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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.

       The  use  of the timezone structure is obsolete; the tz argument should
       normally be specified as NULL.  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 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 timezones has been abandoned.  Under
       Linux, in a call to settimeofday() the tz_dsttime field should be zero.

       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 non-zero.  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.

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

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.

NOTES

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

SEE ALSO

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

COLOPHON

       This  page  is  part of release 3.15 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/.