220 lines
8.7 KiB
HTML
220 lines
8.7 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: Location Counter</title>
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<meta name="description" content="LD: Location Counter">
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<meta name="keywords" content="LD: Location Counter">
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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="Expressions.html#Expressions" rel="up" title="Expressions">
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<link href="Operators.html#Operators" rel="next" title="Operators">
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<link href="Orphan-Sections.html#Orphan-Sections" rel="prev" title="Orphan Sections">
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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.indentedblock {margin-left: 3.2em}
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div.lisp {margin-left: 3.2em}
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div.smalldisplay {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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div.smalllisp {margin-left: 3.2em}
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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="Location-Counter"></a>
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<div class="header">
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<p>
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Next: <a href="Operators.html#Operators" accesskey="n" rel="next">Operators</a>, Previous: <a href="Orphan-Sections.html#Orphan-Sections" accesskey="p" rel="prev">Orphan Sections</a>, Up: <a href="Expressions.html#Expressions" accesskey="u" rel="up">Expressions</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="The-Location-Counter"></a>
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<h4 class="subsection">3.10.5 The Location Counter</h4>
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<a name="index-_002e"></a>
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<a name="index-dot"></a>
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<a name="index-location-counter"></a>
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<a name="index-current-output-location"></a>
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<p>The special linker variable <em>dot</em> ‘<samp>.</samp>’ always contains the
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current output location counter. Since the <code>.</code> always refers to a
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location in an output section, it may only appear in an expression
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within a <code>SECTIONS</code> command. The <code>.</code> symbol may appear
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anywhere that an ordinary symbol is allowed in an expression.
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</p>
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<a name="index-holes"></a>
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<p>Assigning a value to <code>.</code> will cause the location counter to be
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moved. This may be used to create holes in the output section. The
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location counter may not be moved backwards inside an output section,
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and may not be moved backwards outside of an output section if so
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doing creates areas with overlapping LMAs.
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</p>
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<div class="smallexample">
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<pre class="smallexample">SECTIONS
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{
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output :
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{
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file1(.text)
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. = . + 1000;
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file2(.text)
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. += 1000;
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file3(.text)
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} = 0x12345678;
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}
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</pre></div>
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<p>In the previous example, the ‘<samp>.text</samp>’ section from <samp>file1</samp> is
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located at the beginning of the output section ‘<samp>output</samp>’. It is
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followed by a 1000 byte gap. Then the ‘<samp>.text</samp>’ section from
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<samp>file2</samp> appears, also with a 1000 byte gap following before the
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‘<samp>.text</samp>’ section from <samp>file3</samp>. The notation ‘<samp>= 0x12345678</samp>’
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specifies what data to write in the gaps (see <a href="Output-Section-Fill.html#Output-Section-Fill">Output Section Fill</a>).
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</p>
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<a name="index-dot-inside-sections"></a>
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<p>Note: <code>.</code> actually refers to the byte offset from the start of the
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current containing object. Normally this is the <code>SECTIONS</code>
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statement, whose start address is 0, hence <code>.</code> can be used as an
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absolute address. If <code>.</code> is used inside a section description
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however, it refers to the byte offset from the start of that section,
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not an absolute address. Thus in a script like this:
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</p>
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<div class="smallexample">
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<pre class="smallexample">SECTIONS
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{
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. = 0x100
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.text: {
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*(.text)
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. = 0x200
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}
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. = 0x500
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.data: {
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*(.data)
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. += 0x600
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}
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}
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</pre></div>
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<p>The ‘<samp>.text</samp>’ section will be assigned a starting address of 0x100
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and a size of exactly 0x200 bytes, even if there is not enough data in
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the ‘<samp>.text</samp>’ input sections to fill this area. (If there is too
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much data, an error will be produced because this would be an attempt to
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move <code>.</code> backwards). The ‘<samp>.data</samp>’ section will start at 0x500
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and it will have an extra 0x600 bytes worth of space after the end of
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the values from the ‘<samp>.data</samp>’ input sections and before the end of
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the ‘<samp>.data</samp>’ output section itself.
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</p>
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<a name="index-dot-outside-sections"></a>
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<p>Setting symbols to the value of the location counter outside of an
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output section statement can result in unexpected values if the linker
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needs to place orphan sections. For example, given the following:
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</p>
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<div class="smallexample">
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<pre class="smallexample">SECTIONS
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{
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start_of_text = . ;
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.text: { *(.text) }
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end_of_text = . ;
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start_of_data = . ;
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.data: { *(.data) }
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end_of_data = . ;
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}
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</pre></div>
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<p>If the linker needs to place some input section, e.g. <code>.rodata</code>,
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not mentioned in the script, it might choose to place that section
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between <code>.text</code> and <code>.data</code>. You might think the linker
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should place <code>.rodata</code> on the blank line in the above script, but
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blank lines are of no particular significance to the linker. As well,
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the linker doesn’t associate the above symbol names with their
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sections. Instead, it assumes that all assignments or other
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statements belong to the previous output section, except for the
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special case of an assignment to <code>.</code>. I.e., the linker will
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place the orphan <code>.rodata</code> section as if the script was written
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as follows:
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</p>
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<div class="smallexample">
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<pre class="smallexample">SECTIONS
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{
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start_of_text = . ;
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.text: { *(.text) }
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end_of_text = . ;
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start_of_data = . ;
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.rodata: { *(.rodata) }
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.data: { *(.data) }
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end_of_data = . ;
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}
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</pre></div>
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<p>This may or may not be the script author’s intention for the value of
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<code>start_of_data</code>. One way to influence the orphan section
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placement is to assign the location counter to itself, as the linker
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assumes that an assignment to <code>.</code> is setting the start address of
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a following output section and thus should be grouped with that
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section. So you could write:
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</p>
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<div class="smallexample">
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<pre class="smallexample">SECTIONS
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{
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start_of_text = . ;
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.text: { *(.text) }
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end_of_text = . ;
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. = . ;
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start_of_data = . ;
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.data: { *(.data) }
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end_of_data = . ;
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}
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</pre></div>
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<p>Now, the orphan <code>.rodata</code> section will be placed between
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<code>end_of_text</code> and <code>start_of_data</code>.
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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="Operators.html#Operators" accesskey="n" rel="next">Operators</a>, Previous: <a href="Orphan-Sections.html#Orphan-Sections" accesskey="p" rel="prev">Orphan Sections</a>, Up: <a href="Expressions.html#Expressions" accesskey="u" rel="up">Expressions</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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