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