1GETITIMER(2) Linux Programmer's Manual GETITIMER(2)
2
6 getitimer, setitimer - get or set value of an interval timer
7
9 #include <sys/time.h>
10 int getitimer(int which, struct itimerval *value);
12 int setitimer(int which, const struct itimerval *value,
13 struct itimerval *ovalue);
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 {
39 long tv_sec; /* seconds */
40 long tv_usec; /* microseconds */
41 };
42 The function getitimer() fills the structure indicated by value with
44 the current setting for the timer indicated by which (one of
45 ITIMER_REAL, ITIMER_VIRTUAL, or ITIMER_PROF). The element it_value is
46 set to the amount of time remaining on the timer, or zero if the timer
47 is disabled. Similarly, it_interval is set to the reset value. The
48 function setitimer() sets the indicated timer to the value in value.
49 If ovalue is non-zero, the old value of the timer is stored there.
50 Timers decrement from it_value to zero, generate a signal, and reset to
52 it_interval. A timer which is set to zero (it_value is zero or the
53 timer expires and it_interval is zero) stops.
54 Both tv_sec and tv_usec are significant in determining the duration of
56 a timer.
57 Timers will never expire before the requested time, but may expire some
59 (short) time afterwards, which depends on the system timer resolution
60 and on the system load. (But see BUGS below.) Upon expiration, a sig‐
61 nal will be generated and the timer reset. If the timer expires while
62 the process is active (always true for ITIMER_VIRTUAL) the signal will
63 be delivered immediately when generated. Otherwise the delivery will
64 be offset by a small time dependent on the system loading.
65
67 On success, zero is returned. On error, -1 is returned, and errno is
68 set appropriately.
69
71 EFAULT value or ovalue are not valid pointers.
72 EINVAL which is not one of ITIMER_REAL, ITIMER_VIRTUAL, or ITIMER_PROF.
74
76 A child created via fork(2) does not inherit its parent's interval
77 timers. Interval timers are preserved across an execve(2).
78
80 POSIX.1-2001, SVr4, 4.4BSD (this call first appeared in 4.2BSD).
81
83 gettimeofday(2), sigaction(2), signal(2), time(7)
84
86 The generation and delivery of a signal are distinct, and only one
87 instance of each of the signals listed above may be pending for a
88 process. Under very heavy loading, an ITIMER_REAL timer may expire
89 before the signal from a previous expiration has been delivered. The
90 second signal in such an event will be lost.
91 On Linux, timer values are represented in jiffies. If a request is
93 made set a timer with a value whose jiffies representation exceeds
94 MAX_SEC_IN_JIFFIES (defined in include/linux/jiffies.h), then the timer
95 is silently truncated to this ceiling value. On Linux/x86 (where,
96 since kernel 2.6.13, the default jiffy is 0.004 seconds), this means
97 that the ceiling value for a timer is approximately 99.42 days.
98 On certain systems (including x86), Linux kernels before version 2.6.12
100 have a bug which will produce premature timer expirations of up to one
101 jiffy under some circumstances. This bug is fixed in kernel 2.6.12.
102 POSIX.1-2001 says that setitimer() should fail if a tv_usec value is
104 specified that is outside of the range 0 to 999999. However, Linux
105 does not give an error, but instead silently adjusts the corresponding
106 seconds value for the timer. In the future (scheduled for March 2007),
107 this non-conformance will be repaired: existing applications should be
108 fixed now to ensure that they supply a properly formed tv_usec value.
109Linux 2.6.16 2006-04-27 GETITIMER(2)