234 lines
9.6 KiB
HTML
234 lines
9.6 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: PHDRS</title>
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<meta name="description" content="LD: PHDRS">
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<meta name="keywords" content="LD: PHDRS">
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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="VERSION.html#VERSION" rel="next" title="VERSION">
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<link href="MEMORY.html#MEMORY" rel="prev" title="MEMORY">
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<style type="text/css">
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<!--
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kbd {font-style:oblique}
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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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-->
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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="PHDRS"></a>
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<div class="header">
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<p>
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Next: <a href="VERSION.html#VERSION" accesskey="n" rel="next">VERSION</a>, Previous: <a href="MEMORY.html#MEMORY" accesskey="p" rel="prev">MEMORY</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="PHDRS-Command"></a>
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<h3 class="section">3.8 PHDRS Command</h3>
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<a name="index-PHDRS"></a>
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<a name="index-program-headers"></a>
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<a name="index-ELF-program-headers"></a>
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<a name="index-program-segments"></a>
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<a name="index-segments_002c-ELF"></a>
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<p>The ELF object file format uses <em>program headers</em>, also knows as
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<em>segments</em>. The program headers describe how the program should be
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loaded into memory. You can print them out by using the <code>objdump</code>
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program with the ‘<samp>-p</samp>’ option.
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</p>
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<p>When you run an ELF program on a native ELF system, the system loader
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reads the program headers in order to figure out how to load the
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program. This will only work if the program headers are set correctly.
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This manual does not describe the details of how the system loader
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interprets program headers; for more information, see the ELF ABI.
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</p>
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<p>The linker will create reasonable program headers by default. However,
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in some cases, you may need to specify the program headers more
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precisely. You may use the <code>PHDRS</code> command for this purpose. When
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the linker sees the <code>PHDRS</code> command in the linker script, it will
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not create any program headers other than the ones specified.
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</p>
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<p>The linker only pays attention to the <code>PHDRS</code> command when
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generating an ELF output file. In other cases, the linker will simply
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ignore <code>PHDRS</code>.
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</p>
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<p>This is the syntax of the <code>PHDRS</code> command. The words <code>PHDRS</code>,
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<code>FILEHDR</code>, <code>AT</code>, and <code>FLAGS</code> are keywords.
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</p>
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<div class="smallexample">
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<pre class="smallexample">PHDRS
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{
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<var>name</var> <var>type</var> [ FILEHDR ] [ PHDRS ] [ AT ( <var>address</var> ) ]
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[ FLAGS ( <var>flags</var> ) ] ;
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}
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</pre></div>
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<p>The <var>name</var> is used only for reference in the <code>SECTIONS</code> command
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of the linker script. It is not put into the output file. Program
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header names are stored in a separate name space, and will not conflict
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with symbol names, file names, or section names. Each program header
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must have a distinct name. The headers are processed in order and it
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is usual for them to map to sections in ascending load address order.
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</p>
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<p>Certain program header types describe segments of memory which the
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system loader will load from the file. In the linker script, you
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specify the contents of these segments by placing allocatable output
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sections in the segments. You use the ‘<samp>:<var>phdr</var></samp>’ output section
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attribute to place a section in a particular segment. See <a href="Output-Section-Phdr.html#Output-Section-Phdr">Output Section Phdr</a>.
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</p>
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<p>It is normal to put certain sections in more than one segment. This
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merely implies that one segment of memory contains another. You may
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repeat ‘<samp>:<var>phdr</var></samp>’, using it once for each segment which should
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contain the section.
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</p>
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<p>If you place a section in one or more segments using ‘<samp>:<var>phdr</var></samp>’,
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then the linker will place all subsequent allocatable sections which do
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not specify ‘<samp>:<var>phdr</var></samp>’ in the same segments. This is for
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convenience, since generally a whole set of contiguous sections will be
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placed in a single segment. You can use <code>:NONE</code> to override the
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default segment and tell the linker to not put the section in any
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segment at all.
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</p>
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<a name="index-FILEHDR"></a>
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<a name="index-PHDRS-1"></a>
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<p>You may use the <code>FILEHDR</code> and <code>PHDRS</code> keywords after
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the program header type to further describe the contents of the segment.
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The <code>FILEHDR</code> keyword means that the segment should include the ELF
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file header. The <code>PHDRS</code> keyword means that the segment should
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include the ELF program headers themselves. If applied to a loadable
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segment (<code>PT_LOAD</code>), all prior loadable segments must have one of
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these keywords.
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</p>
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<p>The <var>type</var> may be one of the following. The numbers indicate the
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value of the keyword.
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</p>
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<dl compact="compact">
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<dt><code>PT_NULL</code> (0)</dt>
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<dd><p>Indicates an unused program header.
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</p>
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</dd>
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<dt><code>PT_LOAD</code> (1)</dt>
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<dd><p>Indicates that this program header describes a segment to be loaded from
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the file.
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</p>
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</dd>
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<dt><code>PT_DYNAMIC</code> (2)</dt>
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<dd><p>Indicates a segment where dynamic linking information can be found.
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</p>
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</dd>
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<dt><code>PT_INTERP</code> (3)</dt>
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<dd><p>Indicates a segment where the name of the program interpreter may be
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found.
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</p>
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</dd>
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<dt><code>PT_NOTE</code> (4)</dt>
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<dd><p>Indicates a segment holding note information.
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</p>
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</dd>
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<dt><code>PT_SHLIB</code> (5)</dt>
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<dd><p>A reserved program header type, defined but not specified by the ELF
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ABI.
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</p>
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</dd>
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<dt><code>PT_PHDR</code> (6)</dt>
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<dd><p>Indicates a segment where the program headers may be found.
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</p>
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</dd>
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<dt><var>expression</var></dt>
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<dd><p>An expression giving the numeric type of the program header. This may
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be used for types not defined above.
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</p></dd>
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</dl>
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<p>You can specify that a segment should be loaded at a particular address
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in memory by using an <code>AT</code> expression. This is identical to the
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<code>AT</code> command used as an output section attribute (see <a href="Output-Section-LMA.html#Output-Section-LMA">Output Section LMA</a>). The <code>AT</code> command for a program header overrides the
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output section attribute.
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</p>
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<p>The linker will normally set the segment flags based on the sections
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which comprise the segment. You may use the <code>FLAGS</code> keyword to
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explicitly specify the segment flags. The value of <var>flags</var> must be
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an integer. It is used to set the <code>p_flags</code> field of the program
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header.
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</p>
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<p>Here is an example of <code>PHDRS</code>. This shows a typical set of program
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headers used on a native ELF system.
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</p>
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<div class="example">
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<pre class="example">PHDRS
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{
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headers PT_PHDR PHDRS ;
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interp PT_INTERP ;
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text PT_LOAD FILEHDR PHDRS ;
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data PT_LOAD ;
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dynamic PT_DYNAMIC ;
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}
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SECTIONS
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{
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. = SIZEOF_HEADERS;
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.interp : { *(.interp) } :text :interp
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.text : { *(.text) } :text
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.rodata : { *(.rodata) } /* defaults to :text */
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…
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. = . + 0x1000; /* move to a new page in memory */
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.data : { *(.data) } :data
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.dynamic : { *(.dynamic) } :data :dynamic
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…
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}
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</pre></div>
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<hr>
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<div class="header">
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<p>
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Next: <a href="VERSION.html#VERSION" accesskey="n" rel="next">VERSION</a>, Previous: <a href="MEMORY.html#MEMORY" accesskey="p" rel="prev">MEMORY</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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