xref: /freebsd/share/man/man7/clocks.7 (revision ae83180158c4c937f170e31eff311b18c0286a93)
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28.\" $FreeBSD$
29.\" "
30.Dd April 1, 1996
31.Os
32.Dt CLOCKS 7
33.Sh NAME
34.Nm clocks
35.Nd various system timers
36.Sh SYNOPSIS
37.In time.h
38.Sh DESCRIPTION
39.Dv HZ
40is not part of the application interface in
41.Bx .
42.Pp
43There are many different real and virtual (timekeeping) clocks with
44different frequencies:
45.Bl -bullet
46.It
47The scheduling clock.
48This is a real clock with frequency that happens to be 100.
49It is not available to applications.
50.It
51The statistics clock.
52This is a real clock with frequency that happens to be 128.
53It is not directly available to applications.
54.It
55The clock reported by
56.Xr clock 3 .
57This is a virtual clock with a frequency that happens to be 128.
58Its actual frequency is given by the macro
59.Dv CLOCKS_PER_SEC .
60Note that
61.Dv CLOCKS_PER_SEC
62may be floating point.
63Do not use
64.Xr clock 3
65in new programs under
66.Fx .
67It is feeble compared with
68.Xr getrusage 2 .
69It is provided for \*[Ai]
70conformance.
71It is implemented by calling
72.Xr getrusage 2
73and throwing away information and resolution.
74.It
75The clock reported by
76.Xr times 3 .
77This is a virtual clock with a frequency that happens to be 128.
78Its actual frequency is given by the macro
79.Dv CLK_TCK
80(deprecated; do not use) and by
81.Fn sysconf SC_CLK_TCK
82and by
83.Xr sysctl 3 .
84Note that its frequency may be different from
85.Dv CLOCKS_PER_SEC .
86Do not use
87.Xr times 3
88in new programs under
89.Fx .
90It is feeble compared with
91.Xr gettimeofday 2
92together with
93.Xr getrusage 2 .
94It is provided for \*[Px]
95conformance.
96It is implemented by calling
97.Xr gettimeofday 2
98and
99.Xr getrusage 2
100and throwing away information and resolution.
101.It
102The profiling clock.
103This is a real clock with frequency 1024.
104It is used mainly by
105.Xr moncontrol 3 ,
106.Xr kgmon 8
107and
108.Xr gprof 1 .
109Applications should determine its actual frequency using
110.Xr sysctl 3
111or by reading it from the header in the profiling data file.
112.It
113The mc146818a clock.
114This is a real clock with a nominal frequency of 32768.
115It is divided down to give the statistic clock and the profiling clock.
116It is not available to applications.
117.It
118The microseconds clock.
119This is a virtual clock with frequency 1000000.
120It is used for most timekeeping in
121.Bx
122and is exported to applications in
123.Xr getrusage 2 ,
124.Xr gettimeofday 2 ,
125.Xr select 2 ,
126.Xr getitimer 2 ,
127etc.
128This is the clock that should normally be used by
129.Bx
130applications.
131.It
132The i8254 clock.
133This is a real clock/timer with a nominal frequency of 1193182.
134It has three independent time counters to be used.
135It is divided down to give the scheduling clock.
136It is not available to applications.
137.It
138The TSC clock (64-bit register) on fifth-generation or later x86 systems.
139This is a real clock with a frequency that is equivalent to the number of
140cycles per second of the CPU(s).
141Its frequency can be found using the
142.Va machdep.tsc_freq
143sysctl, if it is available.
144It is used to interpolate between values of the scheduling clock.
145It can be accessed using the
146.Dv PMIOTSTAMP
147request of
148.Xr perfmon 4 .
149.El
150.Pp
151Summary: if
152.Dv HZ
153is not 1000000 then the application is probably using the wrong clock.
154.Sh SEE ALSO
155.Xr gprof 1 ,
156.Xr getitimer 2 ,
157.Xr getrusage 2 ,
158.Xr gettimeofday 2 ,
159.Xr select 2 ,
160.Xr clock 3 ,
161.Xr moncontrol 3 ,
162.Xr times 3
163.Sh AUTHORS
164.An -nosplit
165This man page has been written by
166.An J\(:org Wunsch
167after a description posted by
168.An Bruce Evans .
169