DragonFly On-Line Manual Pages

CLOCK_GETTIME(2)	 DragonFly 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 <sys/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() allow the calling process to retrieve or set the value used by a clock which is specified by clock_id. clock_id can be one of the following values: CLOCK_REALTIME, CLOCK_REALTIME_PRECISE, and CLOCK_REALTIME_FAST for time that increments as a wall clock should; CLOCK_MONOTONIC, CLOCK_MONOTONIC_PRECISE, and CLOCK_MONOTONIC_FAST which increments in SI seconds; CLOCK_UPTIME, CLOCK_UPTIME_PRECISE, and 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, and 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, and CLOCK_UPTIME_PRECISE are used to get the most exact value as possi- ble, at the expense of execution time. The clock IDs CLOCK_UPTIME_FAST, CLOCK_MONOTONIC_FAST, CLOCK_REALTIME_FAST, and CLOCK_SECOND make use of kpmap(4) and do not incur any system call overhead after a certain amount of calls. 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 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() 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 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(), etc. functions 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, and CLOCK_SECOND are FreeBSD extensions to the POSIX interface. DragonFly 4.3 March 17, 2015 DragonFly 4.3