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
.
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 by clock_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 to clock_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
ctime(3),
time(3),
clockctl(4)
STANDARDS
The
clock_settime(),
clock_gettime() and
clock_getres() functions conform to
IEEE
Std 1003.1b-1993 (“POSIX.1b”).