112 lines
4.5 KiB
HTML
112 lines
4.5 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
|
<html>
|
|
<!-- This file documents the GNU linker LD
|
|
(Linaro_Binutils-2017.02)
|
|
version 2.27.0.
|
|
|
|
Copyright (C) 1991-2016 Free Software Foundation, Inc.
|
|
|
|
Permission is granted to copy, distribute and/or modify this document
|
|
under the terms of the GNU Free Documentation License, Version 1.3
|
|
or any later version published by the Free Software Foundation;
|
|
with no Invariant Sections, with no Front-Cover Texts, and with no
|
|
Back-Cover Texts. A copy of the license is included in the
|
|
section entitled "GNU Free Documentation License". -->
|
|
<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
|
|
<head>
|
|
<title>LD: Evaluation</title>
|
|
|
|
<meta name="description" content="LD: Evaluation">
|
|
<meta name="keywords" content="LD: Evaluation">
|
|
<meta name="resource-type" content="document">
|
|
<meta name="distribution" content="global">
|
|
<meta name="Generator" content="makeinfo">
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
|
<link href="index.html#Top" rel="start" title="Top">
|
|
<link href="LD-Index.html#LD-Index" rel="index" title="LD Index">
|
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
|
<link href="Expressions.html#Expressions" rel="up" title="Expressions">
|
|
<link href="Expression-Section.html#Expression-Section" rel="next" title="Expression Section">
|
|
<link href="Operators.html#Operators" rel="prev" title="Operators">
|
|
<style type="text/css">
|
|
<!--
|
|
a.summary-letter {text-decoration: none}
|
|
blockquote.smallquotation {font-size: smaller}
|
|
div.display {margin-left: 3.2em}
|
|
div.example {margin-left: 3.2em}
|
|
div.indentedblock {margin-left: 3.2em}
|
|
div.lisp {margin-left: 3.2em}
|
|
div.smalldisplay {margin-left: 3.2em}
|
|
div.smallexample {margin-left: 3.2em}
|
|
div.smallindentedblock {margin-left: 3.2em; font-size: smaller}
|
|
div.smalllisp {margin-left: 3.2em}
|
|
kbd {font-style:oblique}
|
|
pre.display {font-family: inherit}
|
|
pre.format {font-family: inherit}
|
|
pre.menu-comment {font-family: serif}
|
|
pre.menu-preformatted {font-family: serif}
|
|
pre.smalldisplay {font-family: inherit; font-size: smaller}
|
|
pre.smallexample {font-size: smaller}
|
|
pre.smallformat {font-family: inherit; font-size: smaller}
|
|
pre.smalllisp {font-size: smaller}
|
|
span.nocodebreak {white-space:nowrap}
|
|
span.nolinebreak {white-space:nowrap}
|
|
span.roman {font-family:serif; font-weight:normal}
|
|
span.sansserif {font-family:sans-serif; font-weight:normal}
|
|
ul.no-bullet {list-style: none}
|
|
-->
|
|
</style>
|
|
|
|
|
|
</head>
|
|
|
|
<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
|
|
<a name="Evaluation"></a>
|
|
<div class="header">
|
|
<p>
|
|
Next: <a href="Expression-Section.html#Expression-Section" accesskey="n" rel="next">Expression Section</a>, Previous: <a href="Operators.html#Operators" accesskey="p" rel="prev">Operators</a>, Up: <a href="Expressions.html#Expressions" accesskey="u" rel="up">Expressions</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>
|
|
</div>
|
|
<hr>
|
|
<a name="Evaluation-1"></a>
|
|
<h4 class="subsection">3.10.7 Evaluation</h4>
|
|
<a name="index-lazy-evaluation"></a>
|
|
<a name="index-expression-evaluation-order"></a>
|
|
<p>The linker evaluates expressions lazily. It only computes the value of
|
|
an expression when absolutely necessary.
|
|
</p>
|
|
<p>The linker needs some information, such as the value of the start
|
|
address of the first section, and the origins and lengths of memory
|
|
regions, in order to do any linking at all. These values are computed
|
|
as soon as possible when the linker reads in the linker script.
|
|
</p>
|
|
<p>However, other values (such as symbol values) are not known or needed
|
|
until after storage allocation. Such values are evaluated later, when
|
|
other information (such as the sizes of output sections) is available
|
|
for use in the symbol assignment expression.
|
|
</p>
|
|
<p>The sizes of sections cannot be known until after allocation, so
|
|
assignments dependent upon these are not performed until after
|
|
allocation.
|
|
</p>
|
|
<p>Some expressions, such as those depending upon the location counter
|
|
‘<samp>.</samp>’, must be evaluated during section allocation.
|
|
</p>
|
|
<p>If the result of an expression is required, but the value is not
|
|
available, then an error results. For example, a script like the
|
|
following
|
|
</p><div class="smallexample">
|
|
<pre class="smallexample">SECTIONS
|
|
{
|
|
.text 9+this_isnt_constant :
|
|
{ *(.text) }
|
|
}
|
|
</pre></div>
|
|
<p>will cause the error message ‘<samp>non constant expression for initial
|
|
address</samp>’.
|
|
</p>
|
|
|
|
|
|
|
|
</body>
|
|
</html>
|