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
     different frequencies:

     •   The scheduling clock.  This is a real clock with frequency that
         happens to be 100.  It is not available to applications.

     •   The statistics clock.  This is a real clock with frequency that
         happens to be 128.  It is not 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.  Do not use clock(3) in new programs under FreeBSD.  It is
         feeble compared with getrusage(2).  It is provided for ANSI
         conformance.  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
         frequency 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 implemented 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
         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 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 J"rg Wunsch after a description posted by
     Bruce Evans.

FreeBSD 13.1-RELEASE-p6        January 18, 2008        FreeBSD 13.1-RELEASE-p6