getitimer, setitimer - get or set value of an interval timer
#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);
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 */
};
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).
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
appropriately.
- 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.
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 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.
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.
gettimeofday(2),
sigaction(2),
signal(2),
timer_create(2),
timerfd_create(2),
time(7)