1getitimer(2) System Calls Manual getitimer(2)
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6 getitimer, setitimer - get or set value of an interval timer
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9 Standard C library (libc, -lc)
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12 #include <sys/time.h>
13 int getitimer(int which, struct itimerval *curr_value);
15 int setitimer(int which, const struct itimerval *restrict new_value,
16 struct itimerval *_Nullable restrict old_value);
17
19 These system calls provide access to interval timers, that is, timers
20 that initially expire at some point in the future, and (optionally) at
21 regular intervals after that. When a timer expires, a signal is gener‐
22 ated for the calling process, and the timer is reset to the specified
23 interval (if the interval is nonzero).
24 Three types of timers—specified via the which argument—are provided,
26 each of which counts against a different clock and generates a differ‐
27 ent signal on timer expiration:
28 ITIMER_REAL
30 This timer counts down in real (i.e., wall clock) time. At each
31 expiration, a SIGALRM signal is generated.
32 ITIMER_VIRTUAL
34 This timer counts down against the user-mode CPU time consumed
35 by the process. (The measurement includes CPU time consumed by
36 all threads in the process.) At each expiration, a SIGVTALRM
37 signal is generated.
38 ITIMER_PROF
40 This timer counts down against the total (i.e., both user and
41 system) CPU time consumed by the process. (The measurement in‐
42 cludes CPU time consumed by all threads in the process.) At
43 each expiration, a SIGPROF signal is generated.
44 In conjunction with ITIMER_VIRTUAL, this timer can be used to
46 profile user and system CPU time consumed by the process.
47 A process has only one of each of the three types of timers.
49 Timer values are defined by the following structures:
51 struct itimerval {
53 struct timeval it_interval; /* Interval for periodic timer */
54 struct timeval it_value; /* Time until next expiration */
55 };
56 struct timeval {
58 time_t tv_sec; /* seconds */
59 suseconds_t tv_usec; /* microseconds */
60 };
61 getitimer()
63 The function getitimer() places the current value of the timer speci‐
64 fied by which in the buffer pointed to by curr_value.
65 The it_value substructure is populated with the amount of time remain‐
67 ing until the next expiration of the specified timer. This value
68 changes as the timer counts down, and will be reset to it_interval when
69 the timer expires. If both fields of it_value are zero, then this
70 timer is currently disarmed (inactive).
71 The it_interval substructure is populated with the timer interval. If
73 both fields of it_interval are zero, then this is a single-shot timer
74 (i.e., it expires just once).
75 setitimer()
77 The function setitimer() arms or disarms the timer specified by which,
78 by setting the timer to the value specified by new_value. If old_value
79 is non-NULL, the buffer it points to is used to return the previous
80 value of the timer (i.e., the same information that is returned by
81 getitimer()).
82 If either field in new_value.it_value is nonzero, then the timer is
84 armed to initially expire at the specified time. If both fields in
85 new_value.it_value are zero, then the timer is disarmed.
86 The new_value.it_interval field specifies the new interval for the
88 timer; if both of its subfields are zero, the timer is single-shot.
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91 On success, zero is returned. On error, -1 is returned, and errno is
92 set to indicate the error.
93
95 EFAULT new_value, old_value, or curr_value is not valid a pointer.
96 EINVAL which is not one of ITIMER_REAL, ITIMER_VIRTUAL, or ITIMER_PROF;
98 or (since Linux 2.6.22) one of the tv_usec fields in the struc‐
99 ture pointed to by new_value contains a value outside the range
100 [0, 999999].
101
103 The standards are silent on the meaning of the call:
104 setitimer(which, NULL, &old_value);
106 Many systems (Solaris, the BSDs, and perhaps others) treat this as
108 equivalent to:
109 getitimer(which, &old_value);
111 In Linux, this is treated as being equivalent to a call in which the
113 new_value fields are zero; that is, the timer is disabled. Don't use
114 this Linux misfeature: it is nonportable and unnecessary.
115
117 POSIX.1-2008.
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120 POSIX.1-2001, SVr4, 4.4BSD (this call first appeared in 4.2BSD).
121 POSIX.1-2008 marks getitimer() and setitimer() obsolete, recommending
122 the use of the POSIX timers API (timer_gettime(2), timer_settime(2),
123 etc.) instead.
124
126 Timers will never expire before the requested time, but may expire some
127 (short) time afterward, which depends on the system timer resolution
128 and on the system load; see time(7). (But see BUGS below.) If the
129 timer expires while the process is active (always true for ITIMER_VIR‐
130 TUAL), the signal will be delivered immediately when generated.
131 A child created via fork(2) does not inherit its parent's interval
133 timers. Interval timers are preserved across an execve(2).
134 POSIX.1 leaves the interaction between setitimer() and the three inter‐
136 faces alarm(2), sleep(3), and usleep(3) unspecified.
137
139 The generation and delivery of a signal are distinct, and only one in‐
140 stance of each of the signals listed above may be pending for a
141 process. Under very heavy loading, an ITIMER_REAL timer may expire be‐
142 fore the signal from a previous expiration has been delivered. The
143 second signal in such an event will be lost.
144 Before Linux 2.6.16, timer values are represented in jiffies. If a re‐
146 quest is made set a timer with a value whose jiffies representation ex‐
147 ceeds MAX_SEC_IN_JIFFIES (defined in include/linux/jiffies.h), then the
148 timer is silently truncated to this ceiling value. On Linux/i386
149 (where, since Linux 2.6.13, the default jiffy is 0.004 seconds), this
150 means that the ceiling value for a timer is approximately 99.42 days.
151 Since Linux 2.6.16, the kernel uses a different internal representation
152 for times, and this ceiling is removed.
153 On certain systems (including i386), Linux kernels before Linux 2.6.12
155 have a bug which will produce premature timer expirations of up to one
156 jiffy under some circumstances. This bug is fixed in Linux 2.6.12.
157 POSIX.1-2001 says that setitimer() should fail if a tv_usec value is
159 specified that is outside of the range [0, 999999]. However, up to and
160 including Linux 2.6.21, Linux does not give an error, but instead
161 silently adjusts the corresponding seconds value for the timer. From
162 Linux 2.6.22 onward, this nonconformance has been repaired: an improper
163 tv_usec value results in an EINVAL error.
164
166 gettimeofday(2), sigaction(2), signal(2), timer_create(2), timerfd_cre‐
167 ate(2), time(7)
168Linux man-pages 6.04 2023-03-30 getitimer(2)