NAME
clock_gettime,
clock_settime, clock_getres
— get/set/calibrate date and
time
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 four values:
CLOCK_REALTIME for time that increments as a wall
clock should, 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_MONOTONIC
for time that increments at a steady rate (monotonically).
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 superuser 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 superuser 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
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.
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 superuser attempted to set the time.
SEE ALSO
STANDARDS
The clock_gettime(),
clock_settime(), and
clock_getres() functions conform to
IEEE Std 1003.1b-1993
(“POSIX.1b”).