getitimer(2) - Linux manual page (original) (raw)
getitimer(2) System Calls Manual getitimer(2)
NAME top
getitimer, setitimer - get or set value of an interval timerLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <sys/time.h>**
**int getitimer(int** _which_**, struct itimerval ***_currvalue_**);**
**int setitimer(int** _which_**, const struct itimerval *restrict** _newvalue_**,**
**struct itimerval *_Nullable restrict** _oldvalue_**);**DESCRIPTION top
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 currvalue.
The _itvalue_ 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
_itinterval_ when the timer expires. If both fields of _itvalue_
are zero, then this timer is currently disarmed (inactive).
The _itinterval_ substructure is populated with the timer interval.
If both fields of _itinterval_ 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 newvalue. If oldvalue 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 _newvalue.itvalue_ is nonzero, then the timer
is armed to initially expire at the specified time. If both
fields in _newvalue.itvalue_ are zero, then the timer is disarmed.
The _newvalue.itinterval_ field specifies the new interval for the
timer; if both of its subfields are zero, the timer is single-
shot.RETURN VALUE top
On success, zero is returned. On error, -1 is returned, and _[errno](../man3/errno.3.html)_
is set to indicate the error.ERRORS top
**EFAULT** _newvalue_, _oldvalue_, or _currvalue_ 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 _tvusec_
fields in the structure pointed to by _newvalue_ contains a
value outside the range [0, 999999].VERSIONS top
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 _newvalue_ fields are zero; that is, the timer is disabled.
_Don't use this Linux misfeature_: it is nonportable and
unnecessary.STANDARDS top
POSIX.1-2008.HISTORY top
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)](../man2/timer%5Fgettime.2.html),
[timer_settime(2)](../man2/timer%5Fsettime.2.html), etc.) instead.NOTES top
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)](../man7/time.7.html). (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)](../man2/fork.2.html) does not inherit its parent's interval
timers. Interval timers are preserved across an [execve(2)](../man2/execve.2.html).
POSIX.1 leaves the interaction between **setitimer**() and the three
interfaces [alarm(2)](../man2/alarm.2.html), [sleep(3)](../man3/sleep.3.html), and [usleep(3)](../man3/usleep.3.html) unspecified.BUGS top
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 _tvusec_ 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 _tvusec_ value results in an **EINVAL** error.SEE ALSO top
[gettimeofday(2)](../man2/gettimeofday.2.html), [sigaction(2)](../man2/sigaction.2.html), [signal(2)](../man2/signal.2.html), [timer_create(2)](../man2/timer%5Fcreate.2.html),
[timerfd_create(2)](../man2/timerfd%5Fcreate.2.html), [time(7)](../man7/time.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 getitimer(2)
Pages that refer to this page:alarm(2), fork(2), sigaction(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)