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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 five values: CLOCK_REALTIME for time
     that increments as a wall clock should, CLOCK_MONOTONIC which increments
     in SI seconds, CLOCK_UPTIME 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, or CLOCK_PROF for time that
     increments when the CPU is running in user or kernel mode.

     The structure pointed to by tp is defined in #include <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.  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”).