FreeBSD Man Pages

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:

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

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

     o	 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.

     o	 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.

     o	 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.

     o	 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.

     o	 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.

     o	 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.

     o	 The TSC clock (64-bit register) on fifth-generation or later x86 sys-

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

FreeBSD 5.4		       February 24, 2005		   FreeBSD 5.4

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