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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | BUGS | SEE ALSO | COLOPHON |
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getitimer(2) System Calls Manual getitimer(2)
getitimer, setitimer - get or set value of an interval timer
Standard C library (libc, -lc)
#include <sys/time.h>
int getitimer(int which, struct itimerval *curr_value);
int setitimer(int which, const struct itimerval *restrict new_value,
struct itimerval *_Nullable restrict old_value);
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.
On success, zero is returned. On error, -1 is returned, and
errno is set to indicate the error.
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, 999999].
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.
POSIX.1-2008.
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.
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 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.
Before Linux 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 Linux
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 Linux 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, 999999]. However,
up to and including Linux 2.6.21, Linux does not give an error,
but instead silently adjusts the corresponding seconds value for
the timer. From Linux 2.6.22 onward, this nonconformance has
been repaired: an improper tv_usec value results in an EINVAL
error.
gettimeofday(2), sigaction(2), signal(2), timer_create(2),
timerfd_create(2), time(7)
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2024-06-26. If you discover any rendering problems in this HTML
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Linux man-pages 6.9.1 2024-05-02 getitimer(2)
Pages that refer to this page: alarm(2), fork(2), syscalls(2), timer_create(2), timerfd_create(2), __pmaf(3), profil(3), ualarm(3), usleep(3), pthreads(7), signal(7), socket(7), time(7)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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