settimeofday

       #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.  (However, compilation warnings will result if tv is NULL.)

       The  use  of the timezone structure is obsolete; the tz argument should
       normally be specified as NULL.  (See NOTES below.)

       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.  (The tz_dsttime field should be
       zero for this case.)  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.

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.

       add(3).

       Traditionally, the fields of struct timeval were of type long.

   The tz_dsttime field
       On a non-Linux kernel, with glibc, the tz_dsttime field of struct time-
       zone  will  be  set to a nonzero value by gettimeofday() if the current
       timezone has ever had or will have a daylight saving rule applied.   In
       this  sense  it exactly mirrors the meaning of daylight(3) for the cur-
       rent zone.  On Linux, with glibc, the setting of the  tz_dsttime  field
       of  struct timezone has never been used by settimeofday() or gettimeof-
       day().  Thus, the following is purely of historical interest.

       On old systems, the field tz_dsttime contains a symbolic constant (val-
       ues  are given below) that indicates in which part of the year Daylight
       Saving Time is in force.  (Note: this 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  fol-
       lows:

           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      /* Romania */
           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.

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

COLOPHON
       This  page  is  part of release 4.04 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       http://www.kernel.org/doc/man-pages/.

Linux                             2015-12-28                   GETTIMEOFDAY(2)
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