1GETITIMER(2) Linux Programmer's Manual GETITIMER(2)
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6 getitimer, setitimer - get or set value of an interval timer
7
9 #include <sys/time.h>
10 int getitimer(int which, struct itimerval *curr_value);
12 int setitimer(int which, const struct itimerval *new_value,
13 struct itimerval *old_value);
14
16 The system provides each process with three interval timers, each
17 decrementing in a distinct time domain. When any timer expires, a sig‐
18 nal is sent to the process, and the timer (potentially) restarts.
19 ITIMER_REAL decrements in real time, and delivers SIGALRM upon expi‐
21 ration.
22 ITIMER_VIRTUAL decrements only when the process is executing, and
24 delivers SIGVTALRM upon expiration.
25 ITIMER_PROF decrements both when the process executes and when the
27 system is executing on behalf of the process. Coupled
28 with ITIMER_VIRTUAL, this timer is usually used to pro‐
29 file the time spent by the application in user and ker‐
30 nel space. SIGPROF is delivered upon expiration.
31 Timer values are defined by the following structures:
33 struct itimerval {
35 struct timeval it_interval; /* next value */
36 struct timeval it_value; /* current value */
37 };
38 struct timeval {
40 time_t tv_sec; /* seconds */
41 suseconds_t tv_usec; /* microseconds */
42 };
43 The function getitimer() fills the structure pointed to by curr_value
45 with the current setting for the timer specified by which (one of
46 ITIMER_REAL, ITIMER_VIRTUAL, or ITIMER_PROF). The element it_value is
47 set to the amount of time remaining on the timer, or zero if the timer
48 is disabled. Similarly, it_interval is set to the reset value.
49 The function setitimer() sets the specified timer to the value in
51 new_value. If old_value is non-NULL, the old value of the timer is
52 stored there.
53 Timers decrement from it_value to zero, generate a signal, and reset to
55 it_interval. A timer which is set to zero (it_value is zero or the
56 timer expires and it_interval is zero) stops.
57 Both tv_sec and tv_usec are significant in determining the duration of
59 a timer.
60 Timers will never expire before the requested time, but may expire some
62 (short) time afterward, which depends on the system timer resolution
63 and on the system load; see time(7). (But see BUGS below.) Upon expi‐
64 ration, a signal will be generated and the timer reset. If the timer
65 expires while the process is active (always true for ITIMER_VIRTUAL)
66 the signal will be delivered immediately when generated. Otherwise the
67 delivery will be offset by a small time dependent on the system load‐
68 ing.
69
71 On success, zero is returned. On error, -1 is returned, and errno is
72 set appropriately.
73
75 EFAULT new_value, old_value, or curr_value is not valid a pointer.
76 EINVAL which is not one of ITIMER_REAL, ITIMER_VIRTUAL, or ITIMER_PROF;
78 or (since Linux 2.6.22) one of the tv_usec fields in the struc‐
79 ture pointed to by new_value contains a value outside the range
80 0 to 999999.
81
83 POSIX.1-2001, SVr4, 4.4BSD (this call first appeared in 4.2BSD).
84 POSIX.1-2008 marks getitimer() and setitimer() obsolete, recommending
85 the use of the POSIX timers API (timer_gettime(2), timer_settime(2),
86 etc.) instead.
87
89 A child created via fork(2) does not inherit its parent's interval
90 timers. Interval timers are preserved across an execve(2).
91 POSIX.1 leaves the interaction between setitimer() and the three inter‐
93 faces alarm(2), sleep(3), and usleep(3) unspecified.
94 The standards are silent on the meaning of the call:
96 setitimer(which, NULL, &old_value);
98 Many systems (Solaris, the BSDs, and perhaps others) treat this as
100 equivalent to:
101 getitimer(which, &old_value);
103 In Linux, this is treated as being equivalent to a call in which the
105 new_value fields are zero; that is, the timer is disabled. Don't use
106 this Linux misfeature: it is nonportable and unnecessary.
107
109 The generation and delivery of a signal are distinct, and only one
110 instance of each of the signals listed above may be pending for a
111 process. Under very heavy loading, an ITIMER_REAL timer may expire
112 before the signal from a previous expiration has been delivered. The
113 second signal in such an event will be lost.
114 On Linux kernels before 2.6.16, timer values are represented in
116 jiffies. If a request is made set a timer with a value whose jiffies
117 representation exceeds MAX_SEC_IN_JIFFIES (defined in
118 include/linux/jiffies.h), then the timer is silently truncated to this
119 ceiling value. On Linux/i386 (where, since Linux 2.6.13, the default
120 jiffy is 0.004 seconds), this means that the ceiling value for a timer
121 is approximately 99.42 days. Since Linux 2.6.16, the kernel uses a
122 different internal representation for times, and this ceiling is
123 removed.
124 On certain systems (including i386), Linux kernels before version
126 2.6.12 have a bug which will produce premature timer expirations of up
127 to one jiffy under some circumstances. This bug is fixed in kernel
128 2.6.12.
129 POSIX.1-2001 says that setitimer() should fail if a tv_usec value is
131 specified that is outside of the range 0 to 999999. However, in ker‐
132 nels up to and including 2.6.21, Linux does not give an error, but
133 instead silently adjusts the corresponding seconds value for the timer.
134 From kernel 2.6.22 onward, this nonconformance has been repaired: an
135 improper tv_usec value results in an EINVAL error.
136
138 gettimeofday(2), sigaction(2), signal(2), timer_create(2), timerfd_cre‐
139 ate(2), time(7)
140
142 This page is part of release 3.53 of the Linux man-pages project. A
143 description of the project, and information about reporting bugs, can
144 be found at http://www.kernel.org/doc/man-pages/.
145Linux 2012-10-01 GETITIMER(2)