1CLOCK_GETRES(3P) POSIX Programmer's Manual CLOCK_GETRES(3P)
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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.
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13 clock_getres, clock_gettime, clock_settime — clock and timer functions
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16 #include <time.h>
17 int clock_getres(clockid_t clock_id, struct timespec *res);
19 int clock_gettime(clockid_t clock_id, struct timespec *tp);
20 int clock_settime(clockid_t clock_id, const struct timespec *tp);
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23 The clock_getres() function shall return the resolution of any clock.
24 Clock resolutions are implementation-defined and cannot be set by a
25 process. If the argument res is not NULL, the resolution of the speci‐
26 fied clock shall be stored in the location pointed to by res. If res
27 is NULL, the clock resolution is not returned. If the time argument of
28 clock_settime() is not a multiple of res, then the value is truncated
29 to a multiple of res.
30 The clock_gettime() function shall return the current value tp for the
32 specified clock, clock_id.
33 The clock_settime() function shall set the specified clock, clock_id,
35 to the value specified by tp. Time values that are between two consec‐
36 utive non-negative integer multiples of the resolution of the specified
37 clock shall be truncated down to the smaller multiple of the resolu‐
38 tion.
39 A clock may be system-wide (that is, visible to all processes) or per-
41 process (measuring time that is meaningful only within a process). All
42 implementations shall support a clock_id of CLOCK_REALTIME as defined
43 in <time.h>. This clock represents the clock measuring real time for
44 the system. For this clock, the values returned by clock_gettime() and
45 specified by clock_settime() represent the amount of time (in seconds
46 and nanoseconds) since the Epoch. An implementation may also support
47 additional clocks. The interpretation of time values for these clocks
48 is unspecified.
49 If the value of the CLOCK_REALTIME clock is set via clock_settime(),
51 the new value of the clock shall be used to determine the time of expi‐
52 ration for absolute time services based upon the CLOCK_REALTIME clock.
53 This applies to the time at which armed absolute timers expire. If the
54 absolute time requested at the invocation of such a time service is
55 before the new value of the clock, the time service shall expire imme‐
56 diately as if the clock had reached the requested time normally.
57 Setting the value of the CLOCK_REALTIME clock via clock_settime() shall
59 have no effect on threads that are blocked waiting for a relative time
60 service based upon this clock, including the nanosleep() function; nor
61 on the expiration of relative timers based upon this clock. Conse‐
62 quently, these time services shall expire when the requested relative
63 interval elapses, independently of the new or old value of the clock.
64 If the Monotonic Clock option is supported, all implementations shall
66 support a clock_id of CLOCK_MONOTONIC defined in <time.h>. This clock
67 represents the monotonic clock for the system. For this clock, the
68 value returned by clock_gettime() represents the amount of time (in
69 seconds and nanoseconds) since an unspecified point in the past (for
70 example, system start-up time, or the Epoch). This point does not
71 change after system start-up time. The value of the CLOCK_MONOTONIC
72 clock cannot be set via clock_settime(). This function shall fail if
73 it is invoked with a clock_id argument of CLOCK_MONOTONIC.
74 The effect of setting a clock via clock_settime() on armed per-process
76 timers associated with a clock other than CLOCK_REALTIME is implementa‐
77 tion-defined.
78 If the value of the CLOCK_REALTIME clock is set via clock_settime(),
80 the new value of the clock shall be used to determine the time at which
81 the system shall awaken a thread blocked on an absolute
82 clock_nanosleep() call based upon the CLOCK_REALTIME clock. If the
83 absolute time requested at the invocation of such a time service is
84 before the new value of the clock, the call shall return immediately as
85 if the clock had reached the requested time normally.
86 Setting the value of the CLOCK_REALTIME clock via clock_settime() shall
88 have no effect on any thread that is blocked on a relative
89 clock_nanosleep() call. Consequently, the call shall return when the
90 requested relative interval elapses, independently of the new or old
91 value of the clock.
92 Appropriate privileges to set a particular clock are implementation-
94 defined.
95 If _POSIX_CPUTIME is defined, implementations shall support clock ID
97 values obtained by invoking clock_getcpuclockid(), which represent the
98 CPU-time clock of a given process. Implementations shall also support
99 the special clockid_t value CLOCK_PROCESS_CPUTIME_ID, which represents
100 the CPU-time clock of the calling process when invoking one of the
101 clock_*() or timer_*() functions. For these clock IDs, the values
102 returned by clock_gettime() and specified by clock_settime() represent
103 the amount of execution time of the process associated with the clock.
104 Changing the value of a CPU-time clock via clock_settime() shall have
105 no effect on the behavior of the sporadic server scheduling policy (see
106 Scheduling Policies).
107 If _POSIX_THREAD_CPUTIME is defined, implementations shall support
109 clock ID values obtained by invoking pthread_getcpuclockid(), which
110 represent the CPU-time clock of a given thread. Implementations shall
111 also support the special clockid_t value CLOCK_THREAD_CPUTIME_ID, which
112 represents the CPU-time clock of the calling thread when invoking one
113 of the clock_*() or timer_*() functions. For these clock IDs, the val‐
114 ues returned by clock_gettime() and specified by clock_settime() shall
115 represent the amount of execution time of the thread associated with
116 the clock. Changing the value of a CPU-time clock via clock_settime()
117 shall have no effect on the behavior of the sporadic server scheduling
118 policy (see Scheduling Policies).
119
121 A return value of 0 shall indicate that the call succeeded. A return
122 value of −1 shall indicate that an error occurred, and errno shall be
123 set to indicate the error.
124
126 The clock_getres(), clock_gettime(), and clock_settime() functions
127 shall fail if:
128 EINVAL The clock_id argument does not specify a known clock.
130 The clock_gettime() function shall fail if:
132 EOVERFLOW
134 The number of seconds will not fit in an object of type time_t.
135 The clock_settime() function shall fail if:
137 EINVAL The tp argument to clock_settime() is outside the range for the
139 given clock ID.
140 EINVAL The tp argument specified a nanosecond value less than zero or
142 greater than or equal to 1000 million.
143 EINVAL The value of the clock_id argument is CLOCK_MONOTONIC.
145 The clock_settime() function may fail if:
147 EPERM The requesting process does not have appropriate privileges to
149 set the specified clock.
150 The following sections are informative.
152
154 None.
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157 Note that the absolute value of the monotonic clock is meaningless
158 (because its origin is arbitrary), and thus there is no need to set it.
159 Furthermore, realtime applications can rely on the fact that the value
160 of this clock is never set and, therefore, that time intervals measured
161 with this clock will not be affected by calls to clock_settime().
162
164 None.
165
167 None.
168
170 Scheduling Policies, clock_getcpuclockid(), clock_nanosleep(), ctime(),
171 mq_receive(), mq_send(), nanosleep(), pthread_mutex_timedlock(),
172 sem_timedwait(), time(), timer_create(), timer_getoverrun()
173 The Base Definitions volume of POSIX.1‐2008, <time.h>
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177 Portions of this text are reprinted and reproduced in electronic form
178 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
179 -- Portable Operating System Interface (POSIX), The Open Group Base
180 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
181 cal and Electronics Engineers, Inc and The Open Group. (This is
182 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
183 event of any discrepancy between this version and the original IEEE and
184 The Open Group Standard, the original IEEE and The Open Group Standard
185 is the referee document. The original Standard can be obtained online
186 at http://www.unix.org/online.html .
187 Any typographical or formatting errors that appear in this page are
189 most likely to have been introduced during the conversion of the source
190 files to man page format. To report such errors, see https://www.ker‐
191 nel.org/doc/man-pages/reporting_bugs.html .
192IEEE/The Open Group 2013 CLOCK_GETRES(3P)