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174 lines
7.6 KiB
HTML
174 lines
7.6 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Location Counter - 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="Expressions.html#Expressions" title="Expressions">
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<link rel="prev" href="Orphan-Sections.html#Orphan-Sections" title="Orphan Sections">
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<link rel="next" href="Operators.html#Operators" title="Operators">
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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="Location-Counter"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Operators.html#Operators">Operators</a>,
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Previous: <a rel="previous" accesskey="p" href="Orphan-Sections.html#Orphan-Sections">Orphan Sections</a>,
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Up: <a rel="up" accesskey="u" href="Expressions.html#Expressions">Expressions</a>
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<hr>
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</div>
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<h4 class="subsection">3.10.5 The Location Counter</h4>
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<p><a name="index-g_t_002e-539"></a><a name="index-dot-540"></a><a name="index-location-counter-541"></a><a name="index-current-output-location-542"></a>The special linker variable <dfn>dot</dfn> ‘<samp><span class="samp">.</span></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><a name="index-holes-543"></a>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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<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>
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<p class="noindent">In the previous example, the ‘<samp><span class="samp">.text</span></samp>’ section from <samp><span class="file">file1</span></samp> is
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located at the beginning of the output section ‘<samp><span class="samp">output</span></samp>’. It is
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followed by a 1000 byte gap. Then the ‘<samp><span class="samp">.text</span></samp>’ section from
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<samp><span class="file">file2</span></samp> appears, also with a 1000 byte gap following before the
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‘<samp><span class="samp">.text</span></samp>’ section from <samp><span class="file">file3</span></samp>. The notation ‘<samp><span class="samp">= 0x12345678</span></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><a name="index-dot-inside-sections-544"></a>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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<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>
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<p>The ‘<samp><span class="samp">.text</span></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><span class="samp">.text</span></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><span class="samp">.data</span></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><span class="samp">.data</span></samp>’ input sections and before the end of
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the ‘<samp><span class="samp">.data</span></samp>’ output section itself.
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<p><a name="index-dot-outside-sections-545"></a>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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<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>
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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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<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>
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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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<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>
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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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</body></html>
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