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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       The  use  of  the  timezone  structure  is  obsolete;  the  tz argument should normally be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 nonzero.  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, recommending the use of clock_gettime(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 3.35 of the Linux man-pages project.  A  description  of  the
       project,  and information about reporting bugs, can be found at http://man7.org/linux/man-
       pages/.