131 lines
6.2 KiB
HTML
131 lines
6.2 KiB
HTML
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- This file documents the GNU linker LD
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(Linaro_Binutils-2017.02)
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version 2.27.0.
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Copyright (C) 1991-2016 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 1.3
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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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<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>LD: Basic Script Concepts</title>
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<meta name="description" content="LD: Basic Script Concepts">
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<meta name="keywords" content="LD: Basic Script Concepts">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="LD-Index.html#LD-Index" rel="index" title="LD Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Scripts.html#Scripts" rel="up" title="Scripts">
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<link href="Script-Format.html#Script-Format" rel="next" title="Script Format">
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<link href="Scripts.html#Scripts" rel="prev" title="Scripts">
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<style type="text/css">
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<!--
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a.summary-letter {text-decoration: none}
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blockquote.smallquotation {font-size: smaller}
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div.display {margin-left: 3.2em}
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div.example {margin-left: 3.2em}
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div.smallexample {margin-left: 3.2em}
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div.smallindentedblock {margin-left: 3.2em; font-size: smaller}
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kbd {font-style:oblique}
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pre.display {font-family: inherit}
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pre.format {font-family: inherit}
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pre.menu-comment {font-family: serif}
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pre.menu-preformatted {font-family: serif}
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pre.smalldisplay {font-family: inherit; font-size: smaller}
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pre.smallexample {font-size: smaller}
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pre.smallformat {font-family: inherit; font-size: smaller}
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pre.smalllisp {font-size: smaller}
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span.nocodebreak {white-space:nowrap}
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span.nolinebreak {white-space:nowrap}
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span.roman {font-family:serif; font-weight:normal}
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span.sansserif {font-family:sans-serif; font-weight:normal}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Basic-Script-Concepts"></a>
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<div class="header">
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<p>
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Next: <a href="Script-Format.html#Script-Format" accesskey="n" rel="next">Script Format</a>, Up: <a href="Scripts.html#Scripts" accesskey="u" rel="up">Scripts</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>
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</div>
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<hr>
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<a name="Basic-Linker-Script-Concepts"></a>
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<h3 class="section">3.1 Basic Linker Script Concepts</h3>
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<a name="index-linker-script-concepts"></a>
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<p>We need to define some basic concepts and vocabulary in order to
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describe the linker script language.
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</p>
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<p>The linker combines input files into a single output file. The output
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file and each input file are in a special data format known as an
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<em>object file format</em>. Each file is called an <em>object file</em>.
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The output file is often called an <em>executable</em>, but for our
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purposes we will also call it an object file. Each object file has,
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among other things, a list of <em>sections</em>. We sometimes refer to a
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section in an input file as an <em>input section</em>; similarly, a section
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in the output file is an <em>output section</em>.
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</p>
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<p>Each section in an object file has a name and a size. Most sections
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also have an associated block of data, known as the <em>section
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contents</em>. A section may be marked as <em>loadable</em>, which means that
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the contents should be loaded into memory when the output file is run.
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A section with no contents may be <em>allocatable</em>, which means that an
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area in memory should be set aside, but nothing in particular should be
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loaded there (in some cases this memory must be zeroed out). A section
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which is neither loadable nor allocatable typically contains some sort
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of debugging information.
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</p>
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<p>Every loadable or allocatable output section has two addresses. The
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first is the <em>VMA</em>, or virtual memory address. This is the address
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the section will have when the output file is run. The second is the
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<em>LMA</em>, or load memory address. This is the address at which the
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section will be loaded. In most cases the two addresses will be the
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same. An example of when they might be different is when a data section
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is loaded into ROM, and then copied into RAM when the program starts up
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(this technique is often used to initialize global variables in a ROM
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based system). In this case the ROM address would be the LMA, and the
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RAM address would be the VMA.
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</p>
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<p>You can see the sections in an object file by using the <code>objdump</code>
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program with the ‘<samp>-h</samp>’ option.
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</p>
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<p>Every object file also has a list of <em>symbols</em>, known as the
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<em>symbol table</em>. A symbol may be defined or undefined. Each symbol
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has a name, and each defined symbol has an address, among other
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information. If you compile a C or C++ program into an object file, you
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will get a defined symbol for every defined function and global or
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static variable. Every undefined function or global variable which is
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referenced in the input file will become an undefined symbol.
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</p>
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<p>You can see the symbols in an object file by using the <code>nm</code>
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program, or by using the <code>objdump</code> program with the ‘<samp>-t</samp>’
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option.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Script-Format.html#Script-Format" accesskey="n" rel="next">Script Format</a>, Up: <a href="Scripts.html#Scripts" accesskey="u" rel="up">Scripts</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>
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</div>
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</body>
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</html>
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