--- name: "clocks" section: "7" platform: "freebsd" package: "base" version: "16.0-CURRENT-20261027-61c70dc16a79" release: "FreeBSD 16.0-CURRENT (2026-10-27)" platforms: ["freebsd"] url: "https://new.carta.tech/clocks" --- # clocks(7) various system timers ## NAME **clocks** – various system timers ## SYNOPSIS **#include \** ## 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)](/clock/3.md). 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)](/clock/3.md) in new programs under FreeBSD. It is feeble compared with [getrusage(2)](/getrusage/2.md). It is provided for ANSI conformance. It is implemented by calling [getrusage(2)](/getrusage/2.md) and throwing away information and resolution. - The clock reported by [times(3)](/times/3.md). 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)](/sysctl/3.md). Note that its frequency may be different from CLOCKS\_PER\_SEC. Do not use [times(3)](/times/3.md) in new programs under FreeBSD. It is feeble compared with [gettimeofday(2)](/gettimeofday/2.md) together with [getrusage(2)](/getrusage/2.md). It is provided for POSIX conformance. It is implemented by calling [gettimeofday(2)](/gettimeofday/2.md) and [getrusage(2)](/getrusage/2.md) and throwing away information and resolution. - The profiling clock. This is a real clock with frequency 1024. It is used mainly by [moncontrol(3)](/moncontrol/3.md) and [gprof(1)](/gprof/1.md). Applications should determine its actual frequency using [sysctl(3)](/sysctl/3.md) 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)](/getrusage/2.md), [gettimeofday(2)](/gettimeofday/2.md), [select(2)](/select/2.md), [getitimer(2)](/getitimer/2.md), 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. - 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)](/gprof/1.md), [clock\_gettime(2)](/clock_gettime/2.md), [getitimer(2)](/getitimer/2.md), [getrusage(2)](/getrusage/2.md), [gettimeofday(2)](/gettimeofday/2.md), [select(2)](/select/2.md), [clock(3)](/clock/3.md), [moncontrol(3)](/moncontrol/3.md), [times(3)](/times/3.md) ## AUTHORS This manual page was written by Jörg Wunsch after a description posted by Bruce Evans.