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CLOCKS(7)	   FreeBSD 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
	    happens to be 100.	It isn't available to applications.

	   The statistics clock.  This is a real clock with frequency that
	    happens to be 128.	It isn't directly available to applications.

	   The clock reported by clock(3).  This is a virtual clock with a
	    frequency 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.  Don't use clock() in new programs under FreeBSD.
	    It is feeble compared with getrusage(2).  It is provided for ANSI
	    conformance.  It is implemented by calling getrusage() and throw-
	    ing away information and resolution.

	   The clock reported by times(3).  This is a virtual clock with a
	    frequency that happens to be 128.  Its actual frequency is given
	    by the macro CLK_TCK (deprecated; don't use) and by
	    sysconf(SC_CLK_TCK) and by sysctl(3).  Note that its frequency may
	    be different from CLOCKS_PER_SEC.  Don't use times(3) in new pro-
	    grams under FreeBSD.  It is feeble compared with gettimeofday(2)
	    together with getrusage().	It is provided for POSIX conformance.
	    It is implemented by calling gettimeofday() and getrusage() 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).	Appli-
	    cations should determine its actual frequency using sysctl(3) or
	    by reading it from the header in the profiling data file.

	   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.

	   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 fre-
	    quency of 1193182.	It is divided down to give the scheduling
	    clock.  It isn't available to applications.

	   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 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.

     Summary: if `HZ' isn't 1000000 then the application is probably using the
     wrong clock.

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

AUTHORS
     This man page has been written by Jorg Wunsch after a description posted
     by Bruce Evans.

FreeBSD 4.9			 April 1, 1996			   FreeBSD 4.9