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CLOCK_GETTIME(2) FreeBSD System Calls Manual CLOCK_GETTIME(2)
NAME
clock_gettime, clock_settime, clock_getres - get/set/calibrate date and
time
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <time.h>
int
clock_gettime(clockid_t clock_id, struct timespec *tp);
int
clock_settime(clockid_t clock_id, const struct timespec *tp);
int
clock_getres(clockid_t clock_id, struct timespec *tp);
DESCRIPTION
The clock_gettime() and clock_settime() system calls allow the calling
process to retrieve or set the value used by a clock which is specified
by clock_id.
The clock_id argument can be a value obtained from clock_getcpuclockid(3)
or pthread_getcpuclockid(3) as well as the following values:
CLOCK_REALTIME
CLOCK_REALTIME_PRECISE
CLOCK_REALTIME_FAST
CLOCK_REALTIME_COARSE
Increments as a wall clock should.
CLOCK_MONOTONIC
CLOCK_MONOTONIC_PRECISE
CLOCK_MONOTONIC_FAST
CLOCK_MONOTONIC_COARSE
Increments in SI seconds.
CLOCK_UPTIME
CLOCK_UPTIME_PRECISE
CLOCK_UPTIME_FAST
CLOCK_BOOTTIME
Starts at zero when the kernel boots and increments monotonically
in SI seconds while the machine is running.
CLOCK_VIRTUAL
Increments only when the CPU is running in user mode on behalf of
the calling process.
CLOCK_PROF
Increments when the CPU is running in user or kernel mode.
CLOCK_SECOND
Returns the current second without performing a full time counter
query, using an in-kernel cached value of the current second.
CLOCK_PROCESS_CPUTIME_ID
Returns the execution time of the calling process.
CLOCK_THREAD_CPUTIME_ID
Returns the execution time of the calling thread.
The clock IDs CLOCK_REALTIME, CLOCK_MONOTONIC, and CLOCK_UPTIME perform a
for compatibility with other systems. Finally, CLOCK_BOOTTIME is an
alias for CLOCK_UPTIME for compatibility with other systems.
The structure pointed to by tp is defined in <sys/timespec.h> as:
struct timespec {
time_t tv_sec; /* seconds */
long tv_nsec; /* and nanoseconds */
};
Only the super-user may set the time of day, using only CLOCK_REALTIME.
If the system securelevel(7) is greater than 1 (see 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 adjtime(2) system call even
when the system is secure.
The resolution (granularity) of a clock is returned by the clock_getres()
system call. This value is placed in a (non-NULL) *tp.
RETURN VALUES
Upon successful completion, the value 0 is returned; otherwise the
value -1 is returned and the global variable errno is set to indicate the
error.
ERRORS
The following error codes may be set in errno:
[EINVAL] The clock_id or timespec argument was not a valid
value.
[EPERM] A user other than the super-user attempted to set the
time.
SEE ALSO
date(1), adjtime(2), clock_getcpuclockid(3), ctime(3),
pthread_getcpuclockid(3)
STANDARDS
The clock_gettime(), clock_settime(), and clock_getres() system calls
conform to IEEE Std 1003.1b-1993 ("POSIX.1b"). The clock IDs
CLOCK_REALTIME_FAST, CLOCK_REALTIME_PRECISE, CLOCK_MONOTONIC_FAST,
CLOCK_MONOTONIC_PRECISE, CLOCK_UPTIME, CLOCK_UPTIME_FAST,
CLOCK_UPTIME_PRECISE, CLOCK_SECOND are FreeBSD extensions to the POSIX
interface.
HISTORY
The clock_gettime(), clock_settime(), and clock_getres() system calls
first appeared in FreeBSD 3.0.
FreeBSD 14.0-RELEASE-p11 July 8, 2022 FreeBSD 14.0-RELEASE-p11