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clocks - various system timers

 

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.
      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.
 

Sections

Based on BSD UNIX
FreeBSD is an advanced operating system for x86 compatible (including Pentium and Athlon), amd64 compatible (including Opteron, Athlon64, and EM64T), UltraSPARC, IA-64, PC-98 and ARM architectures. It is derived from BSD, the version of UNIX developed at the University of California, Berkeley. It is developed and maintained by a large team of individuals. Additional platforms are in various stages of development.