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           #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


           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           The  use  of the timezone structure is obsolete; the tz argument should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.


           gettimeofday() and settimeofday() return 0 for success, or -1 for fail-
           ure (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.
           Macros  for  operating  on  timeval  structures are described in timer-
           Traditionally, the fields of struct timeval were of type long.
           The tz_dsttime field has never been used under Linux.  Thus,  the  fol-
           lowing is purely of historic 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-
               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.


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

    Linux 2012-04-26 GETTIMEOFDAY(2)


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