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       The use of the timezone structure is obsolete; the tz argument should  normally  be  specified  as  NULL.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

       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/.