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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.

       The  use  of the timezone structure is obsolete; the tz argument should normally be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_dst-
       time field should be zero.

       Under Linux there are some peculiar "warp clock" semantics associated with the set-
       timeofday() 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, recomending the use of clock_get-
       time(2) instead.

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 3.22 of the Linux man-pages project.  A description of
       the  project, and information about reporting bugs, can be found at http://www.ker-
       nel.org/doc/man-pages/.



Linux                             2009-03-25                   GETTIMEOFDAY(2)

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