GETTIMEOFDAY(2) 					     Linux Programmer's Manual						   GETTIMEOFDAY(2)

NAME

       gettimeofday, settimeofday - get / set time

SYNOPSIS

       #include <sys/time.h>

       int gettimeofday(struct timeval *tv, struct timezone *tz);
       int settimeofday(const struct timeval *tv, const struct timezone *tz);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       settimeofday(): _BSD_SOURCE

DESCRIPTION

       The  functions  gettimeofday()  and settimeofday() can get and set the time as well as a timezone.  The tv argument is a struct timeval (as
       specified in <sys/time.h>):

	   struct timeval {
	       time_t	   tv_sec;     /* seconds */
	       suseconds_t tv_usec;    /* microseconds */
	   };

       and gives the number of seconds and microseconds since the Epoch (see time(2)).	The tz argument is a struct timezone:

	   struct timezone {
	       int tz_minuteswest;     /* minutes west of Greenwich */
	       int tz_dsttime;	       /* type of DST correction */
	   };

       If either tv or tz is NULL, the corresponding structure is not set or returned.

       The use of the timezone structure is obsolete; the tz argument should normally be specified as NULL.  The tz_dsttime field has  never  been
       used under Linux; it has not been and will not be supported by libc or glibc.  Each and every occurrence of this field in the kernel source
       (other than the declaration) is a bug.  Thus, the following is purely of historic interest.

       The field tz_dsttime contains a symbolic constant (values are given below) that indicates in which part of the year Daylight Saving Time is
       in force.  (Note: its value is constant throughout the year: it does not indicate that DST is in force, it just selects an algorithm.)  The
       daylight saving time algorithms defined are as follows :

	   DST_NONE	/* not on dst */
	   DST_USA	/* USA style dst */
	   DST_AUST	/* Australian style dst */
	   DST_WET	/* Western European dst */
	   DST_MET	/* Middle European dst */
	   DST_EET	/* Eastern European dst */
	   DST_CAN	/* Canada */
	   DST_GB	/* Great Britain and Eire */
	   DST_RUM	/* Rumania */
	   DST_TUR	/* Turkey */
	   DST_AUSTALT	/* Australian style with shift in 1986 */

       Of course it turned out that the period in which Daylight Saving Time is in force cannot be given by a simple algorithm, one  per  country;
       indeed,	this  period  is  determined  by  unpredictable political decisions.  So this method of representing timezones has been abandoned.
       Under Linux, in a call to settimeofday() the tz_dsttime field should be zero.

       Under Linux there are some peculiar "warp clock" semantics associated with the settimeofday() system call if on the very first call  (after
       booting)  that  has  a non-NULL tz argument, the tv argument is NULL and the tz_minuteswest field is nonzero.  In such a case it is assumed
       that the CMOS clock is on local time, and that it has to be incremented by this amount to get UTC system time.  No doubt it is a  bad  idea
       to use this feature.

       Macros for operating on timeval structures are described in timeradd(3).

RETURN VALUE

       gettimeofday() and settimeofday() return 0 for success, or -1 for failure (in which case errno is set appropriately).

ERRORS

       EFAULT One of tv or tz pointed outside the accessible address space.

       EINVAL Timezone (or something else) is invalid.

       EPERM  The calling process has insufficient privilege to call settimeofday(); under Linux the CAP_SYS_TIME capability is required.

CONFORMING TO

       SVr4,  4.3BSD.	POSIX.1-2001 describes gettimeofday() but not settimeofday().  POSIX.1-2008 marks gettimeofday() as obsolete, recommending
       the use of clock_gettime(2) instead.

NOTES

       Traditionally, the fields of struct timeval were of type long.

SEE ALSO

       date(1), adjtimex(2), time(2), ctime(3), ftime(3), capabilities(7), time(7)

COLOPHON

       This page is part of release 3.27 of the Linux man-pages project.  A description of the project, and information about reporting bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2009-03-25							   GETTIMEOFDAY(2)