1CLOCK_GETRES(3P) POSIX Programmer's Manual CLOCK_GETRES(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 clock_getres, clock_gettime, clock_settime — clock and timer functions
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15 #include <time.h>
16 int clock_getres(clockid_t clock_id, struct timespec *res);
18 int clock_gettime(clockid_t clock_id, struct timespec *tp);
19 int clock_settime(clockid_t clock_id, const struct timespec *tp);
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22 The clock_getres() function shall return the resolution of any clock.
23 Clock resolutions are implementation-defined and cannot be set by a
24 process. If the argument res is not NULL, the resolution of the speci‐
25 fied clock shall be stored in the location pointed to by res. If res
26 is NULL, the clock resolution is not returned. If the time argument of
27 clock_settime() is not a multiple of res, then the value is truncated
28 to a multiple of res.
29 The clock_gettime() function shall return the current value tp for the
31 specified clock, clock_id.
32 The clock_settime() function shall set the specified clock, clock_id,
34 to the value specified by tp. Time values that are between two consec‐
35 utive non-negative integer multiples of the resolution of the specified
36 clock shall be truncated down to the smaller multiple of the resolu‐
37 tion.
38 A clock may be system-wide (that is, visible to all processes) or per-
40 process (measuring time that is meaningful only within a process). All
41 implementations shall support a clock_id of CLOCK_REALTIME as defined
42 in <time.h>. This clock represents the clock measuring real time for
43 the system. For this clock, the values returned by clock_gettime() and
44 specified by clock_settime() represent the amount of time (in seconds
45 and nanoseconds) since the Epoch. An implementation may also support
46 additional clocks. The interpretation of time values for these clocks
47 is unspecified.
48 If the value of the CLOCK_REALTIME clock is set via clock_settime(),
50 the new value of the clock shall be used to determine the time of expi‐
51 ration for absolute time services based upon the CLOCK_REALTIME clock.
52 This applies to the time at which armed absolute timers expire. If the
53 absolute time requested at the invocation of such a time service is
54 before the new value of the clock, the time service shall expire imme‐
55 diately as if the clock had reached the requested time normally.
56 Setting the value of the CLOCK_REALTIME clock via clock_settime() shall
58 have no effect on threads that are blocked waiting for a relative time
59 service based upon this clock, including the nanosleep() function; nor
60 on the expiration of relative timers based upon this clock. Conse‐
61 quently, these time services shall expire when the requested relative
62 interval elapses, independently of the new or old value of the clock.
63 If the Monotonic Clock option is supported, all implementations shall
65 support a clock_id of CLOCK_MONOTONIC defined in <time.h>. This clock
66 represents the monotonic clock for the system. For this clock, the
67 value returned by clock_gettime() represents the amount of time (in
68 seconds and nanoseconds) since an unspecified point in the past (for
69 example, system start-up time, or the Epoch). This point does not
70 change after system start-up time. The value of the CLOCK_MONOTONIC
71 clock cannot be set via clock_settime(). This function shall fail if
72 it is invoked with a clock_id argument of CLOCK_MONOTONIC.
73 The effect of setting a clock via clock_settime() on armed per-process
75 timers associated with a clock other than CLOCK_REALTIME is implementa‐
76 tion-defined.
77 If the value of the CLOCK_REALTIME clock is set via clock_settime(),
79 the new value of the clock shall be used to determine the time at which
80 the system shall awaken a thread blocked on an absolute
81 clock_nanosleep() call based upon the CLOCK_REALTIME clock. If the
82 absolute time requested at the invocation of such a time service is
83 before the new value of the clock, the call shall return immediately as
84 if the clock had reached the requested time normally.
85 Setting the value of the CLOCK_REALTIME clock via clock_settime() shall
87 have no effect on any thread that is blocked on a relative
88 clock_nanosleep() call. Consequently, the call shall return when the
89 requested relative interval elapses, independently of the new or old
90 value of the clock.
91 Appropriate privileges to set a particular clock are implementation-
93 defined.
94 If _POSIX_CPUTIME is defined, implementations shall support clock ID
96 values obtained by invoking clock_getcpuclockid(), which represent the
97 CPU-time clock of a given process. Implementations shall also support
98 the special clockid_t value CLOCK_PROCESS_CPUTIME_ID, which represents
99 the CPU-time clock of the calling process when invoking one of the
100 clock_*() or timer_*() functions. For these clock IDs, the values
101 returned by clock_gettime() and specified by clock_settime() represent
102 the amount of execution time of the process associated with the clock.
103 Changing the value of a CPU-time clock via clock_settime() shall have
104 no effect on the behavior of the sporadic server scheduling policy (see
105 Scheduling Policies).
106 If _POSIX_THREAD_CPUTIME is defined, implementations shall support
108 clock ID values obtained by invoking pthread_getcpuclockid(), which
109 represent the CPU-time clock of a given thread. Implementations shall
110 also support the special clockid_t value CLOCK_THREAD_CPUTIME_ID, which
111 represents the CPU-time clock of the calling thread when invoking one
112 of the clock_*() or timer_*() functions. For these clock IDs, the val‐
113 ues returned by clock_gettime() and specified by clock_settime() shall
114 represent the amount of execution time of the thread associated with
115 the clock. Changing the value of a CPU-time clock via clock_settime()
116 shall have no effect on the behavior of the sporadic server scheduling
117 policy (see Scheduling Policies).
118
120 A return value of 0 shall indicate that the call succeeded. A return
121 value of -1 shall indicate that an error occurred, and errno shall be
122 set to indicate the error.
123
125 The clock_getres(), clock_gettime(), and clock_settime() functions
126 shall fail if:
127 EINVAL The clock_id argument does not specify a known clock.
129 The clock_gettime() function shall fail if:
131 EOVERFLOW
133 The number of seconds will not fit in an object of type time_t.
134 The clock_settime() function shall fail if:
136 EINVAL The tp argument to clock_settime() is outside the range for the
138 given clock ID.
139 EINVAL The tp argument specified a nanosecond value less than zero or
141 greater than or equal to 1000 million.
142 EINVAL The value of the clock_id argument is CLOCK_MONOTONIC.
144 The clock_settime() function may fail if:
146 EPERM The requesting process does not have appropriate privileges to
148 set the specified clock.
149 The following sections are informative.
151
153 None.
154
156 Note that the absolute value of the monotonic clock is meaningless
157 (because its origin is arbitrary), and thus there is no need to set it.
158 Furthermore, realtime applications can rely on the fact that the value
159 of this clock is never set and, therefore, that time intervals measured
160 with this clock will not be affected by calls to clock_settime().
161
163 None.
164
166 None.
167
169 Scheduling Policies, clock_getcpuclockid(), clock_nanosleep(), ctime(),
170 mq_receive(), mq_send(), nanosleep(), pthread_mutex_timedlock(),
171 sem_timedwait(), time(), timer_create(), timer_getoverrun()
172 The Base Definitions volume of POSIX.1‐2017, <time.h>
174
176 Portions of this text are reprinted and reproduced in electronic form
177 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
178 table Operating System Interface (POSIX), The Open Group Base Specifi‐
179 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
180 Electrical and Electronics Engineers, Inc and The Open Group. In the
181 event of any discrepancy between this version and the original IEEE and
182 The Open Group Standard, the original IEEE and The Open Group Standard
183 is the referee document. The original Standard can be obtained online
184 at http://www.opengroup.org/unix/online.html .
185 Any typographical or formatting errors that appear in this page are
187 most likely to have been introduced during the conversion of the source
188 files to man page format. To report such errors, see https://www.ker‐
189 nel.org/doc/man-pages/reporting_bugs.html .
190IEEE/The Open Group 2017 CLOCK_GETRES(3P)