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 *curr_value);
12 int setitimer(int which, const struct itimerval *restrict new_value,
13 struct itimerval *restrict old_value);
14
16 These system calls provide access to interval timers, that is, timers
17 that initially expire at some point in the future, and (optionally) at
18 regular intervals after that. When a timer expires, a signal is gener‐
19 ated for the calling process, and the timer is reset to the specified
20 interval (if the interval is nonzero).
21 Three types of timers—specified via the which argument—are provided,
23 each of which counts against a different clock and generates a differ‐
24 ent signal on timer expiration:
25 ITIMER_REAL
27 This timer counts down in real (i.e., wall clock) time. At each
28 expiration, a SIGALRM signal is generated.
29 ITIMER_VIRTUAL
31 This timer counts down against the user-mode CPU time consumed
32 by the process. (The measurement includes CPU time consumed by
33 all threads in the process.) At each expiration, a SIGVTALRM
34 signal is generated.
35 ITIMER_PROF
37 This timer counts down against the total (i.e., both user and
38 system) CPU time consumed by the process. (The measurement in‐
39 cludes CPU time consumed by all threads in the process.) At
40 each expiration, a SIGPROF signal is generated.
41 In conjunction with ITIMER_VIRTUAL, this timer can be used to
43 profile user and system CPU time consumed by the process.
44 A process has only one of each of the three types of timers.
46 Timer values are defined by the following structures:
48 struct itimerval {
50 struct timeval it_interval; /* Interval for periodic timer */
51 struct timeval it_value; /* Time until next expiration */
52 };
53 struct timeval {
55 time_t tv_sec; /* seconds */
56 suseconds_t tv_usec; /* microseconds */
57 };
58 getitimer()
60 The function getitimer() places the current value of the timer speci‐
61 fied by which in the buffer pointed to by curr_value.
62 The it_value substructure is populated with the amount of time remain‐
64 ing until the next expiration of the specified timer. This value
65 changes as the timer counts down, and will be reset to it_interval when
66 the timer expires. If both fields of it_value are zero, then this
67 timer is currently disarmed (inactive).
68 The it_interval substructure is populated with the timer interval. If
70 both fields of it_interval are zero, then this is a single-shot timer
71 (i.e., it expires just once).
72 setitimer()
74 The function setitimer() arms or disarms the timer specified by which,
75 by setting the timer to the value specified by new_value. If old_value
76 is non-NULL, the buffer it points to is used to return the previous
77 value of the timer (i.e., the same information that is returned by
78 getitimer()).
79 If either field in new_value.it_value is nonzero, then the timer is
81 armed to initially expire at the specified time. If both fields in
82 new_value.it_value are zero, then the timer is disarmed.
83 The new_value.it_interval field specifies the new interval for the
85 timer; if both of its subfields are zero, the timer is single-shot.
86
88 On success, zero is returned. On error, -1 is returned, and errno is
89 set to indicate the error.
90
92 EFAULT new_value, old_value, or curr_value is not valid a pointer.
93 EINVAL which is not one of ITIMER_REAL, ITIMER_VIRTUAL, or ITIMER_PROF;
95 or (since Linux 2.6.22) one of the tv_usec fields in the struc‐
96 ture pointed to by new_value contains a value outside the range
97 0 to 999999.
98
100 POSIX.1-2001, SVr4, 4.4BSD (this call first appeared in 4.2BSD).
101 POSIX.1-2008 marks getitimer() and setitimer() obsolete, recommending
102 the use of the POSIX timers API (timer_gettime(2), timer_settime(2),
103 etc.) instead.
104
106 Timers will never expire before the requested time, but may expire some
107 (short) time afterward, which depends on the system timer resolution
108 and on the system load; see time(7). (But see BUGS below.) If the
109 timer expires while the process is active (always true for ITIMER_VIR‐
110 TUAL), the signal will be delivered immediately when generated.
111 A child created via fork(2) does not inherit its parent's interval
113 timers. Interval timers are preserved across an execve(2).
114 POSIX.1 leaves the interaction between setitimer() and the three inter‐
116 faces alarm(2), sleep(3), and usleep(3) unspecified.
117 The standards are silent on the meaning of the call:
119 setitimer(which, NULL, &old_value);
121 Many systems (Solaris, the BSDs, and perhaps others) treat this as
123 equivalent to:
124 getitimer(which, &old_value);
126 In Linux, this is treated as being equivalent to a call in which the
128 new_value fields are zero; that is, the timer is disabled. Don't use
129 this Linux misfeature: it is nonportable and unnecessary.
130
132 The generation and delivery of a signal are distinct, and only one in‐
133 stance of each of the signals listed above may be pending for a
134 process. Under very heavy loading, an ITIMER_REAL timer may expire be‐
135 fore the signal from a previous expiration has been delivered. The
136 second signal in such an event will be lost.
137 On Linux kernels before 2.6.16, timer values are represented in
139 jiffies. If a request is made set a timer with a value whose jiffies
140 representation exceeds MAX_SEC_IN_JIFFIES (defined in in‐
141 clude/linux/jiffies.h), then the timer is silently truncated to this
142 ceiling value. On Linux/i386 (where, since Linux 2.6.13, the default
143 jiffy is 0.004 seconds), this means that the ceiling value for a timer
144 is approximately 99.42 days. Since Linux 2.6.16, the kernel uses a
145 different internal representation for times, and this ceiling is re‐
146 moved.
147 On certain systems (including i386), Linux kernels before version
149 2.6.12 have a bug which will produce premature timer expirations of up
150 to one jiffy under some circumstances. This bug is fixed in kernel
151 2.6.12.
152 POSIX.1-2001 says that setitimer() should fail if a tv_usec value is
154 specified that is outside of the range 0 to 999999. However, in ker‐
155 nels up to and including 2.6.21, Linux does not give an error, but in‐
156 stead silently adjusts the corresponding seconds value for the timer.
157 From kernel 2.6.22 onward, this nonconformance has been repaired: an
158 improper tv_usec value results in an EINVAL error.
159
161 gettimeofday(2), sigaction(2), signal(2), timer_create(2), timerfd_cre‐
162 ate(2), time(7)
163
165 This page is part of release 5.13 of the Linux man-pages project. A
166 description of the project, information about reporting bugs, and the
167 latest version of this page, can be found at
168 https://www.kernel.org/doc/man-pages/.
169Linux 2021-03-22 GETITIMER(2)