NAME
gettimeofday,
settimeofday —
get/set date and time
SYNOPSIS
#include
<sys/time.h>
int
gettimeofday(struct
timeval *tp, struct
timezone *tzp);
int
settimeofday(const
struct timeval *tp, const
struct timezone *tzp);
DESCRIPTION
The system's notion of the current Greenwich time is obtained with
the
gettimeofday()
call and set with the
settimeofday()
call. The time is expressed in seconds and microseconds since midnight (0
hour), January 1, 1970. The resolution of the system clock is hardware
dependent, and the time may be updated continuously or in
“ticks”. If tp is
NULL, the time will not be returned or set. The
structure pointed to by tp is defined in
<sys/time.h> as:
struct timeval {
time_t tv_sec; /* seconds since Jan. 1, 1970 */
suseconds_t tv_usec; /* and microseconds */
};
The tzp parameter is
historical and timezone information is no longer tracked by the system. All
code should pass NULL for tzp.
For
gettimeofday(),
if tzp is non-NULL an empty
timezone structure will be returned. For
settimeofday(),
if tzp is non-NULL its
contents are ignored.
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.
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
gettimeofday() and
settimeofday() will succeed unless:
- [
EFAULT] - An argument address referenced invalid memory.
In addition, settimeofday() may return the
following errors:
- [
EINVAL] - tp specified a microsecond value less than zero or greater than or equal to 1 million.
- [
EPERM] - A user other than the superuser attempted to set the time.
SEE ALSO
date(1), adjtime(2), clock_gettime(2), getitimer(2), ctime(3), time(3)
STANDARDS
The gettimeofday() function conforms to
IEEE Std 1003.1-2008 (“POSIX.1”).
HISTORY
As predecessors of these functions, former system calls
time() and stime() first
appeared in Version 1 AT&T UNIX, and
ftime() first appeared in
Version 7 AT&T UNIX. The
gettimeofday() and
settimeofday() system calls first appeared in
4.1cBSD.
CAVEATS
Setting the time with settimeofday() is
dangerous; if possible use
adjtime(2) instead. Many daemon programming techniques utilize
time-delta techniques using the results from
gettimeofday() instead of from
clock_gettime(2) on the
CLOCK_MONOTONIC clock. Time jumps can cause these
programs to malfunction in unexpected ways. If the time must be set,
consider rebooting the machine for safety.