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140 lines
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140 lines
6.6 KiB
HTML
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<title>Expression Section - Untitled</title>
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<link rel="up" href="Expressions.html#Expressions" title="Expressions">
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<!--
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This file documents the GNU linker LD
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(GNU Binutils)
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version 2.26.
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Copyright (C) 1991-2015 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 1.3
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or any later version published by the Free Software Foundation;
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<div class="node">
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<a name="Expression-Section"></a>
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<p>
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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>
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</div>
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<h4 class="subsection">3.10.8 The Section of an Expression</h4>
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<p><a name="index-expression-sections-551"></a><a name="index-absolute-expressions-552"></a><a name="index-relative-expressions-553"></a><a name="index-absolute-and-relocatable-symbols-554"></a><a name="index-relocatable-and-absolute-symbols-555"></a><a name="index-symbols_002c-relocatable-and-absolute-556"></a>Addresses and symbols may be section relative, or absolute. A section
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relative symbol is relocatable. If you request relocatable output
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using the ‘<samp><span class="samp">-r</span></samp>’ option, a further link operation may change the
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value of a section relative symbol. On the other hand, an absolute
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symbol will retain the same value throughout any further link
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operations.
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<p>Some terms in linker expressions are addresses. This is true of
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section relative symbols and for builtin functions that return an
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address, such as <code>ADDR</code>, <code>LOADADDR</code>, <code>ORIGIN</code> and
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<code>SEGMENT_START</code>. Other terms are simply numbers, or are builtin
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functions that return a non-address value, such as <code>LENGTH</code>.
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One complication is that unless you set <code>LD_FEATURE ("SANE_EXPR")</code>
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(see <a href="Miscellaneous-Commands.html#Miscellaneous-Commands">Miscellaneous Commands</a>), numbers and absolute symbols are treated
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differently depending on their location, for compatibility with older
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versions of <code>ld</code>. Expressions appearing outside an output
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section definition treat all numbers as absolute addresses.
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Expressions appearing inside an output section definition treat
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absolute symbols as numbers. If <code>LD_FEATURE ("SANE_EXPR")</code> is
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given, then absolute symbols and numbers are simply treated as numbers
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everywhere.
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<p>In the following simple example,
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<pre class="smallexample"> SECTIONS
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{
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. = 0x100;
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__executable_start = 0x100;
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.data :
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{
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. = 0x10;
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__data_start = 0x10;
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*(.data)
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}
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...
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}
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</pre>
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<p>both <code>.</code> and <code>__executable_start</code> are set to the absolute
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address 0x100 in the first two assignments, then both <code>.</code> and
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<code>__data_start</code> are set to 0x10 relative to the <code>.data</code>
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section in the second two assignments.
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<p>For expressions involving numbers, relative addresses and absolute
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addresses, ld follows these rules to evaluate terms:
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<ul>
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<li>Unary operations on an absolute address or number, and binary
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operations on two absolute addresses or two numbers, or between one
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absolute address and a number, apply the operator to the value(s).
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<li>Unary operations on a relative address, and binary operations on two
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relative addresses in the same section or between one relative address
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and a number, apply the operator to the offset part of the address(es).
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<li>Other binary operations, that is, between two relative addresses not
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in the same section, or between a relative address and an absolute
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address, first convert any non-absolute term to an absolute address
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before applying the operator.
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</ul>
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<p>The result section of each sub-expression is as follows:
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<ul>
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<li>An operation involving only numbers results in a number.
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<li>The result of comparisons, ‘<samp><span class="samp">&&</span></samp>’ and ‘<samp><span class="samp">||</span></samp>’ is also a number.
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<li>The result of other binary arithmetic and logical operations on two
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relative addresses in the same section or two absolute addresses
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(after above conversions) is also a number.
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<li>The result of other operations on relative addresses or one
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relative address and a number, is a relative address in the same
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section as the relative operand(s).
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<li>The result of other operations on absolute addresses (after above
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conversions) is an absolute address.
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</ul>
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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 ‘<samp><span class="samp">.data</span></samp>’:
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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 ‘<samp><span class="samp">ABSOLUTE</span></samp>’ were not used, ‘<samp><span class="samp">_edata</span></samp>’ would be relative to the
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‘<samp><span class="samp">.data</span></samp>’ section.
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<p>Using <code>LOADADDR</code> also forces an expression absolute, since this
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particular builtin function returns an absolute address.
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</body></html>
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