xref: /dragonfly/lib/libc/sys/clock_gettime.2 (revision 7bcb6caf) 
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30.\" $FreeBSD: head/lib/libc/sys/clock_gettime.2 292777 2015-12-27 15:37:07Z dchagin $
31.\"
32.Dd July 26, 2017
33.Dt CLOCK_GETTIME 2
34.Os
35.Sh NAME
36.Nm clock_gettime ,
37.Nm clock_settime ,
38.Nm clock_getres
39.Nd get/set/calibrate date and time
40.Sh LIBRARY
41.Lb libc
42.Sh SYNOPSIS
43.In sys/time.h
44.Ft int
45.Fn clock_gettime "clockid_t clock_id" "struct timespec *tp"
46.Ft int
47.Fn clock_settime "clockid_t clock_id" "const struct timespec *tp"
48.Ft int
49.Fn clock_getres "clockid_t clock_id" "struct timespec *tp"
50.Sh DESCRIPTION
51The
52.Fn clock_gettime
53and
54.Fn clock_settime
55system calls allow the calling process to retrieve or set the value
56used by a clock which is specified by
57.Fa clock_id .
58.Pp
59The
60.Fa clock_id
61argument can be a value obtained from
62.Xr clock_getcpuclockid 3
63or
64.Xr pthread_getcpuclockid 3
65as well as the following values:
66.Pp
67.Bl -tag -width indent -compact
68.It Dv CLOCK_REALTIME
69.It Dv CLOCK_REALTIME_PRECISE
70.It Dv CLOCK_REALTIME_FAST
71Increments as a wall clock should.
72.It Dv CLOCK_MONOTONIC
73.It Dv CLOCK_MONOTONIC_PRECISE
74.It Dv CLOCK_MONOTONIC_FAST
75Increments in SI seconds.
76.It Dv CLOCK_UPTIME
77.It Dv CLOCK_UPTIME_PRECISE
78.It Dv CLOCK_UPTIME_FAST
79Starts at zero when the kernel boots and increments
80monotonically in SI seconds while the machine is running.
81.It Dv CLOCK_VIRTUAL
82Increments only when
83the CPU is running in user mode on behalf of the calling process.
84.It Dv CLOCK_PROF
85Increments when the CPU is running in user or kernel mode.
86.It Dv CLOCK_SECOND
87Returns the current second without performing a full time counter
88query, using an in-kernel cached value of the current second.
89.It Dv CLOCK_PROCESS_CPUTIME_ID
90Returns the execution time of the calling process.
91.It Dv CLOCK_THREAD_CPUTIME_ID
92Returns the execution time of the calling thread.
93.El
94.Pp
95The clock IDs
96.Dv CLOCK_REALTIME_FAST ,
97.Dv CLOCK_MONOTONIC_FAST ,
98and
99.Dv CLOCK_UPTIME_FAST
100are analogs of corresponding IDs without _FAST suffix but do not perform
101a full time counter query, so their accuracy is one timer tick.
102Similarly,
103.Dv CLOCK_REALTIME_PRECISE ,
104.Dv CLOCK_MONOTONIC_PRECISE ,
105and
106.Dv CLOCK_UPTIME_PRECISE
107are used to get the most exact value as possible, at the expense of
108execution time.
109.Pp
110The clock IDs
111.Dv CLOCK_UPTIME_FAST ,
112.Dv CLOCK_MONOTONIC_FAST ,
113.Dv CLOCK_REALTIME_FAST ,
114and
115.Dv CLOCK_SECOND
116make use of
117.Xr kpmap 4
118and do not incur any system call overhead after a certain amount of calls.
119.Pp
120The structure pointed to by
121.Fa tp
122is defined in
123.In sys/_timespec.h
124as:
125.Bd -literal
126struct timespec {
127	time_t	tv_sec;		/* seconds */
128	long	tv_nsec;	/* and nanoseconds */
129};
130.Ed
131.Pp
132Only the super-user may set the time of day, using only
133.Dv CLOCK_REALTIME .
134If the system securelevel is greater than 1 (see
135.Xr init 8 ) ,
136the time may only be advanced.
137This limitation is imposed to prevent a malicious super-user
138from setting arbitrary time stamps on files.
139The system time can still be adjusted backwards using the
140.Xr adjtime 2
141system call even when the system is secure.
142.Pp
143The resolution (granularity) of a clock is returned by the
144.Fn clock_getres
145system call.
146This value is placed in a (non-NULL)
147.Fa *tp .
148.Sh RETURN VALUES
149.Rv -std
150.Sh ERRORS
151The following error codes may be set in
152.Va errno :
153.Bl -tag -width Er
154.It Bq Er EINVAL
155The
156.Fa clock_id
157or
158.Fa tp
159argument was not a valid value.
160.It Bq Er EPERM
161A user other than the super-user attempted to set the time.
162.El
163.Sh SEE ALSO
164.Xr date 1 ,
165.Xr adjtime 2 ,
166.Xr clock_getcpuclockid 3 ,
167.Xr ctime 3 ,
168.Xr pthread_getcpuclockid 3 ,
169.Xr timed 8
170.Sh STANDARDS
171The
172.Fn clock_gettime ,
173.Fn clock_settime ,
174and
175.Fn clock_getres
176system calls conform to
177.St -p1003.1b-93 .
178The clock IDs
179.Dv CLOCK_REALTIME_FAST ,
180.Dv CLOCK_REALTIME_PRECISE ,
181.Dv CLOCK_MONOTONIC_FAST ,
182.Dv CLOCK_MONOTONIC_PRECISE ,
183.Dv CLOCK_UPTIME ,
184.Dv CLOCK_UPTIME_FAST ,
185.Dv CLOCK_UPTIME_PRECISE ,
186and
187.Dv CLOCK_SECOND
188are
189.Fx
190extensions to the POSIX interface.
191