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117 lines
6.7 KiB
HTML
117 lines
6.7 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Simple Example - 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="prev" href="Script-Format.html#Script-Format" title="Script Format">
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<link rel="next" href="Simple-Commands.html#Simple-Commands" title="Simple Commands">
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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.smalldisplay { font-family:inherit; font-size:smaller }
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pre.smallformat { font-family:inherit; font-size:smaller }
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pre.smallexample { font-size:smaller }
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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="Simple-Example"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Simple-Commands.html#Simple-Commands">Simple Commands</a>,
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Previous: <a rel="previous" accesskey="p" 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">3.3 Simple Linker Script Example</h3>
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<p><a name="index-linker-script-example-369"></a><a name="index-example-of-linker-script-370"></a>Many linker scripts are fairly simple.
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<p>The simplest possible linker script has just one command:
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‘<samp><span class="samp">SECTIONS</span></samp>’. You use the ‘<samp><span class="samp">SECTIONS</span></samp>’ command to describe the
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memory layout of the output file.
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<p>The ‘<samp><span class="samp">SECTIONS</span></samp>’ command is a powerful command. Here we will
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describe a simple use of it. Let's assume your program consists only of
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code, initialized data, and uninitialized data. These will be in the
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‘<samp><span class="samp">.text</span></samp>’, ‘<samp><span class="samp">.data</span></samp>’, and ‘<samp><span class="samp">.bss</span></samp>’ sections, respectively.
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Let's assume further that these are the only sections which appear in
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your input files.
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<p>For this example, let's say that the code should be loaded at address
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0x10000, and that the data should start at address 0x8000000. Here is a
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linker script which will do that:
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<pre class="smallexample"> SECTIONS
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{
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. = 0x10000;
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.text : { *(.text) }
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. = 0x8000000;
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.data : { *(.data) }
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.bss : { *(.bss) }
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}
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</pre>
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<p>You write the ‘<samp><span class="samp">SECTIONS</span></samp>’ command as the keyword ‘<samp><span class="samp">SECTIONS</span></samp>’,
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followed by a series of symbol assignments and output section
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descriptions enclosed in curly braces.
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<p>The first line inside the ‘<samp><span class="samp">SECTIONS</span></samp>’ command of the above example
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sets the value of the special symbol ‘<samp><span class="samp">.</span></samp>’, which is the location
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counter. If you do not specify the address of an output section in some
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other way (other ways are described later), the address is set from the
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current value of the location counter. The location counter is then
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incremented by the size of the output section. At the start of the
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‘<samp><span class="samp">SECTIONS</span></samp>’ command, the location counter has the value ‘<samp><span class="samp">0</span></samp>’.
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<p>The second line defines an output section, ‘<samp><span class="samp">.text</span></samp>’. The colon is
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required syntax which may be ignored for now. Within the curly braces
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after the output section name, you list the names of the input sections
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which should be placed into this output section. The ‘<samp><span class="samp">*</span></samp>’ is a
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wildcard which matches any file name. The expression ‘<samp><span class="samp">*(.text)</span></samp>’
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means all ‘<samp><span class="samp">.text</span></samp>’ input sections in all input files.
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<p>Since the location counter is ‘<samp><span class="samp">0x10000</span></samp>’ when the output section
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‘<samp><span class="samp">.text</span></samp>’ is defined, the linker will set the address of the
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‘<samp><span class="samp">.text</span></samp>’ section in the output file to be ‘<samp><span class="samp">0x10000</span></samp>’.
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<p>The remaining lines define the ‘<samp><span class="samp">.data</span></samp>’ and ‘<samp><span class="samp">.bss</span></samp>’ sections in
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the output file. The linker will place the ‘<samp><span class="samp">.data</span></samp>’ output section
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at address ‘<samp><span class="samp">0x8000000</span></samp>’. After the linker places the ‘<samp><span class="samp">.data</span></samp>’
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output section, the value of the location counter will be
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‘<samp><span class="samp">0x8000000</span></samp>’ plus the size of the ‘<samp><span class="samp">.data</span></samp>’ output section. The
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effect is that the linker will place the ‘<samp><span class="samp">.bss</span></samp>’ output section
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immediately after the ‘<samp><span class="samp">.data</span></samp>’ output section in memory.
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<p>The linker will ensure that each output section has the required
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alignment, by increasing the location counter if necessary. In this
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example, the specified addresses for the ‘<samp><span class="samp">.text</span></samp>’ and ‘<samp><span class="samp">.data</span></samp>’
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sections will probably satisfy any alignment constraints, but the linker
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may have to create a small gap between the ‘<samp><span class="samp">.data</span></samp>’ and ‘<samp><span class="samp">.bss</span></samp>’
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sections.
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<p>That's it! That's a simple and complete linker script.
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</body></html>
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