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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 *restrict new_value,
struct itimerval *restrict 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 to indicate the
error.
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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