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<title>LD: Miscellaneous Commands</title>
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<a name="Miscellaneous-Commands"></a>
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<div class="header">
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<p>
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Previous: <a href="REGION_005fALIAS.html#REGION_005fALIAS" accesskey="p" rel="previous">REGION_ALIAS</a>, Up: <a href="Simple-Commands.html#Simple-Commands" accesskey="u" rel="up">Simple Commands</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="LD-Index.html#LD-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Other-Linker-Script-Commands"></a>
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<h4 class="subsection">3.4.5 Other Linker Script Commands</h4>
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<p>There are a few other linker scripts commands.
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</p>
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<dl compact="compact">
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<dt><code>ASSERT(<var>exp</var>, <var>message</var>)</code></dt>
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<dd><a name="index-ASSERT"></a>
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<a name="index-assertion-in-linker-script"></a>
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<p>Ensure that <var>exp</var> is non-zero. If it is zero, then exit the linker
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with an error code, and print <var>message</var>.
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</p>
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<p>Note that assertions are checked before the final stages of linking
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take place. This means that expressions involving symbols PROVIDEd
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inside section definitions will fail if the user has not set values
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for those symbols. The only exception to this rule is PROVIDEd
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symbols that just reference dot. Thus an assertion like this:
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</p>
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<div class="smallexample">
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<pre class="smallexample"> .stack :
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{
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PROVIDE (__stack = .);
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PROVIDE (__stack_size = 0x100);
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ASSERT ((__stack > (_end + __stack_size)), "Error: No room left for the stack");
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}
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</pre></div>
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<p>will fail if <code>__stack_size</code> is not defined elsewhere. Symbols
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PROVIDEd outside of section definitions are evaluated earlier, so they
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can be used inside ASSERTions. Thus:
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</p>
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<div class="smallexample">
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<pre class="smallexample"> PROVIDE (__stack_size = 0x100);
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.stack :
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{
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PROVIDE (__stack = .);
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ASSERT ((__stack > (_end + __stack_size)), "Error: No room left for the stack");
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}
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</pre></div>
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<p>will work.
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</p>
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</dd>
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<dt><code>EXTERN(<var>symbol</var> <var>symbol</var> …)</code></dt>
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<dd><a name="index-EXTERN"></a>
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<a name="index-undefined-symbol-in-linker-script"></a>
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<p>Force <var>symbol</var> to be entered in the output file as an undefined
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symbol. Doing this may, for example, trigger linking of additional
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modules from standard libraries. You may list several <var>symbol</var>s for
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each <code>EXTERN</code>, and you may use <code>EXTERN</code> multiple times. This
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command has the same effect as the ‘<samp>-u</samp>’ command-line option.
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</p>
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</dd>
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<dt><code>FORCE_COMMON_ALLOCATION</code></dt>
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<dd><a name="index-FORCE_005fCOMMON_005fALLOCATION"></a>
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<a name="index-common-allocation-in-linker-script"></a>
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<p>This command has the same effect as the ‘<samp>-d</samp>’ command-line option:
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to make <code>ld</code> assign space to common symbols even if a relocatable
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output file is specified (‘<samp>-r</samp>’).
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</p>
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</dd>
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<dt><code>INHIBIT_COMMON_ALLOCATION</code></dt>
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<dd><a name="index-INHIBIT_005fCOMMON_005fALLOCATION"></a>
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<a name="index-common-allocation-in-linker-script-1"></a>
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<p>This command has the same effect as the ‘<samp>--no-define-common</samp>’
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command-line option: to make <code>ld</code> omit the assignment of addresses
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to common symbols even for a non-relocatable output file.
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</p>
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</dd>
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<dt><code>FORCE_GROUP_ALLOCATION</code></dt>
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<dd><a name="index-FORCE_005fGROUP_005fALLOCATION"></a>
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<a name="index-group-allocation-in-linker-script-1"></a>
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<a name="index-section-groups-1"></a>
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<a name="index-COMDAT-1"></a>
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<p>This command has the same effect as the
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‘<samp>--force-group-allocation</samp>’ command-line option: to make
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<code>ld</code> place section group members like normal input sections,
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and to delete the section groups even if a relocatable output file is
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specified (‘<samp>-r</samp>’).
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</p>
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</dd>
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<dt><code>INSERT [ AFTER | BEFORE ] <var>output_section</var></code></dt>
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<dd><a name="index-INSERT"></a>
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<a name="index-insert-user-script-into-default-script"></a>
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<p>This command is typically used in a script specified by ‘<samp>-T</samp>’ to
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augment the default <code>SECTIONS</code> with, for example, overlays. It
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inserts all prior linker script statements after (or before)
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<var>output_section</var>, and also causes ‘<samp>-T</samp>’ to not override the
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default linker script. The exact insertion point is as for orphan
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sections. See <a href="Location-Counter.html#Location-Counter">Location Counter</a>. The insertion happens after the
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linker has mapped input sections to output sections. Prior to the
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insertion, since ‘<samp>-T</samp>’ scripts are parsed before the default
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linker script, statements in the ‘<samp>-T</samp>’ script occur before the
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default linker script statements in the internal linker representation
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of the script. In particular, input section assignments will be made
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to ‘<samp>-T</samp>’ output sections before those in the default script. Here
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is an example of how a ‘<samp>-T</samp>’ script using <code>INSERT</code> might look:
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</p>
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<div class="smallexample">
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<pre class="smallexample">SECTIONS
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{
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OVERLAY :
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{
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.ov1 { ov1*(.text) }
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.ov2 { ov2*(.text) }
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}
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}
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INSERT AFTER .text;
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</pre></div>
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</dd>
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<dt><code>NOCROSSREFS(<var>section</var> <var>section</var> …)</code></dt>
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<dd><a name="index-NOCROSSREFS_0028sections_0029"></a>
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<a name="index-cross-references"></a>
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<p>This command may be used to tell <code>ld</code> to issue an error about any
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references among certain output sections.
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</p>
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<p>In certain types of programs, particularly on embedded systems when
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using overlays, when one section is loaded into memory, another section
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will not be. Any direct references between the two sections would be
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errors. For example, it would be an error if code in one section called
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a function defined in the other section.
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</p>
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<p>The <code>NOCROSSREFS</code> command takes a list of output section names. If
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<code>ld</code> detects any cross references between the sections, it reports
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an error and returns a non-zero exit status. Note that the
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<code>NOCROSSREFS</code> command uses output section names, not input section
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names.
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</p>
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</dd>
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<dt><code>NOCROSSREFS_TO(<var>tosection</var> <var>fromsection</var> …)</code></dt>
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<dd><a name="index-NOCROSSREFS_005fTO_0028tosection-fromsections_0029"></a>
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<a name="index-cross-references-1"></a>
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<p>This command may be used to tell <code>ld</code> to issue an error about any
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references to one section from a list of other sections.
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</p>
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<p>The <code>NOCROSSREFS</code> command is useful when ensuring that two or more
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output sections are entirely independent but there are situations where
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a one-way dependency is needed. For example, in a multi-core application
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there may be shared code that can be called from each core but for safety
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must never call back.
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</p>
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<p>The <code>NOCROSSREFS_TO</code> command takes a list of output section names.
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The first section can not be referenced from any of the other sections.
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If <code>ld</code> detects any references to the first section from any of
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the other sections, it reports an error and returns a non-zero exit
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status. Note that the <code>NOCROSSREFS_TO</code> command uses output section
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names, not input section names.
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</p>
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</dd>
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<dt><code>OUTPUT_ARCH(<var>bfdarch</var>)</code></dt>
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<dd><a name="index-OUTPUT_005fARCH_0028bfdarch_0029"></a>
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<a name="index-machine-architecture"></a>
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<a name="index-architecture"></a>
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<p>Specify a particular output machine architecture. The argument is one
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of the names used by the BFD library (see <a href="BFD.html#BFD">BFD</a>). You can see the
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architecture of an object file by using the <code>objdump</code> program with
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the ‘<samp>-f</samp>’ option.
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</p>
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</dd>
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<dt><code>LD_FEATURE(<var>string</var>)</code></dt>
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<dd><a name="index-LD_005fFEATURE_0028string_0029"></a>
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<p>This command may be used to modify <code>ld</code> behavior. If
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<var>string</var> is <code>"SANE_EXPR"</code> then absolute symbols and numbers
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in a script are simply treated as numbers everywhere.
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See <a href="Expression-Section.html#Expression-Section">Expression Section</a>.
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</p></dd>
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</dl>
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Previous: <a href="REGION_005fALIAS.html#REGION_005fALIAS" accesskey="p" rel="previous">REGION_ALIAS</a>, Up: <a href="Simple-Commands.html#Simple-Commands" accesskey="u" rel="up">Simple Commands</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="LD-Index.html#LD-Index" title="Index" rel="index">Index</a>]</p>
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