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clock_gettime: Catch up with the CLOCK_BOOTTIME changes

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When CLOCK_BOOTTIME was made an alias for CLOCK_MONOTONIC, the man page
wasn't updated. Update to include the changed semantics (it was to be
better compatible with Linux). Clarify the details of these clocks.
Provide a reference to the UTC standard, and a clarifying remark
about leap second encoding (fun fact: POSIX provides no unique
value for these, nor definitive values to use).

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This commit is contained in:
Warner Losh 2024-06-28 10:55:23 -06:00
parent 172328c477
commit de1e91339b
1 changed files with 30 additions and 12 deletions
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42
lib/libsys/clock_gettime.2
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@ -27,7 +27,7 @@
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.Dd October 27, 2023
.Dd June 28, 2024
.Dt CLOCK_GETTIME 2
.Os
.Sh NAME
@ -67,18 +67,26 @@ as well as the following values:
.It Dv CLOCK_REALTIME_PRECISE
.It Dv CLOCK_REALTIME_FAST
.It Dv CLOCK_REALTIME_COARSE
Increments as a wall clock should.
Increments in SI seconds like a wall clock.
It uses a 1970 epoch and implements the UTC timescale.
The count of physical SI seconds since 1970, adjusted by subtracting the number
of positive leap seconds and adding the number of negative leap seconds.
Behavior during a leap second is not defined by and POSIX standard.
.It Dv CLOCK_MONOTONIC
.It Dv CLOCK_MONOTONIC_PRECISE
.It Dv CLOCK_MONOTONIC_FAST
.It Dv CLOCK_MONOTONIC_COARSE
Increments in SI seconds.
.It Dv CLOCK_BOOTTIME
Increments in SI seconds, even while the system is suspended.
Its epoch is unspecified.
The count is not adjusted by leap seconds.
.Fx implements
.It Dv CLOCK_UPTIME
.It Dv CLOCK_UPTIME_PRECISE
.It Dv CLOCK_UPTIME_FAST
.It Dv CLOCK_BOOTTIME
Starts at zero when the kernel boots and increments
monotonically in SI seconds while the machine is running.
Increments monotonically in SI seconds while the machine is running.
The count is not adjusted by leap seconds.
The epoch is unspecified.
.It Dv CLOCK_VIRTUAL
Increments only when
the CPU is running in user mode on behalf of the calling process.
@ -94,6 +102,7 @@ Returns the execution time of the calling thread.
.El
.Pp
The clock IDs
.Dv CLOCK_BOOTTIME ,
.Dv CLOCK_REALTIME ,
.Dv CLOCK_MONOTONIC ,
and
@ -122,7 +131,7 @@ systems.
Finally,
.Dv CLOCK_BOOTTIME
is an alias for
.Dv CLOCK_UPTIME
.Dv CLOCK_MONOTONIC
for compatibility with other systems and is unrelated to the
.Fa kern.boottime
.Xr sysctl 8 .
@ -186,19 +195,28 @@ The
and
.Fn clock_getres
system calls conform to
.St -p1003.1b-93 .
.St -p1003.1-2008 .
The clock IDs
.Dv CLOCK_REALTIME_FAST ,
.Dv CLOCK_REALTIME_PRECISE ,
.Dv CLOCK_BOOTTIME ,
.Dv CLOCK_MONOTONIC_FAST ,
.Dv CLOCK_MONOTONIC_PRECISE ,
.Dv CLOCK_REALTIME_FAST ,
.Dv CLOCK_REALTIME_PRECISE ,
.Dv CLOCK_SECOND
.Dv CLOCK_UPTIME ,
.Dv CLOCK_UPTIME_FAST ,
.Dv CLOCK_UPTIME_PRECISE ,
.Dv CLOCK_SECOND
and
.Dv CLOCK_UPTIME_PRECISE
are
.Fx
extensions to the POSIX interface.
.Pp
UTC is defined by ITU-R TF.460-6, Standard-frequency and time-signal emissions.
However, the
.Vt time_t
type is a simple count that does not provide a unique encoding for leap seconds,
nor a specification for what values should be used to encode a leap second.
.Pp
.Sh HISTORY
The
.Fn clock_gettime ,