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98 lines
4.7 KiB
HTML
98 lines
4.7 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Basic Script Concepts - Untitled</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="Untitled">
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<meta name="generator" content="makeinfo 4.13">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Scripts.html#Scripts" title="Scripts">
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<link rel="next" href="Script-Format.html#Script-Format" title="Script Format">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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This file documents the GNU linker LD
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(GNU Binutils)
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version 2.26.
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Copyright (C) 1991-2015 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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<meta http-equiv="Content-Style-Type" content="text/css">
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<style type="text/css"><!--
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pre.display { font-family:inherit }
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pre.format { font-family:inherit }
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pre.smalllisp { font-size:smaller }
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span.sc { font-variant:small-caps }
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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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--></style>
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</head>
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<body>
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<div class="node">
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<a name="Basic-Script-Concepts"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Script-Format.html#Script-Format">Script Format</a>,
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Up: <a rel="up" accesskey="u" href="Scripts.html#Scripts">Scripts</a>
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<hr>
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</div>
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<h3 class="section">3.1 Basic Linker Script Concepts</h3>
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<p><a name="index-linker-script-concepts-367"></a>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>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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<dfn>object file format</dfn>. Each file is called an <dfn>object file</dfn>.
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The output file is often called an <dfn>executable</dfn>, 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 <dfn>sections</dfn>. We sometimes refer to a
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section in an input file as an <dfn>input section</dfn>; similarly, a section
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in the output file is an <dfn>output section</dfn>.
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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 <dfn>section
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contents</dfn>. A section may be marked as <dfn>loadable</dfn>, 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 <dfn>allocatable</dfn>, 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>Every loadable or allocatable output section has two addresses. The
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first is the <dfn>VMA</dfn>, 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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<dfn>LMA</dfn>, 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>You can see the sections in an object file by using the <code>objdump</code>
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program with the ‘<samp><span class="samp">-h</span></samp>’ option.
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<p>Every object file also has a list of <dfn>symbols</dfn>, known as the
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<dfn>symbol table</dfn>. 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>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><span class="samp">-t</span></samp>’
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option.
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</body></html>
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