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.