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

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.Dd April 1, 1996
.Os
.Dt CLOCKS 7
.Sh NAME
.Nm clocks
.Nd various system timers
.Sh SYNOPSIS
.Fd #include <time.h>
.Sh DESCRIPTION
.Ql 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 -offset XXX
.It
The scheduling clock.  This is a real clock with frequency that
happens to be 100.  It isn't available to applications.
.It
The statistics clock.  This is a real clock with frequency that
happens to be 128.  It isn't 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.  Don't use
.Fn clock
in new programs under
.Fx .
It is feeble compared with
.Xr getrusage 2 .
It is provided for ANSI conformance.  It is implemented by calling
.Fn getrusage
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; don't 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 .
Don't use
.Xr times 3
in new programs under
.Fx .
It is feeble compared with
.Xr gettimeofday 2
together with
.Fn getrusage .
It is provided for POSIX
conformance.  It is implemented by calling
.Fn gettimeofday
and
.Fn getrusage
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 mc14618a 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 isn't 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 is divided down to give the scheduling clock.  It isn't
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 sysctl
.Sy machdep.tsc_freq .
It is used to interpolate between values of the scheduling clock.
It is only available to applications in a purely machine-dependent manner.
.El
.Pp
Summary: if
.Ql HZ
isn't 1000000 then the application is probably using the wrong clock.
.Sh SEE ALSO
.Xr gprof 1 ,
.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 man page has been written by
.An J\(:org Wunsch
after a description posted by
.An Bruce Evans .