setitimer(2)
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 it_interval; /* Interval for periodic timer */
struct timeval it_value; /* Time until next expiration */
};
struct timeval {
time_t tv_sec; /* seconds */
suseconds_t tv_usec; /* microseconds */
};
getitimer()
The function getitimer() places the current value of the timer
specified by which in the buffer pointed to by curr_value.
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).
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()).
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.
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is
set appropriately.
ERRORS
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.
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.
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:
setitimer(which, NULL, &old_value);
Many systems (Solaris, the BSDs, and perhaps others) treat this as
equivalent to:
getitimer(which, &old_value);
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.
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.
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.
SEE ALSO
gettimeofday(2), sigaction(2), signal(2), timer_create(2),
timerfd_create(2), time(7)
COLOPHON
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latest version of this page, can be found at
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