gettimeofday,
settimeofday —
get/set date and time
#include
<sys/time.h>
int
gettimeofday(
struct
timeval *tp,
struct timezone
*tzp);
int
settimeofday(
const
struct timeval *tp,
const struct timezone
*tzp);
Note: timezone is no longer
used; this information is kept outside the kernel.
The system's notion of the current Greenwich time and the current time zone 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 or
tzp is
NULL, the associated time information will
not be returned or set.
The structures pointed to by
tp and
tzp are defined in
<sys/time.h>
as:
struct timeval {
time_t tv_sec; /* seconds since Jan. 1, 1970 */
suseconds_t tv_usec; /* and microseconds */
};
struct timezone {
int tz_minuteswest; /* of Greenwich */
int tz_dsttime; /* type of dst correction to apply */
};
The
timezone structure indicates the local time
zone (measured in minutes of time westward from Greenwich), and a flag that,
if nonzero, indicates that Daylight Saving time applies locally during the
appropriate part of the year.
Only the superuser may set the time of day or time zone. 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.
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.
gettimeofday() and
settimeofday() will succeed unless:
-
-
- [
EFAULT]
- An argument address referenced invalid memory.
In addition,
settimeofday() may return the
following error:
-
-
- [
EPERM]
- A user other than the superuser attempted to set the time.
date(1),
adjtime(2),
clock_gettime(2),
getitimer(2),
ctime(3),
time(3)
The
gettimeofday() function conforms to
IEEE Std 1003.1-2008
(“POSIX.1”).
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.
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.
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