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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 one of the following values: CLOCK_REALTIME,
     CLOCK_REALTIME_PRECISE, CLOCK_REALTIME_FAST for time that increments as a
     wall clock should; CLOCK_MONOTONIC, CLOCK_MONOTONIC_PRECISE,
     CLOCK_MONOTONIC_FAST which increments in SI seconds; CLOCK_UPTIME,
     CLOCK_UPTIME_PRECISE, CLOCK_UPTIME_FAST which starts at zero when the
     kernel boots and increments monotonically in SI seconds while the machine
     is running; CLOCK_VIRTUAL for time that increments only when the CPU is
     running in user mode on behalf of the calling process; CLOCK_PROF for
     time that increments when the CPU is running in user or kernel mode; or
     CLOCK_SECOND which returns the current second without performing a full
     time counter query, using in-kernel cached value of current second.

     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 argument was not a valid value.

     [EFAULT]           The *tp argument address referenced invalid memory.

     [EPERM]            A user other than the super-user attempted to set the
                        time.

SEE ALSO

     date(1), adjtime(2), ctime(3), timed(8)

STANDARDS

     The clock_gettime(), clock_settime(), and clock_getres() system calls
     conform to IEEE Std 1003.1b-1993 (``POSIX.1'').  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.