in which the
new_value
fields are zero; that is, the timer is disabled.
Don't use this Linux misfeature:
it is nonportable and unnecessary.
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.
getitimer()
The function
getitimer()
places the current value of the timer specified by
which
in the buffer pointed to by
curr_value.
setitimer()
The function
setitimer()
arms or disarms the timer specified by
which,
by setting the timer to the value specified by
new_value.
If
old_value
is non-NULL,
the buffer it points to is used to return the previous value of the timer
(i.e., the same information that is returned by
getitimer()).
RETURN VALUE
On success, zero is returned.
On error, -1 is returned, and
errno
is set appropriately.
ERRORS
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
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.)
If the timer expires while the process is active (always true for
ITIMER_VIRTUAL),
the signal will be delivered immediately when generated.
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.
COLOPHON
This page is part of release 5.10 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.