Man

Command Section
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
             Increments as a wall clock should.
     CLOCK_MONOTONIC
     CLOCK_MONOTONIC_PRECISE
     CLOCK_MONOTONIC_FAST
             Increments in SI seconds.
     CLOCK_UPTIME
     CLOCK_UPTIME_PRECISE
     CLOCK_UPTIME_FAST
             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_FAST, CLOCK_MONOTONIC_FAST,
     CLOCK_UPTIME_FAST are analogs of corresponding IDs without _FAST suffix
     but do not perform a full time counter query, so their accuracy is one
     timer tick.  Similarly, CLOCK_REALTIME_PRECISE, CLOCK_MONOTONIC_PRECISE,
     CLOCK_UPTIME_PRECISE are used to get the most exact value as possible, at
     the expense of execution time.

     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 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), timed(8)

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.

FreeBSD 11.1-RELEASE-p4         March 20, 2017         FreeBSD 11.1-RELEASE-p4
Command Section