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RedHat 9 (Linux i386) - man page for gettimeofday (redhat section 2)

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 time-
       zone.  The tv argument is a timeval struct, as specified  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       The use of the timezone struct is obsolete; the tz_dsttime field has never been used under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 supported by libc or glibc.  Each and every occur-
       rence  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  algo-
       rithm.)	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 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.

       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 available 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)


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