CLOCK_GETTIME(2) - FreeBSD 시스템 호출 설명서

 clock_gettime, clock_settime, clock_getres – get/set/calibrate date and
 time

 Standard C Library (libc, -lc)

 #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);

 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 as a wall clock should.
 CLOCK_MONOTONIC
 CLOCK_MONOTONIC_PRECISE
 CLOCK_MONOTONIC_FAST
 CLOCK_MONOTONIC_COARSE
         Increments in SI seconds.
 CLOCK_UPTIME
 CLOCK_UPTIME_PRECISE
 CLOCK_UPTIME_FAST
 CLOCK_BOOTTIME
         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, 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_UPTIME for compatibility with other systems.

 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.

 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.

 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.

 date(1), adjtime(2), clock_getcpuclockid(3), ctime(3),
 pthread_getcpuclockid(3)

 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.

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