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
13 (REALTIME)
14
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);
21
24 The clock_getres() function shall return the resolution of any clock.
25 Clock resolutions are implementation-defined and cannot be set by a
26 process. If the argument res is not NULL, the resolution of the speci‐
27 fied clock shall be stored in the location pointed to by res. If res is
28 NULL, the clock resolution is not returned. If the time argument of
29 clock_settime() is not a multiple of res, then the value is truncated
30 to a multiple of res.
31 The clock_gettime() function shall return the current value tp for the
33 specified clock, clock_id.
34 The clock_settime() function shall set the specified clock, clock_id,
36 to the value specified by tp. Time values that are between two consecu‐
37 tive non-negative integer multiples of the resolution of the specified
38 clock shall be truncated down to the smaller multiple of the resolu‐
39 tion.
40 A clock may be system-wide (that is, visible to all processes) or per-
42 process (measuring time that is meaningful only within a process). All
43 implementations shall support a clock_id of CLOCK_REALTIME as defined
44 in <time.h>. This clock represents the realtime clock for the system.
45 For this clock, the values returned by clock_gettime() and specified by
46 clock_settime() represent the amount of time (in seconds and nanosec‐
47 onds) since the Epoch. An implementation may also support additional
48 clocks. The interpretation of time values for these clocks is unspeci‐
49 fied.
50 If the value of the CLOCK_REALTIME clock is set via clock_settime(),
52 the new value of the clock shall be used to determine the time of expi‐
53 ration for absolute time services based upon the CLOCK_REALTIME clock.
54 This applies to the time at which armed absolute timers expire. If the
55 absolute time requested at the invocation of such a time service is
56 before the new value of the clock, the time service shall expire imme‐
57 diately as if the clock had reached the requested time normally.
58 Setting the value of the CLOCK_REALTIME clock via clock_settime() shall
60 have no effect on threads that are blocked waiting for a relative time
61 service based upon this clock, including the nanosleep() function; nor
62 on the expiration of relative timers based upon this clock. Conse‐
63 quently, these time services shall expire when the requested relative
64 interval elapses, independently of the new or old value of the clock.
65 If the Monotonic Clock option is supported, all implementations shall
67 support a clock_id of CLOCK_MONOTONIC defined in <time.h>. This clock
68 represents the monotonic clock for the system. For this clock, the
69 value returned by clock_gettime() represents the amount of time (in
70 seconds and nanoseconds) since an unspecified point in the past (for
71 example, system start-up time, or the Epoch). This point does not
72 change after system start-up time. The value of the CLOCK_MONOTONIC
73 clock cannot be set via clock_settime(). This function shall fail if
74 it is invoked with a clock_id argument of CLOCK_MONOTONIC.
75 The effect of setting a clock via clock_settime() on armed per-process
77 timers associated with a clock other than CLOCK_REALTIME is implementa‐
78 tion-defined.
79 If the value of the CLOCK_REALTIME clock is set via clock_settime(),
81 the new value of the clock shall be used to determine the time at which
82 the system shall awaken a thread blocked on an absolute
83 clock_nanosleep() call based upon the CLOCK_REALTIME clock. If the
84 absolute time requested at the invocation of such a time service is
85 before the new value of the clock, the call shall return immediately as
86 if the clock had reached the requested time normally.
87 Setting the value of the CLOCK_REALTIME clock via clock_settime() shall
89 have no effect on any thread that is blocked on a relative
90 clock_nanosleep() call. Consequently, the call shall return when the
91 requested relative interval elapses, independently of the new or old
92 value of the clock.
93 The appropriate privilege to set a particular clock is implementation-
95 defined.
96 If _POSIX_CPUTIME is defined, implementations shall support clock ID
98 values obtained by invoking clock_getcpuclockid(), which represent the
99 CPU-time clock of a given process. Implementations shall also support
100 the special clockid_t value CLOCK_PROCESS_CPUTIME_ID, which represents
101 the CPU-time clock of the calling process when invoking one of the
102 clock_*() or timer_*() functions. For these clock IDs, the values
103 returned by clock_gettime() and specified by clock_settime() represent
104 the amount of execution time of the process associated with the clock.
105 Changing the value of a CPU-time clock via clock_settime() shall have
106 no effect on the behavior of the sporadic server scheduling policy (see
107 Scheduling Policies ).
108 If _POSIX_THREAD_CPUTIME is defined, implementations shall support
110 clock ID values obtained by invoking pthread_getcpuclockid(), which
111 represent the CPU-time clock of a given thread. Implementations shall
112 also support the special clockid_t value CLOCK_THREAD_CPUTIME_ID, which
113 represents the CPU-time clock of the calling thread when invoking one
114 of the clock_*() or timer_*() functions. For these clock IDs, the val‐
115 ues returned by clock_gettime() and specified by clock_settime() shall
116 represent the amount of execution time of the thread associated with
117 the clock. Changing the value of a CPU-time clock via clock_settime()
118 shall have no effect on the behavior of the sporadic server scheduling
119 policy (see Scheduling Policies ).
120
122 A return value of 0 shall indicate that the call succeeded. A return
123 value of -1 shall indicate that an error occurred, and errno shall be
124 set to indicate the error.
125
127 The clock_getres(), clock_gettime(), and clock_settime() functions
128 shall fail if:
129 EINVAL The clock_id argument does not specify a known clock.
131 The clock_settime() function shall fail if:
134 EINVAL The tp argument to clock_settime() is outside the range for the
136 given clock ID.
137 EINVAL The tp argument specified a nanosecond value less than zero or
139 greater than or equal to 1000 million.
140 EINVAL The value of the clock_id argument is CLOCK_MONOTONIC.
142 The clock_settime() function may fail if:
145 EPERM The requesting process does not have the appropriate privilege
147 to set the specified clock.
148 The following sections are informative.
151
153 None.
154
156 These functions are part of the Timers option and need not be available
157 on all implementations.
158 Note that the absolute value of the monotonic clock is meaningless
160 (because its origin is arbitrary), and thus there is no need to set it.
161 Furthermore, realtime applications can rely on the fact that the value
162 of this clock is never set and, therefore, that time intervals measured
163 with this clock will not be affected by calls to clock_settime().
164
166 None.
167
169 None.
170
172 clock_getcpuclockid(), clock_nanosleep(), ctime(), mq_timedreceive(),
173 mq_timedsend(), nanosleep(), pthread_mutex_timedlock(), sem_timed‐
174 wait(), time(), timer_create(), timer_getoverrun(), the Base Defini‐
175 tions volume of IEEE Std 1003.1-2001, <time.h>
176
178 Portions of this text are reprinted and reproduced in electronic form
179 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
180 -- Portable Operating System Interface (POSIX), The Open Group Base
181 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
182 Electrical and Electronics Engineers, Inc and The Open Group. 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.opengroup.org/unix/online.html .
187IEEE/The Open Group 2003 CLOCK_GETRES(3P)