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GETTIMEOFDAY(2)            Linux Programmer's Manual           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       The  use  of the timezone structure is obsolete; the tz argument should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 fail-
       ure (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 settime-
              ofday(); under Linux the CAP_SYS_TIME capability is required.
CONFORMING TO
       SVr4, 4.3BSD.  POSIX.1-2001 describes gettimeofday() but not settimeof-
       day().  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 timer-
       add(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 time-
       zone 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  cur-
       rent  zone.   On Linux, with glibc, the setting of the tz_dsttime field
       of struct timezone has never been used by settimeofday() or  gettimeof-
       day().  Thus, the following is purely of historical interest.
       On old systems, the field tz_dsttime contains a symbolic constant (val-
       ues 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 fol-
       lows:
           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 4.15 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/.
Linux                             2017-09-15                   GETTIMEOFDAY(2)