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: Expression Section</title> 18 19<meta name="description" content="LD: Expression Section"> 20<meta name="keywords" content="LD: Expression Section"> 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="Builtin-Functions.html#Builtin-Functions" rel="next" title="Builtin Functions"> 30<link href="Evaluation.html#Evaluation" rel="previous" title="Evaluation"> 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="Expression-Section"></a> 65<div class="header"> 66<p> 67Next: <a href="Builtin-Functions.html#Builtin-Functions" accesskey="n" rel="next">Builtin Functions</a>, Previous: <a href="Evaluation.html#Evaluation" accesskey="p" rel="previous">Evaluation</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="The-Section-of-an-Expression"></a> 71<h4 class="subsection">3.10.8 The Section of an Expression</h4> 72<a name="index-expression-sections"></a> 73<a name="index-absolute-expressions"></a> 74<a name="index-relative-expressions"></a> 75<a name="index-absolute-and-relocatable-symbols"></a> 76<a name="index-relocatable-and-absolute-symbols"></a> 77<a name="index-symbols_002c-relocatable-and-absolute"></a> 78<p>Addresses and symbols may be section relative, or absolute. A section 79relative symbol is relocatable. If you request relocatable output 80using the ‘<samp>-r</samp>’ option, a further link operation may change the 81value of a section relative symbol. On the other hand, an absolute 82symbol will retain the same value throughout any further link 83operations. 84</p> 85<p>Some terms in linker expressions are addresses. This is true of 86section relative symbols and for builtin functions that return an 87address, such as <code>ADDR</code>, <code>LOADADDR</code>, <code>ORIGIN</code> and 88<code>SEGMENT_START</code>. Other terms are simply numbers, or are builtin 89functions that return a non-address value, such as <code>LENGTH</code>. 90One complication is that unless you set <code>LD_FEATURE ("SANE_EXPR")</code> 91(see <a href="Miscellaneous-Commands.html#Miscellaneous-Commands">Miscellaneous Commands</a>), numbers and absolute symbols are treated 92differently depending on their location, for compatibility with older 93versions of <code>ld</code>. Expressions appearing outside an output 94section definition treat all numbers as absolute addresses. 95Expressions appearing inside an output section definition treat 96absolute symbols as numbers. If <code>LD_FEATURE ("SANE_EXPR")</code> is 97given, then absolute symbols and numbers are simply treated as numbers 98everywhere. 99</p> 100<p>In the following simple example, 101</p> 102<div class="smallexample"> 103<pre class="smallexample">SECTIONS 104 { 105 . = 0x100; 106 __executable_start = 0x100; 107 .data : 108 { 109 . = 0x10; 110 __data_start = 0x10; 111 *(.data) 112 } 113 … 114 } 115</pre></div> 116 117<p>both <code>.</code> and <code>__executable_start</code> are set to the absolute 118address 0x100 in the first two assignments, then both <code>.</code> and 119<code>__data_start</code> are set to 0x10 relative to the <code>.data</code> 120section in the second two assignments. 121</p> 122<p>For expressions involving numbers, relative addresses and absolute 123addresses, ld follows these rules to evaluate terms: 124</p> 125<ul> 126<li> Unary operations on an absolute address or number, and binary 127operations on two absolute addresses or two numbers, or between one 128absolute address and a number, apply the operator to the value(s). 129</li><li> Unary operations on a relative address, and binary operations on two 130relative addresses in the same section or between one relative address 131and a number, apply the operator to the offset part of the address(es). 132</li><li> Other binary operations, that is, between two relative addresses not 133in the same section, or between a relative address and an absolute 134address, first convert any non-absolute term to an absolute address 135before applying the operator. 136</li></ul> 137 138<p>The result section of each sub-expression is as follows: 139</p> 140<ul> 141<li> An operation involving only numbers results in a number. 142</li><li> The result of comparisons, ‘<samp>&&</samp>’ and ‘<samp>||</samp>’ is also a number. 143</li><li> The result of other binary arithmetic and logical operations on two 144relative addresses in the same section or two absolute addresses 145(after above conversions) is also a number when 146<code>LD_FEATURE ("SANE_EXPR")</code> or inside an output section definition 147but an absolute address otherwise. 148</li><li> The result of other operations on relative addresses or one 149relative address and a number, is a relative address in the same 150section as the relative operand(s). 151</li><li> The result of other operations on absolute addresses (after above 152conversions) is an absolute address. 153</li></ul> 154 155<p>You can use the builtin function <code>ABSOLUTE</code> to force an expression 156to be absolute when it would otherwise be relative. For example, to 157create an absolute symbol set to the address of the end of the output 158section ‘<samp>.data</samp>’: 159</p><div class="smallexample"> 160<pre class="smallexample">SECTIONS 161 { 162 .data : { *(.data) _edata = ABSOLUTE(.); } 163 } 164</pre></div> 165<p>If ‘<samp>ABSOLUTE</samp>’ were not used, ‘<samp>_edata</samp>’ would be relative to the 166‘<samp>.data</samp>’ section. 167</p> 168<p>Using <code>LOADADDR</code> also forces an expression absolute, since this 169particular builtin function returns an absolute address. 170</p> 171<hr> 172<div class="header"> 173<p> 174Next: <a href="Builtin-Functions.html#Builtin-Functions" accesskey="n" rel="next">Builtin Functions</a>, Previous: <a href="Evaluation.html#Evaluation" accesskey="p" rel="previous">Evaluation</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> 175</div> 176 177 178 179</body> 180</html> 181