NAME
getitimer, setitimer - get or set value of an interval timer SYNOPSIS
#include
int getitimer(int which, struct itimerval *currvalue); int setitimer(int which, const struct itimerval *newvalue, struct itimerval *oldvalue); DESCRIPTION The system provides each process with three interval timers, each decrementing in a distinct time domain. When any timer expires, a sig‐ nal is sent to the process, and the timer (potentially) restarts. ITIMERREAL decrements in real time, and delivers SIGALRM upon expi‐ ration. ITIMERVIRTUAL decrements only when the process is executing, and delivers SIGVTALRM upon expiration. ITIMERPROF decrements both when the process executes and when the system is executing on behalf of the process. Coupled with ITIMERVIRTUAL, this timer is usually used to pro‐ file the time spent by the application in user and ker‐ nel space. SIGPROF is delivered upon expiration. Timer values are defined by the following structures: struct itimerval { struct timeval itinterval; /* next value */ struct timeval itvalue; /* current value */ }; struct timeval { timet tvsec; /* seconds */ susecondst tvusec; /* microseconds */ }; The function getitimer() fills the structure pointed to by currvalue with the current setting for the timer specified by which (one of ITIMERREAL, ITIMERVIRTUAL, or ITIMERPROF). The element itvalue is set to the amount of time remaining on the timer, or zero if the timer is disabled. Similarly, itinterval is set to the reset value. The function setitimer() sets the specified timer to the value in newvalue. If oldvalue is non-NULL, the old value of the timer is stored there. Timers decrement from itvalue to zero, generate a signal, and reset to itinterval. A timer which is set to zero (itvalue is zero or the timer expires and itinterval is zero) stops. Both tvsec and tvusec are significant in determining the duration of a timer. 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.) Upon expi‐ ration, a signal will be generated and the timer reset. If the timer expires while the process is active (always true for ITIMERVIRTUAL) the signal will be delivered immediately when generated. Otherwise the delivery will be offset by a small time dependent on the system load‐ ing. RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set appropriately. ERRORS EFAULT newvalue, oldvalue, or currvalue is not valid a pointer. EINVAL which is not one of ITIMERREAL, ITIMERVIRTUAL, or ITIMERPROF; or (since Linux 2.6.22) one of the tvusec fields in the struc‐ ture pointed to by newvalue 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 (timergettime(2), timersettime(2), etc.) instead. NOTES 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. The standards are silent on the meaning of the call: setitimer(which, NULL, &oldvalue); Many systems (Solaris, the BSDs, and perhaps others) treat this as equivalent to: getitimer(which, &oldvalue); 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. 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 ITIMERREAL 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 MAXSECINJIFFIES (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 tvusec value is specified that is outside of the range 0 to 999999. However, in ker‐ nels 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 tvusec value results in an EINVAL error. SEE ALSO gettimeofday(2), sigaction(2), signal(2), timercreate(2), timerfdcre‐ ate(2), time(7) COLOPHON
This page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2012-10-01 GETITIMER(2)