freebsd-dev/share/man/man7/clocks.7

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.Dd January 18, 2008
.Dt CLOCKS 7
.Os
.Sh NAME
.Nm clocks
.Nd various system timers
.Sh SYNOPSIS
.In time.h
.Sh DESCRIPTION
.Dv HZ
is not part of the application interface in
.Bx .
.Pp
There are many different real and virtual (timekeeping) clocks with
different frequencies:
.Bl -bullet
.It
The scheduling clock.
This is a real clock with frequency that happens to be 100.
It is not available to applications.
.It
The statistics clock.
This is a real clock with frequency that happens to be 128.
It is not directly available to applications.
.It
The clock reported by
.Xr clock 3 .
This is a virtual clock with a frequency that happens to be 128.
Its actual frequency is given by the macro
.Dv CLOCKS_PER_SEC .
Note that
.Dv CLOCKS_PER_SEC
may be floating point.
Do not use
.Xr clock 3
in new programs under
.Fx .
It is feeble compared with
.Xr getrusage 2 .
It is provided for
.Tn ANSI
conformance.
It is implemented by calling
.Xr getrusage 2
and throwing away information and resolution.
.It
The clock reported by
.Xr times 3 .
This is a virtual clock with a frequency that happens to be 128.
Its actual frequency is given by the macro
.Dv CLK_TCK
(deprecated; do not use) and by
.Fn sysconf _SC_CLK_TCK
and by
.Xr sysctl 3 .
Note that its frequency may be different from
.Dv CLOCKS_PER_SEC .
Do not use
.Xr times 3
in new programs under
.Fx .
It is feeble compared with
.Xr gettimeofday 2
together with
.Xr getrusage 2 .
It is provided for
.Tn POSIX
conformance.
It is implemented by calling
.Xr gettimeofday 2
and
.Xr getrusage 2
and throwing away information and resolution.
.It
The profiling clock.
This is a real clock with frequency 1024.
It is used mainly by
.Xr moncontrol 3 ,
.Xr kgmon 8
and
.Xr gprof 1 .
Applications should determine its actual frequency using
.Xr sysctl 3
or by reading it from the header in the profiling data file.
.It
The mc146818a clock.
This is a real clock with a nominal frequency of 32768.
It is divided down to give the statistic clock and the profiling clock.
It is not available to applications.
.It
The microseconds clock.
This is a virtual clock with frequency 1000000.
It is used for most timekeeping in
.Bx
and is exported to applications in
.Xr getrusage 2 ,
.Xr gettimeofday 2 ,
.Xr select 2 ,
.Xr getitimer 2 ,
etc.
This is the clock that should normally be used by
.Bx
applications.
.It
The i8254 clock.
This is a real clock/timer with a nominal frequency of 1193182.
It has three independent time counters to be used.
It is divided down to give the scheduling clock.
It is not available to applications.
.It
The TSC clock (64-bit register) on fifth-generation or later x86 systems.
This is a real clock with a frequency that is equivalent to the number of
cycles per second of the CPU(s).
Its frequency can be found using the
.Va machdep.tsc_freq
sysctl, if it is available.
It is used to interpolate between values of the scheduling clock.
It can be accessed using the
.Dv PMIOTSTAMP
request of
.Xr perfmon 4 .
.It
The ACPI clock.
This is a real clock/timer with a nominal frequency of 3579545.
It is accessed via a 24 or 32 bit register.
Unlike the TSC clock, it maintains a constant tick rate even when the CPU
sleeps or its clock rate changes.
It is not available to applications.
.El
.Pp
Summary: if
.Dv HZ
is not 1000000 then the application is probably using the wrong clock.
.Sh SEE ALSO
.Xr gprof 1 ,
.Xr clock_gettime 2 ,
.Xr getitimer 2 ,
.Xr getrusage 2 ,
.Xr gettimeofday 2 ,
.Xr select 2 ,
.Xr clock 3 ,
.Xr moncontrol 3 ,
.Xr times 3
.Sh AUTHORS
.An -nosplit
This manual page was written by
.An J\(:org Wunsch
after a description posted by
.An Bruce Evans .