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);

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       The use of the timezone structure is obsolete; the tz  argument  should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 day-
       light 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 boot-
       ing) 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)

RETURN VALUE
       gettimeofday() and settimeofday() return 0 for success, or -1 for fail-
       ure (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 settime-
              ofday(); under Linux the CAP_SYS_TIME capability is required.

NOTE
       The prototype for settimeofday() and the defines for timercmp, timeris-
       set,  timerclear, timeradd, timersub are (since glibc2.2.2) only avail-
       able if _BSD_SOURCE is defined.

       Traditionally, the fields of struct timeval were longs.

CONFORMING TO
       SVr4, 4.3BSD.  POSIX.1-2001 describes gettimeofday() but not settimeof-
       day().

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

Linux 2.6.6                       2004-05-27                   GETTIMEOFDAY(2)