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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

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CLOCK_SETTIME(2)					      BSD System Calls Manual						  CLOCK_SETTIME(2)

NAME
clock_settime, clock_gettime, clock_getres -- clock and timer functions LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <time.h> int clock_settime(clockid_t clock_id, const struct timespec *tp); int clock_gettime(clockid_t clock_id, struct timespec *tp); int clock_getres(clockid_t clock_id, struct timespec *res); DESCRIPTION
The clock_settime() function sets the clock identified by clock_id to the absolute time specified by tp. If the time specified by tp is not a multiple of the resolution of the clock, tp is truncated to a multiple of the resolution. The clock_gettime() function stores the time of the clock identified by clock_id into the location specified by tp. The clock_getres() function stores the resolution of the clock identified by clock_id into the location specified by res, unless res is NULL. A clock_id of CLOCK_REALTIME identifies the realtime clock for the system. For this clock, the values specified by clock_settime() and obtained by clock_gettime() represent the amount of time (in seconds and nanoseconds) since 00:00 Universal Coordinated Time, January 1, 1970. A clock_id of CLOCK_MONOTONIC identifies a clock that increases at a steady rate (monotonically). This clock is not affected by calls to adjtime(2) and settimeofday(2) and will fail with an EINVAL error if it's the clock specified in a call to clock_settime(). The origin of the clock is unspecified. If the calling user is not the super-user, then the clock_settime() function in the standard C library will try to use the clockctl(4) device if present, thus making possible for non privileged users to set the system time. If clockctl(4) is not present or not accessible, then clock_settime() reverts to the clock_settime() system call, which is restricted to the super user. RETURN VALUES
A value of 0 is returned on success. Otherwise, a value of -1 is returned and errno is set to indicate the error. ERRORS
The clock_settime(), clock_gettime() and clock_getres() functions will fail if: [EINVAL] The clock_id argument does not specify a known clock. [ENOSYS] The function is not supported by this implementation. The clock_settime() function will fail if: [EINVAL] The tp argument is outside the range for the specified clock, clock_id. [EINVAL] The tp argument specified a nanosecond value less than zero of greater than or equal 1000 million. [EINVAL] The clock_id argument is a clock that can not be adjusted. [EPERM] The calling process does not have the appropriate privilege to set the specified clock, clock_id. The clock_gettime() function will fail if: [EFAULT] The tp argument specifies an address that is not a valid part of the process address space. SEE ALSO
ctime(3), time(3), clockctl(4) STANDARDS
The clock_settime(), clock_gettime() and clock_getres() functions conform to IEEE Std 1003.1b-1993 (``POSIX.1''). BSD
February 11, 1999 BSD
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