Copyright (c) 1980, 1991, 1993
The Regents of the University of California. All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
1. Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.
3. Neither the name of the University nor the names of its contributors
may be used to endorse or promote products derived from this software
without specific prior written permission.
THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
SUCH DAMAGE.
.Dd October 27, 2023 .Dt CLOCK_GETTIME 2 .Os .Sh NAME .Nm clock_gettime , .Nm clock_settime , .Nm clock_getres .Nd get/set/calibrate date and time .Sh LIBRARY .Lb libc .Sh SYNOPSIS n time.h .Ft int .Fn clock_gettime "clockid_t clock_id" "struct timespec *tp" .Ft int .Fn clock_settime "clockid_t clock_id" "const struct timespec *tp" .Ft int .Fn clock_getres "clockid_t clock_id" "struct timespec *tp" .Sh DESCRIPTION The .Fn clock_gettime and .Fn clock_settime system calls allow the calling process to retrieve or set the value used by a clock which is specified by .Fa clock_id .
p The .Fa clock_id argument can be a value obtained from .Xr clock_getcpuclockid 3 or .Xr pthread_getcpuclockid 3 as well as the following values:
p l -tag -width indent -compact t Dv CLOCK_REALTIME t Dv CLOCK_REALTIME_PRECISE t Dv CLOCK_REALTIME_FAST t Dv CLOCK_REALTIME_COARSE Increments as a wall clock should. t Dv CLOCK_MONOTONIC t Dv CLOCK_MONOTONIC_PRECISE t Dv CLOCK_MONOTONIC_FAST t Dv CLOCK_MONOTONIC_COARSE Increments in SI seconds. t Dv CLOCK_UPTIME t Dv CLOCK_UPTIME_PRECISE t Dv CLOCK_UPTIME_FAST t Dv CLOCK_BOOTTIME Starts at zero when the kernel boots and increments monotonically in SI seconds while the machine is running. t Dv CLOCK_VIRTUAL Increments only when the CPU is running in user mode on behalf of the calling process. t Dv CLOCK_PROF Increments when the CPU is running in user or kernel mode. t Dv CLOCK_SECOND Returns the current second without performing a full time counter query, using an in-kernel cached value of the current second. t Dv CLOCK_PROCESS_CPUTIME_ID Returns the execution time of the calling process. t Dv CLOCK_THREAD_CPUTIME_ID Returns the execution time of the calling thread. .El
p The clock IDs .Dv CLOCK_REALTIME , .Dv CLOCK_MONOTONIC , and .Dv CLOCK_UPTIME perform a full time counter query. The clock IDs with the _FAST suffix, i.e., .Dv CLOCK_REALTIME_FAST , .Dv CLOCK_MONOTONIC_FAST , and .Dv CLOCK_UPTIME_FAST , do not perform a full time counter query, so their accuracy is one timer tick. Similarly, .Dv CLOCK_REALTIME_PRECISE , .Dv CLOCK_MONOTONIC_PRECISE , and .Dv CLOCK_UPTIME_PRECISE are used to get the most exact value as possible, at the expense of execution time. The clock IDs .Dv CLOCK_REALTIME_COARSE and .Dv CLOCK_MONOTONIC_COARSE are aliases of corresponding IDs with _FAST suffix for compatibility with other systems. Finally, .Dv CLOCK_BOOTTIME is an alias for .Dv CLOCK_UPTIME for compatibility with other systems and is unrelated to the .Fa kern.boottime .Xr sysctl 8 .
p The structure pointed to by .Fa tp is defined in n sys/timespec.h as: d -literal struct timespec { time_t tv_sec; /* seconds */ long tv_nsec; /* and nanoseconds */ }; .Ed
p Only the super-user may set the time of day, using only .Dv CLOCK_REALTIME . If the system .Xr securelevel 7 is greater than 1 (see .Xr init 8 ) , the time may only be advanced. This limitation is imposed to prevent a malicious super-user from setting arbitrary time stamps on files. The system time can still be adjusted backwards using the .Xr adjtime 2 system call even when the system is secure.
p The resolution (granularity) of a clock is returned by the .Fn clock_getres system call. This value is placed in a (non-NULL) .Fa *tp . .Sh RETURN VALUES .Rv -std .Sh ERRORS The following error codes may be set in .Va errno : l -tag -width Er t Bq Er EINVAL The .Fa clock_id or .Fa timespec argument was not a valid value. t Bq Er EPERM A user other than the super-user attempted to set the time. .El .Sh SEE ALSO .Xr date 1 , .Xr adjtime 2 , .Xr clock_getcpuclockid 3 , .Xr ctime 3 , .Xr pthread_getcpuclockid 3 .Sh STANDARDS The .Fn clock_gettime , .Fn clock_settime , and .Fn clock_getres system calls conform to .St -p1003.1b-93 . The clock IDs .Dv CLOCK_REALTIME_FAST , .Dv CLOCK_REALTIME_PRECISE , .Dv CLOCK_MONOTONIC_FAST , .Dv CLOCK_MONOTONIC_PRECISE , .Dv CLOCK_UPTIME , .Dv CLOCK_UPTIME_FAST , .Dv CLOCK_UPTIME_PRECISE , .Dv CLOCK_SECOND are .Fx extensions to the POSIX interface. .Sh HISTORY The .Fn clock_gettime , .Fn clock_settime , and .Fn clock_getres system calls first appeared in .Fx 3.0 .