409 lines
8.7 KiB
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409 lines
8.7 KiB
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of GETTIMEOFDAY</TITLE>
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</HEAD><BODY>
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<H1>GETTIMEOFDAY</H1>
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Section: Linux Programmer's Manual (2)<BR>Updated: 2019-03-06<BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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gettimeofday, settimeofday - get / set time
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<PRE>
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<B>#include <<A HREF="file:///usr/include/sys/time.h">sys/time.h</A>></B>
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<B>int gettimeofday(struct timeval *</B><I>tv</I><B>, struct timezone *</B><I>tz</I><B>);</B>
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<B>int settimeofday(const struct timeval *</B><I>tv</I><B>, const struct timezone *</B><I>tz</I><B>);</B>
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</PRE>
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<P>
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Feature Test Macro Requirements for glibc (see
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<B><A HREF="/cgi-bin/man/man2html?7+feature_test_macros">feature_test_macros</A></B>(7)):
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<P>
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<B>settimeofday</B>():
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<BR> Since glibc 2.19:
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<BR> _DEFAULT_SOURCE
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<BR> Glibc 2.19 and earlier:
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<BR> _BSD_SOURCE
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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The functions
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<B>gettimeofday</B>()
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and
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<B>settimeofday</B>()
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can get and set the time as well as a timezone.
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<P>
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The
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<I>tv</I>
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argument is a
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<I>struct timeval</I>
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(as specified in
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<I><<A HREF="file:///usr/include/sys/time.h">sys/time.h</A>></I>):
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<P>
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struct timeval {
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<BR> time_t tv_sec; /* seconds */
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<BR> suseconds_t tv_usec; /* microseconds */
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};
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<P>
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and gives the number of seconds and microseconds since the Epoch (see
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<B><A HREF="/cgi-bin/man/man2html?2+time">time</A></B>(2)).
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<P>
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The
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<I>tz</I>
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argument is a
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<I>struct timezone</I>:
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<P>
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struct timezone {
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<BR> int tz_minuteswest; /* minutes west of Greenwich */
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<BR> int tz_dsttime; /* type of DST correction */
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};
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<P>
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If either
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<I>tv</I>
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or
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<I>tz</I>
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is NULL, the corresponding structure is not set or returned.
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(However, compilation warnings will result if
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<I>tv</I>
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is NULL.)
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<P>
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The use of the
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<I>timezone</I>
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structure is obsolete; the
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<I>tz</I>
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argument should normally be specified as NULL.
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(See NOTES below.)
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<P>
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Under Linux, there are some peculiar "warp clock" semantics associated
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with the
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<B>settimeofday</B>()
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system call if on the very first call (after booting)
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that has a non-NULL
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<I>tz</I>
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argument, the
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<I>tv</I>
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argument is NULL and the
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<I>tz_minuteswest</I>
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field is nonzero.
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(The
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<I>tz_dsttime</I>
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field should be zero for this case.)
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In such a case it is assumed that the CMOS clock
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is on local time, and that it has to be incremented by this amount
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to get UTC system time.
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No doubt it is a bad idea to use this feature.
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<A NAME="lbAE"> </A>
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<H2>RETURN VALUE</H2>
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<B>gettimeofday</B>()
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and
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<B>settimeofday</B>()
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return 0 for success, or -1 for failure (in which case
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<I>errno</I>
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is set appropriately).
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<A NAME="lbAF"> </A>
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<H2>ERRORS</H2>
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<DL COMPACT>
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<DT id="1"><B>EFAULT</B>
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<DD>
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One of
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<I>tv</I>
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or
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<I>tz</I>
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pointed outside the accessible address space.
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<DT id="2"><B>EINVAL</B>
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<DD>
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(<B>settimeofday</B>()):
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<I>timezone</I>
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is invalid.
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<DT id="3"><B>EINVAL</B>
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<DD>
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(<B>settimeofday</B>()):
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<I>tv.tv_sec</I>
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is negative or
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<I>tv.tv_usec</I>
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is outside the range [0..999,999].
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<DT id="4"><B>EINVAL</B> (since Linux 4.3)
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<DD>
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(<B>settimeofday</B>()):
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An attempt was made to set the time to a value less than
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the current value of the
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<B>CLOCK_MONOTONIC</B>
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clock (see
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<B><A HREF="/cgi-bin/man/man2html?2+clock_gettime">clock_gettime</A></B>(2)).
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<DT id="5"><B>EPERM</B>
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<DD>
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The calling process has insufficient privilege to call
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<B>settimeofday</B>();
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under Linux the
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<B>CAP_SYS_TIME</B>
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capability is required.
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</DL>
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<A NAME="lbAG"> </A>
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<H2>CONFORMING TO</H2>
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SVr4, 4.3BSD.
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POSIX.1-2001 describes
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<B>gettimeofday</B>()
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but not
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<B>settimeofday</B>().
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POSIX.1-2008 marks
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<B>gettimeofday</B>()
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as obsolete, recommending the use of
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<B><A HREF="/cgi-bin/man/man2html?2+clock_gettime">clock_gettime</A></B>(2)
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instead.
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<A NAME="lbAH"> </A>
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<H2>NOTES</H2>
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The time returned by
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<B>gettimeofday</B>()
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<I>is</I>
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affected by discontinuous jumps in the system time
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(e.g., if the system administrator manually changes the system time).
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If you need a monotonically increasing clock, see
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<B><A HREF="/cgi-bin/man/man2html?2+clock_gettime">clock_gettime</A></B>(2).
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<P>
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Macros for operating on
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<I>timeval</I>
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structures are described in
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<B><A HREF="/cgi-bin/man/man2html?3+timeradd">timeradd</A></B>(3).
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<P>
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Traditionally, the fields of
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<I>struct timeval</I>
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were of type
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<I>long</I>.
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<A NAME="lbAI"> </A>
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<H3>C library/kernel differences</H3>
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On some architectures, an implementation of
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<B>gettimeofday</B>()
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is provided in the
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<B><A HREF="/cgi-bin/man/man2html?7+vdso">vdso</A></B>(7).
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<A NAME="lbAJ"> </A>
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<H3>The tz_dsttime field</H3>
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On a non-Linux kernel, with glibc, the
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<I>tz_dsttime</I>
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field of
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<I>struct timezone</I>
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will be set to a nonzero value by
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<B>gettimeofday</B>()
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if the current timezone has ever had or will have a daylight saving
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rule applied.
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In this sense it exactly mirrors the meaning of
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<B><A HREF="/cgi-bin/man/man2html?3+daylight">daylight</A></B>(3)
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for the current zone.
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On Linux, with glibc, the setting of the
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<I>tz_dsttime</I>
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field of
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<I>struct timezone</I>
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has never been used by
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<B>settimeofday</B>()
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or
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<B>gettimeofday</B>().
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Thus, the following is purely of historical interest.
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<P>
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On old systems, the field
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<I>tz_dsttime</I>
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contains a symbolic constant (values are given below)
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that indicates in which part of the year Daylight Saving Time
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is in force.
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(Note: this value is constant throughout the year:
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it does not indicate that DST is in force, it just selects an
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algorithm.)
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The daylight saving time algorithms defined are as follows:
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<P>
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<B>DST_NONE</B> /* not on DST */
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<B>DST_USA</B> /* USA style DST */
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<B>DST_AUST</B> /* Australian style DST */
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<B>DST_WET</B> /* Western European DST */
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<B>DST_MET</B> /* Middle European DST */
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<B>DST_EET</B> /* Eastern European DST */
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<B>DST_CAN</B> /* Canada */
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<B>DST_GB</B> /* Great Britain and Eire */
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<B>DST_RUM</B> /* Romania */
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<B>DST_TUR</B> /* Turkey */
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<B>DST_AUSTALT</B> /* Australian style with shift in 1986 */
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<P>
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Of course it turned out that the period in which
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Daylight Saving Time is in force cannot be given
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by a simple algorithm, one per country; indeed,
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this period is determined by unpredictable political
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decisions.
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So this method of representing timezones
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has been abandoned.
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<A NAME="lbAK"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?1+date">date</A></B>(1),
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<B><A HREF="/cgi-bin/man/man2html?2+adjtimex">adjtimex</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+clock_gettime">clock_gettime</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+time">time</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?3+ctime">ctime</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+ftime">ftime</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+timeradd">timeradd</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?7+capabilities">capabilities</A></B>(7),
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<B><A HREF="/cgi-bin/man/man2html?7+time">time</A></B>(7),
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<B><A HREF="/cgi-bin/man/man2html?7+vdso">vdso</A></B>(7),
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<B><A HREF="/cgi-bin/man/man2html?8+hwclock">hwclock</A></B>(8)
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<A NAME="lbAL"> </A>
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<H2>COLOPHON</H2>
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This page is part of release 5.05 of the Linux
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<I>man-pages</I>
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project.
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A description of the project,
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information about reporting bugs,
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and the latest version of this page,
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can be found at
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<A HREF="https://www.kernel.org/doc/man-pages/.">https://www.kernel.org/doc/man-pages/.</A>
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<P>
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<HR>
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<A NAME="index"> </A><H2>Index</H2>
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<DL>
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<DT id="6"><A HREF="#lbAB">NAME</A><DD>
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<DT id="7"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="8"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="9"><A HREF="#lbAE">RETURN VALUE</A><DD>
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<DT id="10"><A HREF="#lbAF">ERRORS</A><DD>
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<DT id="11"><A HREF="#lbAG">CONFORMING TO</A><DD>
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<DT id="12"><A HREF="#lbAH">NOTES</A><DD>
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<DL>
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<DT id="13"><A HREF="#lbAI">C library/kernel differences</A><DD>
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<DT id="14"><A HREF="#lbAJ">The tz_dsttime field</A><DD>
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</DL>
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<DT id="15"><A HREF="#lbAK">SEE ALSO</A><DD>
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<DT id="16"><A HREF="#lbAL">COLOPHON</A><DD>
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</DL>
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<HR>
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This document was created by
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<A HREF="/cgi-bin/man/man2html">man2html</A>,
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using the manual pages.<BR>
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Time: 00:05:32 GMT, March 31, 2021
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