532 lines
13 KiB
HTML
532 lines
13 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of CLOCK_GETRES</TITLE>
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</HEAD><BODY>
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<H1>CLOCK_GETRES</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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clock_getres, clock_gettime, clock_settime - clock and time functions
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<B>#include <<A HREF="file:///usr/include/time.h">time.h</A>></B>
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<P>
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<B>int clock_getres(clockid_t </B><I>clk_id</I><B>, struct timespec *</B><I>res</I><B>);</B>
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<P>
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<B>int clock_gettime(clockid_t </B><I>clk_id</I><B>, struct timespec *</B><I>tp</I><B>);</B>
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<P>
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<B>int clock_settime(clockid_t </B><I>clk_id</I><B>, const struct timespec *</B><I>tp</I><B>);</B>
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<P>
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Link with <I>-lrt</I> (only for glibc versions before 2.17).
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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>clock_getres</B>(),
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<B>clock_gettime</B>(),
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<B>clock_settime</B>():
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<DL COMPACT><DT id="1"><DD>
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_POSIX_C_SOURCE >= 199309L
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</DL>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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The function
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<B>clock_getres</B>()
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finds the resolution (precision) of the specified clock
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<I>clk_id</I>,
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and, if
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<I>res</I>
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is non-NULL, stores it in the <I>struct timespec</I> pointed to by
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<I>res</I>.
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The resolution of clocks depends on the implementation and cannot be
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configured by a particular process.
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If the time value pointed to by the argument
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<I>tp</I>
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of
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<B>clock_settime</B>()
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is not a multiple of
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<I>res</I>,
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then it is truncated to a multiple of
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<I>res</I>.
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<P>
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The functions
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<B>clock_gettime</B>()
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and
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<B>clock_settime</B>()
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retrieve and set the time of the specified clock
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<I>clk_id</I>.
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<P>
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The
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<I>res</I>
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and
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<I>tp</I>
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arguments are
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<I>timespec</I>
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structures, as specified in
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<I><<A HREF="file:///usr/include/time.h">time.h</A>></I>:
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<P>
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struct timespec {
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<BR> time_t tv_sec; /* seconds */
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<BR> long tv_nsec; /* nanoseconds */
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};
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<P>
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The
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<I>clk_id</I>
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argument is the identifier of the particular clock on which to act.
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A clock may be system-wide and hence visible for all processes, or
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per-process if it measures time only within a single process.
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<P>
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All implementations support the system-wide real-time clock,
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which is identified by
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<B>CLOCK_REALTIME</B>.
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Its time represents seconds and nanoseconds since the Epoch.
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When its time is changed, timers for a relative interval are
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unaffected, but timers for an absolute point in time are affected.
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<P>
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More clocks may be implemented.
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The interpretation of the
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corresponding time values and the effect on timers is unspecified.
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<P>
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Sufficiently recent versions of glibc and the Linux kernel
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support the following clocks:
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<DL COMPACT>
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<DT id="2"><B>CLOCK_REALTIME</B>
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<DD>
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System-wide clock that measures real (i.e., wall-clock) time.
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Setting this clock requires appropriate privileges.
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This clock is affected by discontinuous jumps in the system time
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(e.g., if the system administrator manually changes the clock),
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and by the incremental adjustments performed by
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<B><A HREF="/cgi-bin/man/man2html?3+adjtime">adjtime</A></B>(3)
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and NTP.
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<DT id="3"><B>CLOCK_REALTIME_COARSE</B> (since Linux 2.6.32; Linux-specific)
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<DD>
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A faster but less precise version of
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<B>CLOCK_REALTIME</B>.
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Use when you need very fast, but not fine-grained timestamps.
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Requires per-architecture support,
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and probably also architecture support for this flag in the
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<B><A HREF="/cgi-bin/man/man2html?7+vdso">vdso</A></B>(7).
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<DT id="4"><B>CLOCK_MONOTONIC</B>
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<DD>
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Clock that cannot be set and represents monotonic time since---as described
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by POSIX---"some unspecified point in the past".
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On Linux, that point corresponds to the number of seconds that the system
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has been running since it was booted.
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<DT id="5"><DD>
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The
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<B>CLOCK_MONOTONIC</B>
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clock is not affected by discontinuous jumps in the system time
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(e.g., if the system administrator manually changes the clock),
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but is affected by the incremental adjustments performed by
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<B><A HREF="/cgi-bin/man/man2html?3+adjtime">adjtime</A></B>(3)
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and NTP.
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This clock does not count time that the system is suspended.
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<DT id="6"><B>CLOCK_MONOTONIC_COARSE</B> (since Linux 2.6.32; Linux-specific)
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<DD>
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A faster but less precise version of
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<B>CLOCK_MONOTONIC</B>.
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Use when you need very fast, but not fine-grained timestamps.
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Requires per-architecture support,
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and probably also architecture support for this flag in the
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<B><A HREF="/cgi-bin/man/man2html?7+vdso">vdso</A></B>(7).
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<DT id="7"><B>CLOCK_MONOTONIC_RAW</B> (since Linux 2.6.28; Linux-specific)
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<DD>
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Similar to
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<B>CLOCK_MONOTONIC</B>,
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but provides access to a raw hardware-based time
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that is not subject to NTP adjustments or
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the incremental adjustments performed by
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<B><A HREF="/cgi-bin/man/man2html?3+adjtime">adjtime</A></B>(3).
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This clock does not count time that the system is suspended.
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<DT id="8"><B>CLOCK_BOOTTIME</B> (since Linux 2.6.39; Linux-specific)
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<DD>
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Identical to
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<B>CLOCK_MONOTONIC</B>,
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except it also includes any time that the system is suspended.
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This allows applications to get a suspend-aware monotonic clock
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without having to deal with the complications of
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<B>CLOCK_REALTIME</B>,
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which may have discontinuities if the time is changed using
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<B><A HREF="/cgi-bin/man/man2html?2+settimeofday">settimeofday</A></B>(2)
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or similar.
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<DT id="9"><B>CLOCK_PROCESS_CPUTIME_ID</B> (since Linux 2.6.12)
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<DD>
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Per-process CPU-time clock
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(measures CPU time consumed by all threads in the process).
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<DT id="10"><B>CLOCK_THREAD_CPUTIME_ID</B> (since Linux 2.6.12)
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<DD>
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Thread-specific CPU-time clock.
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</DL>
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<A NAME="lbAE"> </A>
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<H2>RETURN VALUE</H2>
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<B>clock_gettime</B>(),
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<B>clock_settime</B>(),
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and
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<B>clock_getres</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="11"><B>EFAULT</B>
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<DD>
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<I>tp</I>
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points outside the accessible address space.
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<DT id="12"><B>EINVAL</B>
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<DD>
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The
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<I>clk_id</I>
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specified is not supported on this system.
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<DT id="13"><B>EINVAL</B>
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<DD>
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(<B>clock_settime</B>()):
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<I>tp.tv_sec</I>
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is negative or
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<I>tp.tv_nsec</I>
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is outside the range [0..999,999,999].
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<DT id="14"><B>EINVAL</B> (since Linux 4.3)
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<DD>
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A call to
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<B>clock_settime</B>()
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with a
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<I>clk_id</I>
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of
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<B>CLOCK_REALTIME</B>
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attempted 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.
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<DT id="15"><B>EPERM</B>
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<DD>
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<B>clock_settime</B>()
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does not have permission to set the clock indicated.
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</DL>
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<A NAME="lbAG"> </A>
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<H2>VERSIONS</H2>
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These system calls first appeared in Linux 2.6.
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<A NAME="lbAH"> </A>
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<H2>ATTRIBUTES</H2>
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For an explanation of the terms used in this section, see
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<B><A HREF="/cgi-bin/man/man2html?7+attributes">attributes</A></B>(7).
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<TABLE BORDER>
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<TR VALIGN=top><TD><B>Interface</B></TD><TD><B>Attribute</B></TD><TD><B>Value</B><BR></TD></TR>
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<TR VALIGN=top><TD>
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<B>clock_getres</B>(),
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<B>clock_gettime</B>(),
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<B>clock_settime</B>()
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</TD><TD>Thread safety</TD><TD>MT-Safe<BR></TD></TR>
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</TABLE>
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<P>
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<A NAME="lbAI"> </A>
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<H2>CONFORMING TO</H2>
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POSIX.1-2001, POSIX.1-2008, SUSv2.
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<A NAME="lbAJ"> </A>
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<H2>AVAILABILITY</H2>
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On POSIX systems on which these functions are available, the symbol
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<B>_POSIX_TIMERS</B>
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is defined in <I><<A HREF="file:///usr/include/unistd.h">unistd.h</A>></I> to a value greater than 0.
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The symbols
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<B>_POSIX_MONOTONIC_CLOCK</B>,
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<B>_POSIX_CPUTIME</B>,
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<B>_POSIX_THREAD_CPUTIME</B>
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indicate that
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<B>CLOCK_MONOTONIC</B>,
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<B>CLOCK_PROCESS_CPUTIME_ID</B>,
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<B>CLOCK_THREAD_CPUTIME_ID</B>
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are available.
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(See also
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<B><A HREF="/cgi-bin/man/man2html?3+sysconf">sysconf</A></B>(3).)
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<A NAME="lbAK"> </A>
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<H2>NOTES</H2>
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POSIX.1 specifies the following:
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<DL COMPACT><DT id="16"><DD>
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<P>
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Setting the value of the
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<B>CLOCK_REALTIME</B>
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clock via
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<B>clock_settime</B>()
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shall have no effect on threads that are blocked waiting for a relative time
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service based upon this clock, including the
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<B>nanosleep</B>()
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function; nor on the expiration of relative timers based upon this clock.
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Consequently, these time services shall expire when the requested relative
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interval elapses, independently of the new or old value of the clock.
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</DL>
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<A NAME="lbAL"> </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>clock_gettime</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="lbAM"> </A>
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<H3>Historical note for SMP systems</H3>
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Before Linux added kernel support for
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<B>CLOCK_PROCESS_CPUTIME_ID</B>
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and
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<B>CLOCK_THREAD_CPUTIME_ID</B>,
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glibc implemented these clocks on many platforms using timer
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registers from the CPUs
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(TSC on i386, AR.ITC on Itanium).
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These registers may differ between CPUs and as a consequence
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these clocks may return
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<B>bogus results</B>
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if a process is migrated to another CPU.
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<P>
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If the CPUs in an SMP system have different clock sources, then
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there is no way to maintain a correlation between the timer registers since
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each CPU will run at a slightly different frequency.
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If that is the case, then
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<I>clock_getcpuclockid(0)</I>
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will return
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<B>ENOENT</B>
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to signify this condition.
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The two clocks will then be useful only if it
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can be ensured that a process stays on a certain CPU.
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<P>
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The processors in an SMP system do not start all at exactly the same
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time and therefore the timer registers are typically running at an offset.
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Some architectures include code that attempts to limit these offsets on bootup.
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However, the code cannot guarantee to accurately tune the offsets.
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Glibc contains no provisions to deal with these offsets (unlike the Linux
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Kernel).
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Typically these offsets are small and therefore the effects may be
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negligible in most cases.
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<P>
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Since glibc 2.4,
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the wrapper functions for the system calls described in this page avoid
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the abovementioned problems by employing the kernel implementation of
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<B>CLOCK_PROCESS_CPUTIME_ID</B>
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and
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<B>CLOCK_THREAD_CPUTIME_ID</B>,
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on systems that provide such an implementation
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(i.e., Linux 2.6.12 and later).
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<A NAME="lbAN"> </A>
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<H2>BUGS</H2>
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According to POSIX.1-2001, a process with "appropriate privileges" may set the
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<B>CLOCK_PROCESS_CPUTIME_ID</B>
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and
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<B>CLOCK_THREAD_CPUTIME_ID</B>
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clocks using
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<B>clock_settime</B>().
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On Linux, these clocks are not settable
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(i.e., no process has "appropriate privileges").
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<A NAME="lbAO"> </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+gettimeofday">gettimeofday</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+settimeofday">settimeofday</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+adjtime">adjtime</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+clock_getcpuclockid">clock_getcpuclockid</A></B>(3),
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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+pthread_getcpuclockid">pthread_getcpuclockid</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+sysconf">sysconf</A></B>(3),
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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="lbAP"> </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="17"><A HREF="#lbAB">NAME</A><DD>
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<DT id="18"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="19"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="20"><A HREF="#lbAE">RETURN VALUE</A><DD>
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<DT id="21"><A HREF="#lbAF">ERRORS</A><DD>
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<DT id="22"><A HREF="#lbAG">VERSIONS</A><DD>
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<DT id="23"><A HREF="#lbAH">ATTRIBUTES</A><DD>
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<DT id="24"><A HREF="#lbAI">CONFORMING TO</A><DD>
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<DT id="25"><A HREF="#lbAJ">AVAILABILITY</A><DD>
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<DT id="26"><A HREF="#lbAK">NOTES</A><DD>
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<DL>
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<DT id="27"><A HREF="#lbAL">C library/kernel differences</A><DD>
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<DT id="28"><A HREF="#lbAM">Historical note for SMP systems</A><DD>
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</DL>
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<DT id="29"><A HREF="#lbAN">BUGS</A><DD>
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<DT id="30"><A HREF="#lbAO">SEE ALSO</A><DD>
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<DT id="31"><A HREF="#lbAP">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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</BODY>
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</HTML>
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