gettimeofday(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

GETTIMEOFDAY(2)         Linux Programmer's Manual        GETTIMEOFDAY(2)

NAME         top

       gettimeofday, settimeofday - get / set time

SYNOPSIS         top

       #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         top

       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       The use of the timezone structure is obsolete; the tz argument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         top

       gettimeofday() and settimeofday() return 0 for success, or -1 for
       failure (in which case errno is set appropriately).

ERRORS         top

       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         top

       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         top

       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         top

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

COLOPHON         top

       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/.

Linux                          2019-03-06                GETTIMEOFDAY(2)

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