NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

GETTIMEOFDAY(2)           Linux Programmer's Manual          GETTIMEOFDAY(2)

NAME         top

       gettimeofday, settimeofday - get / set time

SYNOPSIS         top

       #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         top

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       The use of the timezone structure is obsolete; the tz argument should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

       gettimeofday() and settimeofday() return 0 for success, or -1 for
       failure (in which case errno is set appropriately).

ERRORS         top

       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         top

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

NOTES         top

       The time returned by gettimeofday() is affected by discontinuous
       jumps in the system time (e.g., if the system administrator manually
       changes the system time).  If you need a monotonically increasing
       clock, see clock_gettime(2).

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

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

       The tz_dsttime field has never been used under Linux.  Thus, the
       following is purely of historic interest.

       On old systems, 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: 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 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      /* 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         top

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

COLOPHON         top

       This page is part of release 3.75 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                            2012-04-26                  GETTIMEOFDAY(2)