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 in SI seconds like a wall clock.  It uses a 1970 epoch
             and implements the UTC timescale.  The count of physical SI
             seconds since 1970, adjusted by subtracting the number of
             positive leap seconds and adding the number of negative leap
             seconds.  Behavior during a leap second is not defined by any
             POSIX standard.
     CLOCK_MONOTONIC
     CLOCK_MONOTONIC_PRECISE
     CLOCK_MONOTONIC_FAST
     CLOCK_MONOTONIC_COARSE
     CLOCK_BOOTTIME
             Increments in SI seconds, even while the system is suspended.
             Its epoch is unspecified.  The count is not adjusted by leap
             seconds.  FreeBSD implements
     CLOCK_UPTIME
     CLOCK_UPTIME_PRECISE
     CLOCK_UPTIME_FAST
             Increments monotonically in SI seconds while the machine is
             running.  The count is not adjusted by leap seconds.  The epoch
             is unspecified.
     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.
     CLOCK_TAI
             Increments in SI seconds like a wall clock.  It uses a 1970 epoch
             and implements the TAI timescale.  Similar to CLOCK_REALTIME, but
             without leap seconds.  It will increase monotonically during a
             leap second.  Will return EINVAL if the current offset between
             TAI and UTC is not known, which may be the case early in boot
             before NTP or another time daemon has synchronized.

     The clock IDs CLOCK_BOOTTIME, CLOCK_REALTIME, CLOCK_TAI, CLOCK_MONOTONIC,
     and CLOCK_UPTIME perform a full time counter query.  The clock IDs with
     the _FAST suffix, i.e., CLOCK_REALTIME_FAST, CLOCK_MONOTONIC_FAST, and
     CLOCK_UPTIME_FAST, do not perform a full time counter query, so their
     accuracy is one timer tick.  Similarly, CLOCK_REALTIME_PRECISE,
     CLOCK_MONOTONIC_PRECISE, and CLOCK_UPTIME_PRECISE are used to get the
     most exact value as possible, at the expense of execution time.  The
     clock IDs CLOCK_REALTIME_COARSE and CLOCK_MONOTONIC_COARSE are aliases of
     corresponding IDs with _FAST suffix for compatibility with other systems.
     Finally, CLOCK_BOOTTIME is an alias for CLOCK_MONOTONIC for compatibility
     with other systems and is unrelated to the kern.boottime sysctl(8).

     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 1003.1-2008 ("POSIX.1").  The clock IDs
     CLOCK_BOOTTIME, CLOCK_MONOTONIC_FAST, CLOCK_MONOTONIC_PRECISE,
     CLOCK_REALTIME_FAST, CLOCK_REALTIME_PRECISE, CLOCK_SECOND, CLOCK_TAI,
     CLOCK_UPTIME, CLOCK_UPTIME_FAST, and CLOCK_UPTIME_PRECISE are FreeBSD
     extensions to the POSIX interface.

     UTC is defined by ITU-R TF.460-6, Standard-frequency and time-signal
     emissions.  However, the time_t type is a simple count that does not
     provide a unique encoding for leap seconds, nor a specification for what
     values should be used to encode a leap second.

HISTORY

     The clock_gettime(), clock_settime(), and clock_getres() system calls
     first appeared in FreeBSD 3.0.

FreeBSD 15.1-STABLE-HBSD        August 10, 2024               CLOCK_GETTIME(2)