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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 argu-
       ment 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       The  use  of  the  timezone  structure  is obsolete; the tz argument should normally be specified as NULL.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 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      /* 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 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.

       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 appropri-
       ately).

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 obsolete, recomending 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 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2009-03-25                   GETTIMEOFDAY(2)