81 lines
3.9 KiB
HTML
81 lines
3.9 KiB
HTML
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<html lang="en">
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<head>
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<title>Expression Section - Untitled</title>
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<meta name="description" content="Untitled">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Expressions.html#Expressions" title="Expressions">
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<link rel="prev" href="Evaluation.html#Evaluation" title="Evaluation">
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<link rel="next" href="Builtin-Functions.html#Builtin-Functions" title="Builtin Functions">
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This file documents the GNU linker LD
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(GNU Binutils)
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version 2.19.
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Copyright (C) 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000,
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2001, 2002, 2003, 2004, 2005, 2006, 2007 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.1
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or any later version published by the Free Software Foundation;
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with no Invariant Sections, with no Front-Cover Texts, and with no
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Back-Cover Texts. A copy of the license is included in the
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<style type="text/css"><!--
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</head>
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<body>
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<div class="node">
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<p>
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<a name="Expression-Section"></a>Next: <a rel="next" accesskey="n" href="Builtin-Functions.html#Builtin-Functions">Builtin Functions</a>,
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Previous: <a rel="previous" accesskey="p" href="Evaluation.html#Evaluation">Evaluation</a>,
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Up: <a rel="up" accesskey="u" href="Expressions.html#Expressions">Expressions</a>
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<hr><br>
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</div>
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<h4 class="subsection">3.10.7 The Section of an Expression</h4>
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<p><a name="index-expression-sections-480"></a><a name="index-absolute-expressions-481"></a><a name="index-relative-expressions-482"></a><a name="index-absolute-and-relocatable-symbols-483"></a><a name="index-relocatable-and-absolute-symbols-484"></a><a name="index-symbols_002c-relocatable-and-absolute-485"></a>When the linker evaluates an expression, the result is either absolute
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or relative to some section. A relative expression is expressed as a
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fixed offset from the base of a section.
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<p>The position of the expression within the linker script determines
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whether it is absolute or relative. An expression which appears within
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an output section definition is relative to the base of the output
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section. An expression which appears elsewhere will be absolute.
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<p>A symbol set to a relative expression will be relocatable if you request
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relocatable output using the <span class="samp">-r</span> option. That means that a
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further link operation may change the value of the symbol. The symbol's
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section will be the section of the relative expression.
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<p>A symbol set to an absolute expression will retain the same value
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through any further link operation. The symbol will be absolute, and
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will not have any particular associated section.
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<p>You can use the builtin function <code>ABSOLUTE</code> to force an expression
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to be absolute when it would otherwise be relative. For example, to
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create an absolute symbol set to the address of the end of the output
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section <span class="samp">.data</span>:
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<pre class="smallexample"> SECTIONS
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{
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.data : { *(.data) _edata = ABSOLUTE(.); }
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}
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</pre>
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<p class="noindent">If <span class="samp">ABSOLUTE</span> were not used, <span class="samp">_edata</span> would be relative to the
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<span class="samp">.data</span> section.
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</body></html>
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