GETTIMEOFDAY(2)           (2019-03-06)            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():
              Since glibc 2.19:
                  _DEFAULT_SOURCE
              Glibc 2.19 and earlier:
                  _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          The use of the timezone structure is obsolete; the tz argu-
          ment should normally be specified as NULL.  (See NOTES
          below.)

          Under Linux, there are some peculiar "warp clock" semantics

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          associated with the settimeofday() system call if on the
          very first call (after booting) that has a non-NULL tz argu-
          ment, 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 failure (in which case errno is set appropriately).

     ERRORS
          EFAULT
               One of tv or tz pointed outside the accessible address
               space.

          EINVAL
               (settimeofday()): timezone is invalid.

          EINVAL
               (settimeofday()): tv.tv_sec is negative or tv.tv_usec
               is outside the range [0..999,999].

          EINVAL (since Linux 4.3)
               (settimeofday()): An attempt was made to set the time
               to a value less than the current value of the
               CLOCK_MONOTONIC clock (see clock_gettime(2)).

          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
          The time returned by gettimeofday() is affected by discon-
          tinuous jumps in the system time (e.g., if the system admin-
          istrator 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.

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        C library/kernel differences
          On some architectures, an implementation of gettimeofday()
          is provided in the vdso(7).

        The tz_dsttime field
          On a non-Linux kernel, with glibc, the tz_dsttime field of
          struct timezone 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 current
          zone.  On Linux, with glibc, the setting of the tz_dsttime
          field of struct timezone has never been used by
          settimeofday() or gettimeofday().  Thus, the following is
          purely of historical 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 algo-
          rithm.)  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 algo-
          rithm, one per country; indeed, this period is determined by
          unpredictable political decisions.  So this method of repre-
          senting 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),
          hwclock(8)

     COLOPHON
          This page is part of release 5.10 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 https://www.kernel.org/doc/man-pages/.

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