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Linux 2.6 - man page for settimeofday (linux 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);

   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.	(However,
       compilation warnings will result if tv is NULL.)

       The use of the timezone structure is obsolete; the tz argument should normally  be  speci-
       fied as NULL.  (See NOTES below.)

       Under  Linux there are some peculiar "warp clock" semantics associated with the settimeof-
       day() system call if on the very first call (after booting) that has a non-NULL	tz  argu-
       ment,  the  tv  argument is NULL and the tz_minuteswest field is nonzero.  (The tz_dsttime
       field should be zero for this case.)  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.

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
       The  time returned by gettimeofday() is affected by discontinuous jumps in the system time
       (e.g., if the system administrator manually changes the system time).  If you need a mono-
       tonically increasing clock, see clock_gettime(2).

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

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

       The  tz_dsttime	field  has never been used under Linux.  Thus, the following is purely of
       historic interest.

       On old systems, 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: this
       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	/* Romania */
	   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.

SEE ALSO
       date(1), adjtimex(2), clock_gettime(2), time(2), ctime(3), ftime(3), timeradd(3), capabil-
       ities(7), time(7)

COLOPHON
       This  page  is  part of release 3.55 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					    2012-04-26				  GETTIMEOFDAY(2)


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