1<html lang="en"> 2<head> 3<title>Expression Section - Untitled</title> 4<meta http-equiv="Content-Type" content="text/html"> 5<meta name="description" content="Untitled"> 6<meta name="generator" content="makeinfo 4.13"> 7<link title="Top" rel="start" href="index.html#Top"> 8<link rel="up" href="Expressions.html#Expressions" title="Expressions"> 9<link rel="prev" href="Evaluation.html#Evaluation" title="Evaluation"> 10<link rel="next" href="Builtin-Functions.html#Builtin-Functions" title="Builtin Functions"> 11<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> 12<!-- 13This file documents the GNU linker LD 14(Sourcery CodeBench Lite 2013.11-24) 15version 2.23.52. 16 17Copyright (C) 1991-2013 Free Software Foundation, Inc. 18 19Permission is granted to copy, distribute and/or modify this document 20under the terms of the GNU Free Documentation License, Version 1.3 21or any later version published by the Free Software Foundation; 22with no Invariant Sections, with no Front-Cover Texts, and with no 23Back-Cover Texts. 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A section 52relative symbol is relocatable. If you request relocatable output 53using the ‘<samp><span class="samp">-r</span></samp>’ option, a further link operation may change the 54value of a section relative symbol. On the other hand, an absolute 55symbol will retain the same value throughout any further link 56operations. 57 58 <p>Some terms in linker expressions are addresses. This is true of 59section relative symbols and for builtin functions that return an 60address, such as <code>ADDR</code>, <code>LOADADDR</code>, <code>ORIGIN</code> and 61<code>SEGMENT_START</code>. Other terms are simply numbers, or are builtin 62functions that return a non-address value, such as <code>LENGTH</code>. 63One complication is that unless you set <code>LD_FEATURE ("SANE_EXPR")</code> 64(see <a href="Miscellaneous-Commands.html#Miscellaneous-Commands">Miscellaneous Commands</a>), numbers and absolute symbols are treated 65differently depending on their location, for compatibility with older 66versions of <code>ld</code>. Expressions appearing outside an output 67section definition treat all numbers as absolute addresses. 68Expressions appearing inside an output section definition treat 69absolute symbols as numbers. If <code>LD_FEATURE ("SANE_EXPR")</code> is 70given, then absolute symbols and numbers are simply treated as numbers 71everywhere. 72 73 <p>In the following simple example, 74 75<pre class="smallexample"> SECTIONS 76 { 77 . = 0x100; 78 __executable_start = 0x100; 79 .data : 80 { 81 . = 0x10; 82 __data_start = 0x10; 83 *(.data) 84 } 85 ... 86 } 87</pre> 88 <p>both <code>.</code> and <code>__executable_start</code> are set to the absolute 89address 0x100 in the first two assignments, then both <code>.</code> and 90<code>__data_start</code> are set to 0x10 relative to the <code>.data</code> 91section in the second two assignments. 92 93 <p>For expressions involving numbers, relative addresses and absolute 94addresses, ld follows these rules to evaluate terms: 95 96 <ul> 97<li>Unary operations on an absolute address or number, and binary 98operations on two absolute addresses or two numbers, or between one 99absolute address and a number, apply the operator to the value(s). 100<li>Unary operations on a relative address, and binary operations on two 101relative addresses in the same section or between one relative address 102and a number, apply the operator to the offset part of the address(es). 103<li>Other binary operations, that is, between two relative addresses not 104in the same section, or between a relative address and an absolute 105address, first convert any non-absolute term to an absolute address 106before applying the operator. 107</ul> 108 109 <p>The result section of each sub-expression is as follows: 110 111 <ul> 112<li>An operation involving only numbers results in a number. 113<li>The result of comparisons, ‘<samp><span class="samp">&&</span></samp>’ and ‘<samp><span class="samp">||</span></samp>’ is also a number. 114<li>The result of other binary arithmetic and logical operations on two 115relative addresses in the same section or two absolute addresses 116(after above conversions) is also a number. 117<li>The result of other operations on relative addresses or one 118relative address and a number, is a relative address in the same 119section as the relative operand(s). 120<li>The result of other operations on absolute addresses (after above 121conversions) is an absolute address. 122</ul> 123 124 <p>You can use the builtin function <code>ABSOLUTE</code> to force an expression 125to be absolute when it would otherwise be relative. For example, to 126create an absolute symbol set to the address of the end of the output 127section ‘<samp><span class="samp">.data</span></samp>’: 128<pre class="smallexample"> SECTIONS 129 { 130 .data : { *(.data) _edata = ABSOLUTE(.); } 131 } 132</pre> 133 <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 134‘<samp><span class="samp">.data</span></samp>’ section. 135 136 <p>Using <code>LOADADDR</code> also forces an expression absolute, since this 137particular builtin function returns an absolute address. 138 139 </body></html> 140 141