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       gettimeofday, settimeofday - get / set time


       #include <sys/time.h>
       #include <time.h>

       int gettimeofday(struct timeval *tv, struct timezone *tz);
       int settimeofday(const struct timeval *tv , const struct timezone *tz);


       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 W of Greenwich */
               int  tz_dsttime;     /* type of dst correction */

       The  use  of  the timezone struct is obsolete: the tz_dsttime field has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 time zones  has  been  abandoned.  Under
       Linux, in a call to settimeofday() the tz_dsttime field should be zero.

       Under Linux there is some peculiar ‘warp clock’ semantics associated to
       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.

       The following macros are defined to operate on a struct timeval:

       #define       timerisset(tvp)\
               ((tvp)->tv_sec || (tvp)->tv_usec)
       #define       timercmp(tvp, uvp, cmp)\
               ((tvp)->tv_sec cmp (uvp)->tv_sec ||\
               (tvp)->tv_sec == (uvp)->tv_sec &&\
               (tvp)->tv_usec cmp (uvp)->tv_usec)
       #define       timerclear(tvp)\
               ((tvp)->tv_sec = (tvp)->tv_usec = 0)

       If either tv or tz is null, the corresponding structure is not  set  or

       Only the superuser may use settimeofday().


       gettimeofday()  and  settimeofday()  return  0  for  success, or -1 for
       failure (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
              settimeofday();  under  Linux  the  CAP_SYS_TIME  capability  is


       The  prototype  for  settimeofday()  and  the  defines  for   timercmp,
       timerisset,  timerclear, timeradd, timersub are (since glibc2.2.2) only
       available if _BSD_SOURCE is defined (either explicitly, or  implicitly,
       by not defining _POSIX_SOURCE or compiling with the -ansi flag).

       Traditionally, the fields of struct timeval were longs.


       SVr4,  4.3BSD.  POSIX  1003.1-2001  describes  gettimeofday()  but  not


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