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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.