xref: /freebsd/lib/libsys/clock_gettime.2 (revision a91a246563dffa876a52f53a98de4af9fa364c52)
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30.Dd October 27, 2023
31.Dt CLOCK_GETTIME 2
32.Os
33.Sh NAME
34.Nm clock_gettime ,
35.Nm clock_settime ,
36.Nm clock_getres
37.Nd get/set/calibrate date and time
38.Sh LIBRARY
39.Lb libc
40.Sh SYNOPSIS
41.In time.h
42.Ft int
43.Fn clock_gettime "clockid_t clock_id" "struct timespec *tp"
44.Ft int
45.Fn clock_settime "clockid_t clock_id" "const struct timespec *tp"
46.Ft int
47.Fn clock_getres "clockid_t clock_id" "struct timespec *tp"
48.Sh DESCRIPTION
49The
50.Fn clock_gettime
51and
52.Fn clock_settime
53system calls allow the calling process to retrieve or set the value
54used by a clock which is specified by
55.Fa clock_id .
56.Pp
57The
58.Fa clock_id
59argument can be a value obtained from
60.Xr clock_getcpuclockid 3
61or
62.Xr pthread_getcpuclockid 3
63as well as the following values:
64.Pp
65.Bl -tag -width indent -compact
66.It Dv CLOCK_REALTIME
67.It Dv CLOCK_REALTIME_PRECISE
68.It Dv CLOCK_REALTIME_FAST
69.It Dv CLOCK_REALTIME_COARSE
70Increments as a wall clock should.
71.It Dv CLOCK_MONOTONIC
72.It Dv CLOCK_MONOTONIC_PRECISE
73.It Dv CLOCK_MONOTONIC_FAST
74.It Dv CLOCK_MONOTONIC_COARSE
75Increments in SI seconds.
76.It Dv CLOCK_UPTIME
77.It Dv CLOCK_UPTIME_PRECISE
78.It Dv CLOCK_UPTIME_FAST
79.It Dv CLOCK_BOOTTIME
80Starts at zero when the kernel boots and increments
81monotonically in SI seconds while the machine is running.
82.It Dv CLOCK_VIRTUAL
83Increments only when
84the CPU is running in user mode on behalf of the calling process.
85.It Dv CLOCK_PROF
86Increments when the CPU is running in user or kernel mode.
87.It Dv CLOCK_SECOND
88Returns the current second without performing a full time counter
89query, using an in-kernel cached value of the current second.
90.It Dv CLOCK_PROCESS_CPUTIME_ID
91Returns the execution time of the calling process.
92.It Dv CLOCK_THREAD_CPUTIME_ID
93Returns the execution time of the calling thread.
94.El
95.Pp
96The clock IDs
97.Fa CLOCK_REALTIME ,
98.Fa CLOCK_MONOTONIC ,
99and
100.Fa CLOCK_UPTIME
101perform a full time counter query.
102The clock IDs with the _FAST suffix, i.e.,
103.Fa CLOCK_REALTIME_FAST ,
104.Fa CLOCK_MONOTONIC_FAST ,
105and
106.Fa CLOCK_UPTIME_FAST ,
107do not perform
108a full time counter query, so their accuracy is one timer tick.
109Similarly,
110.Fa CLOCK_REALTIME_PRECISE ,
111.Fa CLOCK_MONOTONIC_PRECISE ,
112and
113.Fa CLOCK_UPTIME_PRECISE
114are used to get the most exact value as possible, at the expense of
115execution time.
116The clock IDs
117.Fa CLOCK_REALTIME_COARSE
118and
119.Fa CLOCK_MONOTONIC_COARSE
120are aliases of corresponding IDs with _FAST suffix for compatibility with other
121systems.
122Finally,
123.Fa CLOCK_BOOTTIME
124is an alias for
125.Fa CLOCK_UPTIME
126for compatibility with other systems and is unrelated to the
127.Fa kern.boottime
128.Xr sysctl 8 .
129.Pp
130The structure pointed to by
131.Fa tp
132is defined in
133.In sys/timespec.h
134as:
135.Bd -literal
136struct timespec {
137	time_t	tv_sec;		/* seconds */
138	long	tv_nsec;	/* and nanoseconds */
139};
140.Ed
141.Pp
142Only the super-user may set the time of day, using only
143.Fa CLOCK_REALTIME .
144If the system
145.Xr securelevel 7
146is greater than 1 (see
147.Xr init 8 ) ,
148the time may only be advanced.
149This limitation is imposed to prevent a malicious super-user
150from setting arbitrary time stamps on files.
151The system time can still be adjusted backwards using the
152.Xr adjtime 2
153system call even when the system is secure.
154.Pp
155The resolution (granularity) of a clock is returned by the
156.Fn clock_getres
157system call.
158This value is placed in a (non-NULL)
159.Fa *tp .
160.Sh RETURN VALUES
161.Rv -std
162.Sh ERRORS
163The following error codes may be set in
164.Va errno :
165.Bl -tag -width Er
166.It Bq Er EINVAL
167The
168.Fa clock_id
169or
170.Fa timespec
171argument
172was not a valid value.
173.It Bq Er EPERM
174A user other than the super-user attempted to set the time.
175.El
176.Sh SEE ALSO
177.Xr date 1 ,
178.Xr adjtime 2 ,
179.Xr clock_getcpuclockid 3 ,
180.Xr ctime 3 ,
181.Xr pthread_getcpuclockid 3
182.Sh STANDARDS
183The
184.Fn clock_gettime ,
185.Fn clock_settime ,
186and
187.Fn clock_getres
188system calls conform to
189.St -p1003.1b-93 .
190The clock IDs
191.Fa CLOCK_REALTIME_FAST ,
192.Fa CLOCK_REALTIME_PRECISE ,
193.Fa CLOCK_MONOTONIC_FAST ,
194.Fa CLOCK_MONOTONIC_PRECISE ,
195.Fa CLOCK_UPTIME ,
196.Fa CLOCK_UPTIME_FAST ,
197.Fa CLOCK_UPTIME_PRECISE ,
198.Fa CLOCK_SECOND
199are
200.Fx
201extensions to the POSIX interface.
202.Sh HISTORY
203The
204.Fn clock_gettime ,
205.Fn clock_settime ,
206and
207.Fn clock_getres
208system calls first appeared in
209.Fx 3.0 .
210