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 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.
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.