.\" Copyright 7/93 by Darren Senn .\" Based on a similar page Copyright 1992 by Rick Faith .\" May be freely distributed .\" Modified Tue Oct 22 00:22:35 EDT 1996 by Eric S. Raymond .\" 2005-04-06 mtk, Matthias Lang .\" Noted MAX_SEC_IN_JIFFIES ceiling .TH GETITIMER 2 2012-10-01 "Linux" "Linux Programmer's Manual" .SH NAME getitimer, setitimer \- get or set value of an interval timer .SH SYNOPSIS .nf .B #include .sp .BI "int getitimer(int " which ", struct itimerval *" curr_value ); .br .BI "int setitimer(int " which ", const struct itimerval *" new_value , .BI " struct itimerval *" old_value ); .fi .SH DESCRIPTION The system provides each process with three interval timers, each decrementing in a distinct time domain. When any timer expires, a signal is sent to the process, and the timer (potentially) restarts. .TP 1.5i .B ITIMER_REAL decrements in real time, and delivers .B SIGALRM upon expiration. .TP .B ITIMER_VIRTUAL decrements only when the process is executing, and delivers .B SIGVTALRM upon expiration. .TP .B ITIMER_PROF decrements both when the process executes and when the system is executing on behalf of the process. Coupled with .BR ITIMER_VIRTUAL , this timer is usually used to profile the time spent by the application in user and kernel space. .B SIGPROF is delivered upon expiration. .LP Timer values are defined by the following structures: .PD 0 .in +4n .nf struct itimerval { struct timeval it_interval; /* next value */ struct timeval it_value; /* current value */ }; struct timeval { time_t tv_sec; /* seconds */ suseconds_t tv_usec; /* microseconds */ }; .fi .in .PD .LP The function .BR getitimer () fills the structure pointed to by .I curr_value with the current setting for the timer specified by .I which (one of .BR ITIMER_REAL , .BR ITIMER_VIRTUAL , or .BR ITIMER_PROF ). The element .I it_value is set to the amount of time remaining on the timer, or zero if the timer is disabled. Similarly, .I it_interval is set to the reset value. The function .BR setitimer () sets the specified timer to the value in .IR new_value . If .I old_value is non-NULL, the old value of the timer is stored there. .LP Timers decrement from .I it_value to zero, generate a signal, and reset to .IR it_interval . A timer which is set to zero .RI ( it_value is zero or the timer expires and .I it_interval is zero) stops. .LP Both .I tv_sec and .I tv_usec are significant in determining the duration of a timer. .LP 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 .BR time (7). (But see BUGS below.) Upon expiration, a signal will be generated and the timer reset. If the timer expires while the process is active (always true for .BR ITIMER_VIRTUAL ) the signal will be delivered immediately when generated. Otherwise the delivery will be offset by a small time dependent on the system loading. .SH "RETURN VALUE" On success, zero is returned. On error, \-1 is returned, and .I errno is set appropriately. .SH ERRORS .TP .B EFAULT .IR new_value , .IR old_value , or .I curr_value is not valid a pointer. .TP .B EINVAL .I which is not one of .BR ITIMER_REAL , .BR ITIMER_VIRTUAL , or .BR ITIMER_PROF ; or (since Linux 2.6.22) one of the .I tv_usec fields in the structure pointed to by .I new_value contains a value outside the range 0 to 999999. .SH "CONFORMING TO" POSIX.1-2001, SVr4, 4.4BSD (this call first appeared in 4.2BSD). POSIX.1-2008 marks .BR getitimer () and .BR setitimer () obsolete, recommending the use of the POSIX timers API .RB ( timer_gettime (2), .BR timer_settime (2), etc.) instead. .SH NOTES A child created via .BR fork (2) does not inherit its parent's interval timers. Interval timers are preserved across an .BR execve (2). POSIX.1 leaves the interaction between .BR setitimer () and the three interfaces .BR alarm (2), .BR sleep (3), and .BR 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 .I new_value fields are zero; that is, the timer is disabled. .IR "Don't use this Linux misfeature" : it is nonportable and unnecessary. .SH 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 .B 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 .B MAX_SEC_IN_JIFFIES (defined in .IR 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. .\" 4 Jul 2005: It looks like this bug may remain in 2.4.x. .\" http://lkml.org/lkml/2005/7/1/165 POSIX.1-2001 says that .BR setitimer () should fail if a .I 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 .I tv_usec value results in an .B EINVAL error. .\" Bugzilla report 25 Apr 2006: .\" http://bugzilla.kernel.org/show_bug.cgi?id=6443 .\" "setitimer() should reject noncanonical arguments" .SH "SEE ALSO" .BR gettimeofday (2), .BR sigaction (2), .BR signal (2), .BR timer_create (2), .BR timerfd_create (2), .BR time (7) .SH COLOPHON This page is part of release 3.44 of the Linux .I man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.