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Man Pages
GETTIMEOFDAY(2) FreeBSD System Calls Manual GETTIMEOFDAY(2)

gettimeofday, settimeofday
get/set date and time

Standard C Library (libc, -lc)

#include <sys/time.h>
int
gettimeofday(struct timeval *tp, struct timezone *tzp);
int
settimeofday(const struct timeval *tp, const struct timezone *tzp);

The system's notion of the current Greenwich time and the current time zone is obtained with the gettimeofday() system call, and set with the settimeofday() system 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 */ 
	suseconds_t	tv_usec;	/* and microseconds */ 
}; 
 
struct timezone { 
	int	tz_minuteswest; /* minutes west of Greenwich */ 
	int	tz_dsttime;	/* type of dst correction */ 
};
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 super-user may set the time of day or time zone. If the system is running at securelevel >= 2 (see init(8)), the time may only be advanced or retarded by a maximum of one second. This limitation is imposed to prevent a malicious super-user from setting arbitrary time stamps on files. The system time can be adjusted backwards without restriction 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.

The following error codes may be set in errno:
[]
The supplied timeval value is invalid.
[]
A user other than the super-user attempted to set the time.

date(1), adjtime(2), clock_gettime(2), ctime(3), timeradd(3), clocks(7), timed(8)

The gettimeofday() system call appeared in 4.2BSD.
August 4, 2018 FreeBSD 12.0-RELEASE

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