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 specified
by clock_id.
The clock_id argument can be one of five values: CLOCK_REALTIME for time
that increments as a wall clock should, CLOCK_MONOTONIC which increments
in SI seconds, CLOCK_UPTIME 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, or CLOCK_PROF for time that
increments when the CPU is running in user or kernel mode.
The structure pointed to by tp is defined in #include <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. 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”).