GETTIMEOFDAY(2)

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
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 {
    	long	tv_sec;		/* seconds since Jan. 1, 1970 */
    	long	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
A 0 return value indicates that the call succeeded. A -1 return value indicates an error occurred, and in this case an error code is stored into the global variable errno.

ERRORS
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.

SEE ALSO
date(1), adjtime(2), clock_gettime(2), getitimer(2), ctime(3), time(3)

STANDARDS
The gettimeofday() function conforms to p1003.1-2008.

HISTORY
As predecessors of these functions, former system calls time() and stime() first appeared in AT&T UNIX, and ftime() first appeared in 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.