settimeofday(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   The  use  of the timezone structure is obsolete; the tz argument should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 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,
   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.

   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

   This  page  is  part of release 4.09 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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