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146 lines
6.4 KiB
HTML
146 lines
6.4 KiB
HTML
4 years ago
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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 BFD library.
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Copyright (C) 1991-2019 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 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being "GNU General Public License" and "Funding
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Free Software", the Front-Cover texts being (a) (see below), and with
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the Back-Cover Texts being (b) (see below). A copy of the license is
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included in the section entitled "GNU Free Documentation License".
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Section Output (Untitled Document)</title>
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<meta name="description" content="Section Output (Untitled Document)">
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<meta name="keywords" content="Section Output (Untitled Document)">
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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="BFD-Index.html#BFD-Index" rel="index" title="BFD Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Sections.html#Sections" rel="up" title="Sections">
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<link href="typedef-asection.html#typedef-asection" rel="next" title="typedef asection">
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<link href="Section-Input.html#Section-Input" rel="prev" title="Section Input">
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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.indentedblock {margin-right: 0em}
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blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
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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.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.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.nolinebreak {white-space: nowrap}
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span.roman {font-family: initial; 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">
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<a name="Section-Output"></a>
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<div class="header">
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<p>
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Next: <a href="typedef-asection.html#typedef-asection" accesskey="n" rel="next">typedef asection</a>, Previous: <a href="Section-Input.html#Section-Input" accesskey="p" rel="prev">Section Input</a>, Up: <a href="Sections.html#Sections" accesskey="u" rel="up">Sections</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="BFD-Index.html#BFD-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Section-output"></a>
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<h4 class="subsection">2.6.2 Section output</h4>
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<p>To write a new object style BFD, the various sections to be
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written have to be created. They are attached to the BFD in
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the same way as input sections; data is written to the
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sections using <code>bfd_set_section_contents</code>.
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</p>
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<p>Any program that creates or combines sections (e.g., the assembler
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and linker) must use the <code>asection</code> fields <code>output_section</code> and
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<code>output_offset</code> to indicate the file sections to which each
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section must be written. (If the section is being created from
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scratch, <code>output_section</code> should probably point to the section
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itself and <code>output_offset</code> should probably be zero.)
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</p>
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<p>The data to be written comes from input sections attached
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(via <code>output_section</code> pointers) to
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the output sections. The output section structure can be
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considered a filter for the input section: the output section
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determines the vma of the output data and the name, but the
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input section determines the offset into the output section of
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the data to be written.
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</p>
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<p>E.g., to create a section "O", starting at 0x100, 0x123 long,
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containing two subsections, "A" at offset 0x0 (i.e., at vma
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0x100) and "B" at offset 0x20 (i.e., at vma 0x120) the <code>asection</code>
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structures would look like:
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</p>
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<div class="example">
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<pre class="example"> section name "A"
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output_offset 0x00
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size 0x20
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output_section -----------> section name "O"
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| vma 0x100
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section name "B" | size 0x123
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output_offset 0x20 |
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size 0x103 |
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output_section --------|
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</pre></div>
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<a name="Link-orders"></a>
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<h4 class="subsection">2.6.3 Link orders</h4>
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<p>The data within a section is stored in a <em>link_order</em>.
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These are much like the fixups in <code>gas</code>. The link_order
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abstraction allows a section to grow and shrink within itself.
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</p>
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<p>A link_order knows how big it is, and which is the next
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link_order and where the raw data for it is; it also points to
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a list of relocations which apply to it.
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</p>
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<p>The link_order is used by the linker to perform relaxing on
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final code. The compiler creates code which is as big as
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necessary to make it work without relaxing, and the user can
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select whether to relax. Sometimes relaxing takes a lot of
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time. The linker runs around the relocations to see if any
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are attached to data which can be shrunk, if so it does it on
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a link_order by link_order basis.
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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="typedef-asection.html#typedef-asection" accesskey="n" rel="next">typedef asection</a>, Previous: <a href="Section-Input.html#Section-Input" accesskey="p" rel="prev">Section Input</a>, Up: <a href="Sections.html#Sections" accesskey="u" rel="up">Sections</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="BFD-Index.html#BFD-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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