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39 .Nd get/set/calibrate date and time
45 .Fn clock_gettime "clockid_t clock_id" "struct timespec *tp"
47 .Fn clock_settime "clockid_t clock_id" "const struct timespec *tp"
49 .Fn clock_getres "clockid_t clock_id" "struct timespec *tp"
55 system calls allow the calling process to retrieve or set the value
56 used by a clock which is specified by
61 argument can be a value obtained from
62 .Xr clock_getcpuclockid 3
64 .Xr pthread_getcpuclockid 3
65 as well as the following values:
67 .Bl -tag -width indent -compact
69 .It Dv CLOCK_REALTIME_PRECISE
70 .It Dv CLOCK_REALTIME_FAST
71 .It Dv CLOCK_REALTIME_COARSE
72 Increments as a wall clock should.
73 .It Dv CLOCK_MONOTONIC
74 .It Dv CLOCK_MONOTONIC_PRECISE
75 .It Dv CLOCK_MONOTONIC_FAST
76 .It Dv CLOCK_MONOTONIC_COARSE
77 Increments in SI seconds.
79 .It Dv CLOCK_UPTIME_PRECISE
80 .It Dv CLOCK_UPTIME_FAST
82 Starts at zero when the kernel boots and increments
83 monotonically in SI seconds while the machine is running.
86 the CPU is running in user mode on behalf of the calling process.
88 Increments when the CPU is running in user or kernel mode.
90 Returns the current second without performing a full time counter
91 query, using an in-kernel cached value of the current second.
92 .It Dv CLOCK_PROCESS_CPUTIME_ID
93 Returns the execution time of the calling process.
94 .It Dv CLOCK_THREAD_CPUTIME_ID
95 Returns the execution time of the calling thread.
100 .Fa CLOCK_MONOTONIC ,
103 perform a full time counter query.
104 The clock IDs with the _FAST suffix, i.e.,
105 .Fa CLOCK_REALTIME_FAST ,
106 .Fa CLOCK_MONOTONIC_FAST ,
108 .Fa CLOCK_UPTIME_FAST ,
110 a full time counter query, so their accuracy is one timer tick.
112 .Fa CLOCK_REALTIME_PRECISE ,
113 .Fa CLOCK_MONOTONIC_PRECISE ,
115 .Fa CLOCK_UPTIME_PRECISE
116 are used to get the most exact value as possible, at the expense of
119 .Fa CLOCK_REALTIME_COARSE
121 .Fa CLOCK_MONOTONIC_COARSE
122 are aliases of corresponding IDs with _FAST suffix for compatibility with other
128 for compatibility with other systems.
130 The structure pointed to by
137 time_t tv_sec; /* seconds */
138 long tv_nsec; /* and nanoseconds */
142 Only the super-user may set the time of day, using only
146 is greater than 1 (see
148 the time may only be advanced.
149 This limitation is imposed to prevent a malicious super-user
150 from setting arbitrary time stamps on files.
151 The system time can still be adjusted backwards using the
153 system call even when the system is secure.
155 The resolution (granularity) of a clock is returned by the
158 This value is placed in a (non-NULL)
163 The following error codes may be set in
172 was not a valid value.
174 A user other than the super-user attempted to set the time.
179 .Xr clock_getcpuclockid 3 ,
181 .Xr pthread_getcpuclockid 3
188 system calls conform to
191 .Fa CLOCK_REALTIME_FAST ,
192 .Fa CLOCK_REALTIME_PRECISE ,
193 .Fa CLOCK_MONOTONIC_FAST ,
194 .Fa CLOCK_MONOTONIC_PRECISE ,
196 .Fa CLOCK_UPTIME_FAST ,
197 .Fa CLOCK_UPTIME_PRECISE ,
201 extensions to the POSIX interface.
208 system calls first appeared in