1TIMER_GETOVERRUN(2) Linux Programmer's Manual TIMER_GETOVERRUN(2)
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6 timer_getoverrun - get overrun count for a POSIX per-process timer
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9 #include <time.h>
10 int timer_getoverrun(timer_t timerid);
12 Link with -lrt.
14 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
16 timer_getoverrun():
18 _POSIX_C_SOURCE >= 199309L
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21 timer_getoverrun() returns the "overrun count" for the timer referred
22 to by timerid. An application can use the overrun count to accurately
23 calculate the number of timer expirations that would have occurred over
24 a given time interval. Timer overruns can occur both when receiving
25 expiration notifications via signals (SIGEV_SIGNAL), and via threads
26 (SIGEV_THREAD).
27 When expiration notifications are delivered via a signal, overruns can
29 occur as follows. Regardless of whether or not a real-time signal is
30 used for timer notifications, the system queues at most one signal per
31 timer. (This is the behavior specified by POSIX.1. The alternative,
32 queuing one signal for each timer expiration, could easily result in
33 overflowing the allowed limits for queued signals on the system.) Be‐
34 cause of system scheduling delays, or because the signal may be tempo‐
35 rarily blocked, there can be a delay between the time when the notifi‐
36 cation signal is generated and the time when it is delivered (e.g.,
37 caught by a signal handler) or accepted (e.g., using sigwaitinfo(2)).
38 In this interval, further timer expirations may occur. The timer over‐
39 run count is the number of additional timer expirations that occurred
40 between the time when the signal was generated and when it was deliv‐
41 ered or accepted.
42 Timer overruns can also occur when expiration notifications are deliv‐
44 ered via invocation of a thread, since there may be an arbitrary delay
45 between an expiration of the timer and the invocation of the notifica‐
46 tion thread, and in that delay interval, additional timer expirations
47 may occur.
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50 On success, timer_getoverrun() returns the overrun count of the speci‐
51 fied timer; this count may be 0 if no overruns have occurred. On fail‐
52 ure, -1 is returned, and errno is set to indicate the error.
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55 EINVAL timerid is not a valid timer ID.
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58 This system call is available since Linux 2.6.
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61 POSIX.1-2001, POSIX.1-2008.
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64 When timer notifications are delivered via signals (SIGEV_SIGNAL), on
65 Linux it is also possible to obtain the overrun count via the si_over‐
66 run field of the siginfo_t structure (see sigaction(2)). This allows
67 an application to avoid the overhead of making a system call to obtain
68 the overrun count, but is a nonportable extension to POSIX.1.
69 POSIX.1 discusses timer overruns only in the context of timer notifica‐
71 tions using signals.
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74 POSIX.1 specifies that if the timer overrun count is equal to or
75 greater than an implementation-defined maximum, DELAYTIMER_MAX, then
76 timer_getoverrun() should return DELAYTIMER_MAX. However, before Linux
77 4.19, if the timer overrun value exceeds the maximum representable in‐
78 teger, the counter cycles, starting once more from low values. Since
79 Linux 4.19, timer_getoverrun() returns DELAYTIMER_MAX (defined as
80 INT_MAX in <limits.h>) in this case (and the overrun value is reset to
81 0).
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84 See timer_create(2).
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87 clock_gettime(2), sigaction(2), signalfd(2), sigwaitinfo(2), timer_cre‐
88 ate(2), timer_delete(2), timer_settime(2), signal(7), time(7)
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91 This page is part of release 5.12 of the Linux man-pages project. A
92 description of the project, information about reporting bugs, and the
93 latest version of this page, can be found at
94 https://www.kernel.org/doc/man-pages/.
95Linux 2021-03-22 TIMER_GETOVERRUN(2)