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);

DESCRIPTION

       The functions gettimeofday and settimeofday can get and set the time as well as a timezone.  The tv argument is a timeval struct, as speci-
       fied  in /usr/include/sys/time.h:

       struct timeval {
	       long tv_sec;	   /* seconds */
	       long tv_usec;  /* microseconds */
       };

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

       struct timezone {
	       int  tz_minuteswest; /* minutes W of Greenwich */
	       int  tz_dsttime;     /* type of dst correction */
       };

       The use of the timezone struct is obsolete; the tz_dsttime field has never been used under Linux - it has not been and  will  not  be  sup-
       ported  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 time zones has been abandoned. Under
       Linux, in a call to settimeofday the tz_dsttime field should be zero.

       Under Linux there is some peculiar `warp clock' semantics associated to the settimeofday system call if on the very first call (after boot-
       ing)  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.

       The following macros are defined to operate on a struct timeval :
       #define	     timerisset(tvp)
	       ((tvp)->tv_sec || (tvp)->tv_usec)
       #define	     timercmp(tvp, uvp, cmp)
	       ((tvp)->tv_sec cmp (uvp)->tv_sec ||
	       (tvp)->tv_sec == (uvp)->tv_sec &&
	       (tvp)->tv_usec cmp (uvp)->tv_usec)
       #define	     timerclear(tvp)
	       ((tvp)->tv_sec = (tvp)->tv_usec = 0)

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

       Only the super user may use settimeofday.

RETURN VALUE

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

ERRORS

       EPERM  settimeofday is called by someone other than the superuser.

       EINVAL Timezone (or something else) is invalid.

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

NOTE

       The  prototype for settimeofday and the defines for timercmp, timerisset, timerclear, timeradd, timersub are (since glibc2.2.2) only avail-
       able if _BSD_SOURCE is defined (either explicitly, or implicitly, by not defining _POSIX_SOURCE or compiling with the -ansi flag).

CONFORMING TO

       SVr4, BSD 4.3

SEE ALSO

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

Linux 2.0.32							    1997-12-10							   GETTIMEOFDAY(2)