GETTIMEOFDAY man page on Archlinux

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

       gettimeofday, settimeofday - get / set time

       #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

       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       The  use	 of the timezone structure is obsolete; the tz argument should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.

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

       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.

       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.

       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

       Macros  for  operating  on  timeval  structures are described in timer‐

       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
       algorithm.)   The  daylight  saving time algorithms defined are as fol‐

	   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.

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

       This page is part of release 3.65 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at

Linux				  2012-04-26		       GETTIMEOFDAY(2)

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