clock_settime(2) [freebsd man page]

CLOCK_GETTIME(2)					      BSD 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 speci-
     fied 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 incre-
     ments 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.

BSD
								 December 29, 2009							       BSD

CLOCK_GETTIME(3)					   BSD Library Functions Manual 					  CLOCK_GETTIME(3)

NAME

     clock_gettime, clock_settime, clock_getres, clock_gettime_nsec_np -- get/set date and time

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);

     uint64_t
     clock_gettime_nsec_np(clockid_t clock_id);

DESCRIPTION

     The clock_gettime() and clock_settime() functions allow the calling process to retrieve or set the value used by a clock which is specified
     by clock_id.

     clock_id can be a value from one of 5 predefined values:

     CLOCK_REALTIME	the system's real time (i.e. wall time) clock, expressed as the amount of time since the Epoch.  This is the same as the
			value returned by gettimeofday(2).

     CLOCK_MONOTONIC	clock that increments monotonically, tracking the time since an arbitrary point, and will continue to increment while the
			system is asleep.

     CLOCK_MONOTONIC_RAW
			clock that increments monotonically, tracking the time since an arbitrary point like CLOCK_MONOTONIC.  However, this clock
			is unaffected by frequency or time adjustments.  It should not be compared to other system time sources.

     CLOCK_MONOTONIC_RAW_APPROX
			like CLOCK_MONOTONIC_RAW, but reads a value cached by the system at context switch.  This can be read faster, but at a
			loss of accuracy as it may return values that are milliseconds old.

     CLOCK_UPTIME_RAW	clock that increments monotonically, in the same manner as CLOCK_MONOTONIC_RAW, but that does not increment while the sys-
			tem is asleep.	The returned value is identical to the result of mach_absolute_time() after the appropriate mach_timebase
			conversion is applied.

     CLOCK_UPTIME_RAW_APPROX
			like CLOCK_UPTIME_RAW, but reads a value cached by the system at context switch.  This can be read faster, but at a loss
			of accuracy as it may return values that are milliseconds old.

     CLOCK_PROCESS_CPUTIME_ID
			clock that tracks the amount of CPU (in user- or kernel-mode) used by the calling process.

     CLOCK_THREAD_CPUTIME_ID
			clock that tracks the amount of CPU (in user- or kernel-mode) used by the calling thread.

     The structure pointed to by tp is defined in <sys/time.h> as:

	   struct timespec {
	       time_t  tv_sec;	   /* seconds */
	       long    tv_nsec;    /* and nanoseconds */
	   };

     Only the CLOCK_REALTIME clock can be set, and only the superuser may do so.

     The resolution of a clock is returned by the clock_getres() call.	This value is placed in a (non-null) *tp.  This value may be smaller than
     the actual precision of the underlying clock, but represents a lower bound on the resolution.

     As a non-portable extension, the clock_gettime_nsec_np() function will return the clock value in 64-bit nanoseconds.

RETURN VALUES

     A 0 return value indicates that the call succeeded.  A -1 return value indicates an error occurred, and in this case an error code is stored
     into the global variable errno.  For clock_gettime_nsec_np() a return value of non-0 indicates success.  A 0 return value indicates an error
     occurred and an error code is stored in errno.

ERRORS

     clock_gettime(), clock_settime(), clock_getres(), and clock_gettime_nsec_np() will fail if:

     [EINVAL]		clock_id is not a valid value.

     [EFAULT]		The tp argument address referenced invalid memory.

     In addition, clock_settime() may return the following errors:

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

     [EINVAL]		clock_id specifies a clock that isn't settable, tp specifies a nanosecond value less than zero or greater than 1000 mil-
			lion, or a value outside the range of the specified clock.

SEE ALSO

     date(1), getitimer(2), gettimeofday(2),

HISTORY

     These functions first appeared in Mac OSX 10.12

STANDARDS

     The clock_gettime(), clock_settime(), and clock_getres() system calls conform to IEEE Std 1003.1b-1993 (``POSIX.1'').
     cleck_gettime_nsec_np() is a non-portable Darwin extension.  The clock IDs CLOCK_MONOTONIC_RAW and CLOCK_UPTIME_RAW are extensions to the
     POSIX interface.

BSD
								 January 26, 2016							       BSD