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);

   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.

       The use of the timezone structure is obsolete; the tz argument should normally be  specified  as  NULL.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 occur-
       rence of this field in the kernel source (other than the declaration) is a bug.  Thus, the following is  purely  of  his-
       toric interest.

       The  field tz_dsttime contains a symbolic constant (values are given below) that indicates in which part of the year Day-
       light 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
       timezones has been abandoned.  Under Linux, in a call to settimeofday() the tz_dsttime field should be zero.

       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.   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.

       Macros for operating on timeval structures are described in timeradd(3).

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

CONFORMING TO
       SVr4,  4.3BSD.  POSIX.1-2001 describes gettimeofday() but not settimeofday().  POSIX.1-2008 marks gettimeofday() as obso-
       lete, recommending the use of clock_gettime(2) instead.

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

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

COLOPHON
       This page is part of release 3.25 of the Linux man-pages project.  A description of the project,  and  information  about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                                                      2009-03-25                                            GETTIMEOFDAY(2)