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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       The  use	 of the	timezone structure is obsolete;	the tz argument	should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 specified as	NULL.  (See NOTES below.)

       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.  (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 fail-
       ure (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 settime-
	      ofday(); under Linux the CAP_SYS_TIME capability is required.

CONFORMING TO
       SVr4, 4.3BSD.  POSIX.1-2001 describes gettimeofday() but	not settimeof-
       day().  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	monotonically  increasing  clock,  see
       clock_gettime(2).

       Macros  for  operating  on  timeval  structures are described in	timer-
       add(3).

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

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

       On old systems, the field tz_dsttime contains a symbolic	constant (val-
       ues 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 al-
       gorithm.)  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; in-
       deed,  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), capabilities(7), time(7)

COLOPHON
       This  page  is  part of release 3.53 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)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

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