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Document _FAST and _PRECISE clocks.

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Submitted by:	Valentin Nechayev <netch segfault kiev ua>
MFC after:	3 days
This commit is contained in:
Konstantin Belousov 2009-12-29 15:58:10 +00:00
parent c34169d468
commit d439b6c223
1 changed files with 40 additions and 10 deletions
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50
lib/libc/sys/clock_gettime.2
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@ -59,25 +59,44 @@ used by a clock which is specified by
The
.Fa clock_id
argument
can be one of five values:
.Dv CLOCK_REALTIME
can be one of the following values:
.Dv CLOCK_REALTIME ,
.Dv CLOCK_REALTIME_PRECISE ,
.Dv CLOCK_REALTIME_FAST
for time that increments as
a wall clock should,
.Dv CLOCK_MONOTONIC
which increments in SI seconds,
.Dv CLOCK_UPTIME
a wall clock should;
.Dv CLOCK_MONOTONIC ,
.Dv CLOCK_MONOTONIC_PRECISE ,
.Dv CLOCK_MONOTONIC_FAST
which increments in SI seconds;
.Dv CLOCK_UPTIME ,
.Dv CLOCK_UPTIME_PRECISE ,
.Dv CLOCK_UPTIME_FAST
which starts at zero when the kernel boots and increments
monotonically in SI seconds while the machine is running,
monotonically in SI seconds while the machine is running;
.Dv CLOCK_VIRTUAL
for time that increments only when
the CPU is running in user mode on behalf of the calling process,
the CPU is running in user mode on behalf of the calling process;
.Dv CLOCK_PROF
for time that increments when the CPU is running in user or
kernel mode, or
kernel mode; or
.Dv CLOCK_SECOND
which returns the current second without performing a full time counter
query, using in-kernel cached value of current second.
.Pp
The clock IDs
.Fa CLOCK_REALTIME_FAST ,
.Fa CLOCK_MONOTONIC_FAST ,
.Fa CLOCK_UPTIME_FAST
are analogs of corresponding IDs without _FAST suffix but do not perform
a full time counter query, so their accuracy is one timer tick.
Similarly,
.Fa CLOCK_REALTIME_PRECISE ,
.Fa CLOCK_MONOTONIC_PRECISE ,
.Fa CLOCK_UPTIME_PRECISE
are used to get the most exact value as possible, at the expense of
execution time.
.Pp
The structure pointed to by
.Fa tp
is defined in
@ -91,7 +110,8 @@ struct timespec {
};
.Ed
.Pp
Only the super-user may set the time of day.
Only the super-user may set the time of day, using only
.Fa CLOCK_REALTIME .
If the system securelevel is greater than 1 (see
.Xr init 8 ) ,
the time may only be advanced.
@ -137,3 +157,13 @@ and
.Fn clock_getres
system calls conform to
.St -p1003.1b-93 .
The clock IDs
.Fa CLOCK_REALTIME_FAST ,
.Fa CLOCK_REALTIME_PRECISE ,
.Fa CLOCK_MONOTONIC_FAST ,
.Fa CLOCK_MONOTONIC_PRECISE ,
.Fa CLOCK_UPTIME ,
.Fa CLOCK_UPTIME_FAST ,
.Fa CLOCK_UPTIME_PRECISE ,
.Fa CLOCK_SECOND
are FreeBSD extensions to the POSIX interface.