1GETITIMER(2) Linux Programmer's Manual GETITIMER(2)
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6 getitimer, setitimer - get or set value of an interval timer
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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 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 This timer counts down in real (i.e., wall clock) time.
27 At each expiration, a SIGALRM signal is generated.
28 ITIMER_VIRTUAL This timer counts down against the user-mode CPU time
30 consumed by the process. (The measurement includes CPU
31 time consumed by all threads in the process.) At each
32 expiration, a SIGVTALRM signal is generated.
33 ITIMER_PROF This timer counts down against the total (i.e., both
35 user and system) CPU time consumed by the process. (The
36 measurement includes CPU time consumed by all threads in
37 the process.) At each expiration, a SIGPROF signal is
38 generated.
39 In conjunction with ITIMER_VIRTUAL, this timer can be
41 used to profile user and system CPU time consumed by the
42 process.
43 A process has only one of each of the three types of timers.
45 Timer values are defined by the following structures:
47 struct itimerval {
49 struct timeval it_interval; /* Interval for periodic timer */
50 struct timeval it_value; /* Time until next expiration */
51 };
52 struct timeval {
54 time_t tv_sec; /* seconds */
55 suseconds_t tv_usec; /* microseconds */
56 };
57 getitimer()
59 The function getitimer() places the current value of the timer speci‐
60 fied by which in the buffer pointed to by curr_value.
61 The it_value substructure is populated with the amount of time remain‐
63 ing until the next expiration of the specified timer. This value
64 changes as the timer counts down, and will be reset to it_interval when
65 the timer expires. If both fields of it_value are zero, then this
66 timer is currently disarmed (inactive).
67 The it_interval substructure is populated with the timer interval. If
69 both fields of it_interval are zero, then this is a single-shot timer
70 (i.e., it expires just once).
71 setitimer()
73 The function setitimer() arms or disarms the timer specified by which,
74 by setting the timer to the value specified by new_value. If old_value
75 is non-NULL, the buffer it points to is used to return the previous
76 value of the timer (i.e., the same information that is returned by
77 getitimer()).
78 If either field in new_value.it_value is nonzero, then the timer is
80 armed to initially expire at the specified time. If both fields in
81 new_value.it_value are zero, then the timer is disarmed.
82 The new_value.it_interval field specifies the new interval for the
84 timer; if both of its subfields are zero, the timer is single-shot.
85
87 On success, zero is returned. On error, -1 is returned, and errno is
88 set appropriately.
89
91 EFAULT new_value, old_value, or curr_value is not valid a pointer.
92 EINVAL which is not one of ITIMER_REAL, ITIMER_VIRTUAL, or ITIMER_PROF;
94 or (since Linux 2.6.22) one of the tv_usec fields in the struc‐
95 ture pointed to by new_value contains a value outside the range
96 0 to 999999.
97
99 POSIX.1-2001, SVr4, 4.4BSD (this call first appeared in 4.2BSD).
100 POSIX.1-2008 marks getitimer() and setitimer() obsolete, recommending
101 the use of the POSIX timers API (timer_gettime(2), timer_settime(2),
102 etc.) instead.
103
105 Timers will never expire before the requested time, but may expire some
106 (short) time afterward, which depends on the system timer resolution
107 and on the system load; see time(7). (But see BUGS below.) If the
108 timer expires while the process is active (always true for ITIMER_VIR‐
109 TUAL), the signal will be delivered immediately when generated.
110 A child created via fork(2) does not inherit its parent's interval
112 timers. Interval timers are preserved across an execve(2).
113 POSIX.1 leaves the interaction between setitimer() and the three inter‐
115 faces alarm(2), sleep(3), and usleep(3) unspecified.
116 The standards are silent on the meaning of the call:
118 setitimer(which, NULL, &old_value);
120 Many systems (Solaris, the BSDs, and perhaps others) treat this as
122 equivalent to:
123 getitimer(which, &old_value);
125 In Linux, this is treated as being equivalent to a call in which the
127 new_value fields are zero; that is, the timer is disabled. Don't use
128 this Linux misfeature: it is nonportable and unnecessary.
129
131 The generation and delivery of a signal are distinct, and only one
132 instance of each of the signals listed above may be pending for a
133 process. Under very heavy loading, an ITIMER_REAL timer may expire
134 before the signal from a previous expiration has been delivered. The
135 second signal in such an event will be lost.
136 On Linux kernels before 2.6.16, timer values are represented in
138 jiffies. If a request is made set a timer with a value whose jiffies
139 representation exceeds MAX_SEC_IN_JIFFIES (defined in
140 include/linux/jiffies.h), then the timer is silently truncated to this
141 ceiling value. On Linux/i386 (where, since Linux 2.6.13, the default
142 jiffy is 0.004 seconds), this means that the ceiling value for a timer
143 is approximately 99.42 days. Since Linux 2.6.16, the kernel uses a
144 different internal representation for times, and this ceiling is
145 removed.
146 On certain systems (including i386), Linux kernels before version
148 2.6.12 have a bug which will produce premature timer expirations of up
149 to one jiffy under some circumstances. This bug is fixed in kernel
150 2.6.12.
151 POSIX.1-2001 says that setitimer() should fail if a tv_usec value is
153 specified that is outside of the range 0 to 999999. However, in ker‐
154 nels up to and including 2.6.21, Linux does not give an error, but
155 instead silently adjusts the corresponding seconds value for the timer.
156 From kernel 2.6.22 onward, this nonconformance has been repaired: an
157 improper tv_usec value results in an EINVAL error.
158
160 gettimeofday(2), sigaction(2), signal(2), timer_create(2), timerfd_cre‐
161 ate(2), time(7)
162
164 This page is part of release 4.16 of the Linux man-pages project. A
165 description of the project, information about reporting bugs, and the
166 latest version of this page, can be found at
167 https://www.kernel.org/doc/man-pages/.
168Linux 2017-09-15 GETITIMER(2)