CLOCKS(7) BSD Miscellaneous Information Manual CLOCKS(7)NAMEclocks — 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
DragonFly. It is feeble compared with getrusage(2). It is pro‐
vided for ANSI conformance. It is implemented by calling
getrusage() 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; 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 DragonFly. 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 hw.tsc_frequency and its presence
via hw.tsc_present. It is used to interpolate between values of
the scheduling clock. It is only available to applications in a
purely machine-dependent manner.
· The HPET (High Precision Event Timers). Only main counter is used
currently. This CPU timer is expected to be faster than ACPI-
fast24 and ACPI-safe, so it should be given higher priority. HPET
is not enabled by default. To enable it, you should add
debug.acpi.enabled="hpet" to your /boot/loader.conf. If the HPET
is detected and attached, kern.cputimer.name will report HPET.
Summary: if ‘HZ’ isn't 1000000 then the application is probably using the
wrong clock.
SEE ALSOgprof(1), getitimer(2), getrusage(2), gettimeofday(2), select(2),
clock(3), moncontrol(3), times(3), loader.conf(5)AUTHORS
This man page has been written by Jörg Wunsch after a description posted
by Bruce Evans.
BSD May 2, 2009 BSD
