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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 Increments as a wall clock should.
72 .It Dv CLOCK_MONOTONIC
73 .It Dv CLOCK_MONOTONIC_PRECISE
74 .It Dv CLOCK_MONOTONIC_FAST
75 Increments in SI seconds.
77 .It Dv CLOCK_UPTIME_PRECISE
78 .It Dv CLOCK_UPTIME_FAST
79 Starts at zero when the kernel boots and increments
80 monotonically in SI seconds while the machine is running.
83 the CPU is running in user mode on behalf of the calling process.
85 Increments when the CPU is running in user or kernel mode.
87 Returns the current second without performing a full time counter
88 query, using an in-kernel cached value of the current second.
89 .It Dv CLOCK_PROCESS_CPUTIME_ID
90 Returns the execution time of the calling process.
91 .It Dv CLOCK_THREAD_CPUTIME_ID
92 Returns the execution time of the calling thread.
96 .Fa CLOCK_REALTIME_FAST ,
97 .Fa CLOCK_MONOTONIC_FAST ,
99 are analogs of corresponding IDs without _FAST suffix but do not perform
100 a full time counter query, so their accuracy is one timer tick.
102 .Fa CLOCK_REALTIME_PRECISE ,
103 .Fa CLOCK_MONOTONIC_PRECISE ,
104 .Fa CLOCK_UPTIME_PRECISE
105 are used to get the most exact value as possible, at the expense of
108 The structure pointed to by
115 time_t tv_sec; /* seconds */
116 long tv_nsec; /* and nanoseconds */
120 Only the super-user may set the time of day, using only
122 If the system securelevel is greater than 1 (see
124 the time may only be advanced.
125 This limitation is imposed to prevent a malicious super-user
126 from setting arbitrary time stamps on files.
127 The system time can still be adjusted backwards using the
129 system call even when the system is secure.
131 The resolution (granularity) of a clock is returned by the
134 This value is placed in a (non-NULL)
139 The following error codes may be set in
148 was not a valid value.
150 A user other than the super-user attempted to set the time.
155 .Xr clock_getcpuclockid 3 ,
157 .Xr pthread_getcpuclockid 3 ,
165 system calls conform to
168 .Fa CLOCK_REALTIME_FAST ,
169 .Fa CLOCK_REALTIME_PRECISE ,
170 .Fa CLOCK_MONOTONIC_FAST ,
171 .Fa CLOCK_MONOTONIC_PRECISE ,
173 .Fa CLOCK_UPTIME_FAST ,
174 .Fa CLOCK_UPTIME_PRECISE ,
176 are FreeBSD extensions to the POSIX interface.
183 system calls first appeared in