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 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. It's 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 throwing away information and resolution.

•   The clock reported by times(3). This is a virtual clock with a frequency that happens to be 128. It's 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 programs 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). Applications 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 with a nominal frequency of 1193182. It is divided down to give the scheduling clock. It isn't available to applications.

•   the i586 clock on i586 systems. This is a real clock with a frequency of up to 200000000. It is used to interpolate between values of the scheduling clock. It isn't available to applications.

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 Joerg Wunsch after a description posted by Bruce Evans.