gettimeofday, settimeofday - get / set time


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

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


       gettimeofday  and settimeofday can set the time as well as
       a timezone.  tv is a  timeval  struct,  as  specified   in

       struct timeval {
               long tv_sec;        /* seconds */
               long tv_usec;  /* microseconds */

       and tz is a 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       The use of the timezone struct is obsolete; the tz_dsttime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 con­
       stant  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 */

       Saving Time is in force cannot be given by a simple  algo­
       rithm,  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

       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 argu­
       ment, 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.

       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 returned.

       Only the super user may use settimeofday.


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


       EPERM  settimeofday  is  called  by someone other than the

       EINVAL Timezone (or something else) is invalid.

       EFAULT One of tv or tz  pointed  outside  your  accessible
              address space.


       SVr4, BSD 4.3


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