clocks man page on FreeBSD

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CLOCKS(7)	     BSD Miscellaneous Information Manual	     CLOCKS(7)

NAME
     clocks — various system timers

SYNOPSIS
     #include <time.h>

DESCRIPTION
     HZ is not part of the application interface in BSD.

     There are many different real and virtual (timekeeping) clocks with dif‐
     ferent frequencies:

     ·	 The scheduling clock.	This is a real clock with frequency that hap‐
	 pens to be 100.  It is not available to applications.

     ·	 The statistics clock.	This is a real clock with frequency that hap‐
	 pens to be 128.  It is not directly available to applications.

     ·	 The clock reported by clock(3).  This is a virtual clock with a fre‐
	 quency that happens to be 128.	 Its actual frequency is given by the
	 macro CLOCKS_PER_SEC.	Note that CLOCKS_PER_SEC may be floating
	 point.	 Do not use clock(3) in new programs under FreeBSD.  It is
	 feeble compared with getrusage(2).  It is provided for ANSI confor‐
	 mance.	 It is implemented by calling getrusage(2) and throwing away
	 information and resolution.

     ·	 The clock reported by times(3).  This is a virtual clock with a fre‐
	 quency that happens to be 128.	 Its actual frequency is given by the
	 macro CLK_TCK (deprecated; do not use) and by sysconf(_SC_CLK_TCK)
	 and by sysctl(3).  Note that its frequency may be different from
	 CLOCKS_PER_SEC.  Do not use times(3) in new programs under FreeBSD.
	 It is feeble compared with gettimeofday(2) together with
	 getrusage(2).	It is provided for POSIX conformance.  It is imple‐
	 mented by calling gettimeofday(2) and getrusage(2) and throwing away
	 information and resolution.

     ·	 The profiling clock.  This is a real clock with frequency 1024.  It
	 is used mainly by moncontrol(3), kgmon(8) and gprof(1).  Applications
	 should determine its actual frequency using sysctl(3) or by reading
	 it from the header in the profiling data file.

     ·	 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.

     ·	 The microseconds clock.  This is a virtual clock with frequency
	 1000000.  It is used for most timekeeping in BSD and is exported to
	 applications in getrusage(2), gettimeofday(2), select(2),
	 getitimer(2), etc.  This is the clock that should normally be used by
	 BSD applications.

     ·	 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.

     ·	 The TSC clock (64-bit register) on fifth-generation or later x86 sys‐
	 tems.	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 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 PMIOTSTAMP request of perfmon(4).

     ·	 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.

     Summary: if HZ is not 1000000 then the application is probably using the
     wrong clock.

SEE ALSO
     gprof(1), clock_gettime(2), getitimer(2), getrusage(2), gettimeofday(2),
     select(2), clock(3), moncontrol(3), times(3)

AUTHORS
     This manual page was written by Jörg Wunsch after a description posted by
     Bruce Evans.

BSD			       January 18, 2008				   BSD
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