1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<!-- This file documents the GNU linker LD 4(GNU Toolchain for the A-profile Architecture 10.3-2021.07 (arm-10.29)) 5version 2.36.1. 6 7Copyright (C) 1991-2021 Free Software Foundation, Inc. 8 9Permission is granted to copy, distribute and/or modify this document 10under the terms of the GNU Free Documentation License, Version 1.3 11or any later version published by the Free Software Foundation; 12with no Invariant Sections, with no Front-Cover Texts, and with no 13Back-Cover Texts. A copy of the license is included in the 14section entitled "GNU Free Documentation License". --> 15<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> 16<head> 17<title>LD: Evaluation</title> 18 19<meta name="description" content="LD: Evaluation"> 20<meta name="keywords" content="LD: Evaluation"> 21<meta name="resource-type" content="document"> 22<meta name="distribution" content="global"> 23<meta name="Generator" content="makeinfo"> 24<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> 25<link href="index.html#Top" rel="start" title="Top"> 26<link href="LD-Index.html#LD-Index" rel="index" title="LD Index"> 27<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> 28<link href="Expressions.html#Expressions" rel="up" title="Expressions"> 29<link href="Expression-Section.html#Expression-Section" rel="next" title="Expression Section"> 30<link href="Operators.html#Operators" rel="previous" title="Operators"> 31<style type="text/css"> 32<!-- 33a.summary-letter {text-decoration: none} 34blockquote.smallquotation {font-size: smaller} 35div.display {margin-left: 3.2em} 36div.example {margin-left: 3.2em} 37div.indentedblock {margin-left: 3.2em} 38div.lisp {margin-left: 3.2em} 39div.smalldisplay {margin-left: 3.2em} 40div.smallexample {margin-left: 3.2em} 41div.smallindentedblock {margin-left: 3.2em; font-size: smaller} 42div.smalllisp {margin-left: 3.2em} 43kbd {font-style:oblique} 44pre.display {font-family: inherit} 45pre.format {font-family: inherit} 46pre.menu-comment {font-family: serif} 47pre.menu-preformatted {font-family: serif} 48pre.smalldisplay {font-family: inherit; font-size: smaller} 49pre.smallexample {font-size: smaller} 50pre.smallformat {font-family: inherit; font-size: smaller} 51pre.smalllisp {font-size: smaller} 52span.nocodebreak {white-space:nowrap} 53span.nolinebreak {white-space:nowrap} 54span.roman {font-family:serif; font-weight:normal} 55span.sansserif {font-family:sans-serif; font-weight:normal} 56ul.no-bullet {list-style: none} 57--> 58</style> 59 60 61</head> 62 63<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> 64<a name="Evaluation"></a> 65<div class="header"> 66<p> 67Next: <a href="Expression-Section.html#Expression-Section" accesskey="n" rel="next">Expression Section</a>, Previous: <a href="Operators.html#Operators" accesskey="p" rel="previous">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> 68</div> 69<hr> 70<a name="Evaluation-1"></a> 71<h4 class="subsection">3.10.7 Evaluation</h4> 72<a name="index-lazy-evaluation"></a> 73<a name="index-expression-evaluation-order"></a> 74<p>The linker evaluates expressions lazily. It only computes the value of 75an expression when absolutely necessary. 76</p> 77<p>The linker needs some information, such as the value of the start 78address of the first section, and the origins and lengths of memory 79regions, in order to do any linking at all. These values are computed 80as soon as possible when the linker reads in the linker script. 81</p> 82<p>However, other values (such as symbol values) are not known or needed 83until after storage allocation. Such values are evaluated later, when 84other information (such as the sizes of output sections) is available 85for use in the symbol assignment expression. 86</p> 87<p>The sizes of sections cannot be known until after allocation, so 88assignments dependent upon these are not performed until after 89allocation. 90</p> 91<p>Some expressions, such as those depending upon the location counter 92‘<samp>.</samp>’, must be evaluated during section allocation. 93</p> 94<p>If the result of an expression is required, but the value is not 95available, then an error results. For example, a script like the 96following 97</p><div class="smallexample"> 98<pre class="smallexample">SECTIONS 99 { 100 .text 9+this_isnt_constant : 101 { *(.text) } 102 } 103</pre></div> 104<p>will cause the error message ‘<samp>non constant expression for initial 105address</samp>’. 106</p> 107 108 109 110</body> 111</html> 112