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x SuSE Linux 13.1-RELEASEx
getitimer(2) System Calls Manual getitimer(2)
NAME
getitimer, setitimer - get or set value of an interval timer
LIBRARY
Standard C library (libc, -lc)
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 *_Nullable 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 gener-
ated 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 differ-
ent 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 in-
cludes 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 speci-
fied by which in the buffer pointed to by curr_value.
The it_value substructure is populated with the amount of time remain-
ing 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 struc-
ture pointed to by new_value contains a value outside the range
[0, 999999].
VERSIONS
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.
STANDARDS
POSIX.1-2008.
HISTORY
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_VIR-
TUAL), 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 inter-
faces alarm(2), sleep(3), and usleep(3) unspecified.
BUGS
The generation and delivery of a signal are distinct, and only one in-
stance of each of the signals listed above may be pending for a
process. Under very heavy loading, an ITIMER_REAL timer may expire be-
fore 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 re-
quest is made set a timer with a value whose jiffies representation ex-
ceeds 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.
SEE ALSO
gettimeofday(2), sigaction(2), signal(2), timer_create(2), timerfd_cre-
ate(2), time(7)
Linux man-pages 6.04 2023-03-30 getitimer(2)
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