NAME
clock_settime
,
clock_gettime
, clock_getres
— clock and timer
functions
LIBRARY
library “libc”
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
Theclock_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
.
The following clock_id values are supported:
CLOCK_REALTIME
- identifies the realtime clock for the system. For this clock, the values
specified by
clock_settime
() and obtained byclock_gettime
() represent the amount of time (in seconds and nanoseconds) since 00:00 Universal Coordinated Time, January 1, 1970. 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 toclock_settime
(). The origin of the clock is unspecified. CLOCK_VIRTUAL
- identifies a clock that increments only when the CPU is running in user mode on behalf of the calling process.
CLOCK_PROF
- identifies a clock that increments when the CPU is running in user or kernel mode on behalf of the calling process.
CLOCK_PROCESS_CPUTIME_ID
- identifies a per process clock based on tick values. This clock is not settable.
CLOCK_THREAD_CPUTIME_ID
- identifies a per thread clock based on tick values. This clock is not settable.
If the calling user is not the super-user,
the
clock_settime
()
system call will fail, and the clock_settime
()
function in the standard C library will try to use the
clockctl(4) device if present, thus making it possible for non
privileged users to set the system time. If
clockctl(4) is not present or not accessible, then
clock_settime
() returns
EPERM
.
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; or the tp argument specified a nanosecond value less than zero of greater than or equal 1000 million; or 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
STANDARDS
The clock_settime
(),
clock_gettime
() and
clock_getres
() functions conform to
IEEE Std 1003.1b-1993
(“POSIX.1b”).