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83 lines
3.5 KiB
HTML
83 lines
3.5 KiB
HTML
15 years ago
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<html lang="en">
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<head>
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<title>Evaluation - Untitled</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="Untitled">
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<meta name="generator" content="makeinfo 4.7">
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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="Operators.html#Operators" title="Operators">
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<link rel="next" href="Expression-Section.html#Expression-Section" title="Expression Section">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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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 2.19.
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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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section entitled ``GNU Free Documentation License''.-->
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<meta http-equiv="Content-Style-Type" content="text/css">
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<style type="text/css"><!--
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pre.display { font-family:inherit }
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pre.format { font-family:inherit }
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pre.smalldisplay { font-family:inherit; font-size:smaller }
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pre.smallformat { font-family:inherit; font-size:smaller }
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pre.smallexample { font-size:smaller }
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pre.smalllisp { font-size:smaller }
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span.sc { font-variant:small-caps }
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span.roman { font-family: serif; font-weight: normal; }
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--></style>
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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="Evaluation"></a>Next: <a rel="next" accesskey="n" href="Expression-Section.html#Expression-Section">Expression Section</a>,
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Previous: <a rel="previous" accesskey="p" href="Operators.html#Operators">Operators</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">3.10.6 Evaluation</h4>
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<p><a name="index-lazy-evaluation-478"></a><a name="index-expression-evaluation-order-479"></a>The linker evaluates expressions lazily. It only computes the value of
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an expression when absolutely necessary.
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<p>The linker needs some information, such as the value of the start
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address of the first section, and the origins and lengths of memory
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regions, in order to do any linking at all. These values are computed
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as soon as possible when the linker reads in the linker script.
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<p>However, other values (such as symbol values) are not known or needed
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until after storage allocation. Such values are evaluated later, when
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other information (such as the sizes of output sections) is available
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for use in the symbol assignment expression.
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<p>The sizes of sections cannot be known until after allocation, so
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assignments dependent upon these are not performed until after
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allocation.
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<p>Some expressions, such as those depending upon the location counter
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<span class="samp">.</span>, must be evaluated during section allocation.
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<p>If the result of an expression is required, but the value is not
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available, then an error results. For example, a script like the
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following
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<pre class="smallexample"> SECTIONS
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{
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.text 9+this_isnt_constant :
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{ *(.text) }
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}
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</pre>
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<p class="noindent">will cause the error message <span class="samp">non constant expression for initial
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address</span>.
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</body></html>
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