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       The  use  of  the  timezone  structure  is  obsolete;  the  tz argument should normally be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

       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 obsolete, recommending the use of clock_gettime(2) instead.

NOTES

       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.

   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 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

       date(1),   adjtimex(2),   clock_gettime(2),   time(2),  ctime(3),  ftime(3),  timeradd(3),
       capabilities(7), time(7), vdso(7), hwclock(8)

COLOPHON

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       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at https://www.kernel.org/doc/man-pages/.