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38 .Nd get/set/calibrate date and time
44 .Fn clock_gettime "clockid_t clock_id" "struct timespec *tp"
46 .Fn clock_settime "clockid_t clock_id" "const struct timespec *tp"
48 .Fn clock_getres "clockid_t clock_id" "struct timespec *tp"
54 allow the calling process to retrieve or set the value used by a clock
59 can be one of the following values:
61 .Dv CLOCK_REALTIME_PRECISE ,
63 .Dv CLOCK_REALTIME_FAST
64 for time that increments as a wall clock should;
66 .Dv CLOCK_MONOTONIC_PRECISE ,
68 .Dv CLOCK_MONOTONIC_FAST
69 which increments in SI seconds;
71 .Dv CLOCK_UPTIME_PRECISE ,
74 which starts at zero when the kernel boots and increments
75 monotonically in SI seconds while the machine is running;
77 for time that increments only when
78 the CPU is running in user mode on behalf of the calling process;
80 for time that increments when the CPU is running in user or
83 which returns the current second without performing a full time counter
84 query, using in-kernel cached value of current second.
87 .Dv CLOCK_REALTIME_FAST ,
88 .Dv CLOCK_MONOTONIC_FAST ,
91 are analogs of corresponding IDs without _FAST suffix but do not perform
92 a full time counter query, so their accuracy is one timer tick.
94 .Dv CLOCK_REALTIME_PRECISE ,
95 .Dv CLOCK_MONOTONIC_PRECISE ,
97 .Dv CLOCK_UPTIME_PRECISE
98 are used to get the most exact value as possible, at the expense of
102 .Dv CLOCK_UPTIME_FAST ,
103 .Dv CLOCK_MONOTONIC_FAST ,
104 .Dv CLOCK_REALTIME_FAST ,
109 and do not incur any system call overhead after a certain amount of calls.
111 The structure pointed to by
118 time_t tv_sec; /* seconds */
119 long tv_nsec; /* and nanoseconds */
123 Only the super-user may set the time of day.
124 If the system securelevel is greater than 1 (see
126 the time may only be advanced.
127 This limitation is imposed to prevent a malicious super-user
128 from setting arbitrary time stamps on files.
129 The system time can still be adjusted backwards using the
131 system call even when the system is secure.
133 The resolution (granularity) of a clock is returned by the
136 This value is placed in a (non-NULL)
141 The following error codes may be set in
147 was not a valid value.
151 argument address referenced invalid memory.
153 A user other than the super-user attempted to set the time.
167 .Dv CLOCK_REALTIME_FAST ,
168 .Dv CLOCK_REALTIME_PRECISE ,
169 .Dv CLOCK_MONOTONIC_FAST ,
170 .Dv CLOCK_MONOTONIC_PRECISE ,
172 .Dv CLOCK_UPTIME_FAST ,
173 .Dv CLOCK_UPTIME_PRECISE ,
178 extensions to the POSIX interface.