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133 lines
6.3 KiB
HTML
133 lines
6.3 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>BFD information loss (Untitled Document)</title>
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<meta name="description" content="BFD information loss (Untitled Document)">
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<meta name="keywords" content="BFD information loss (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="What-BFD-Version-2-Can-Do.html#What-BFD-Version-2-Can-Do" rel="up" title="What BFD Version 2 Can Do">
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<link href="Canonical-format.html#Canonical-format" rel="next" title="Canonical format">
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<link href="What-BFD-Version-2-Can-Do.html#What-BFD-Version-2-Can-Do" rel="prev" title="What BFD Version 2 Can Do">
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<style type="text/css">
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a.summary-letter {text-decoration: none}
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div.example {margin-left: 3.2em}
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kbd {font-style: oblique}
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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="BFD-information-loss"></a>
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<div class="header">
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<p>
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Next: <a href="Canonical-format.html#Canonical-format" accesskey="n" rel="next">Canonical format</a>, Up: <a href="What-BFD-Version-2-Can-Do.html#What-BFD-Version-2-Can-Do" accesskey="u" rel="up">What BFD Version 2 Can Do</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="Information-Loss"></a>
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<h4 class="subsection">1.3.1 Information Loss</h4>
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<p><em>Information can be lost during output.</em> The output formats
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supported by BFD do not provide identical facilities, and
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information which can be described in one form has nowhere to go in
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another format. One example of this is alignment information in
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<code>b.out</code>. There is nowhere in an <code>a.out</code> format file to store
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alignment information on the contained data, so when a file is linked
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from <code>b.out</code> and an <code>a.out</code> image is produced, alignment
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information will not propagate to the output file. (The linker will
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still use the alignment information internally, so the link is performed
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correctly).
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</p>
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<p>Another example is COFF section names. COFF files may contain an
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unlimited number of sections, each one with a textual section name. If
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the target of the link is a format which does not have many sections (e.g.,
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<code>a.out</code>) or has sections without names (e.g., the Oasys format), the
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link cannot be done simply. You can circumvent this problem by
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describing the desired input-to-output section mapping with the linker command
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language.
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</p>
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<p><em>Information can be lost during canonicalization.</em> The BFD
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internal canonical form of the external formats is not exhaustive; there
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are structures in input formats for which there is no direct
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representation internally. This means that the BFD back ends
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cannot maintain all possible data richness through the transformation
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between external to internal and back to external formats.
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</p>
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<p>This limitation is only a problem when an application reads one
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format and writes another. Each BFD back end is responsible for
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maintaining as much data as possible, and the internal BFD
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canonical form has structures which are opaque to the BFD core,
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and exported only to the back ends. When a file is read in one format,
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the canonical form is generated for BFD and the application. At the
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same time, the back end saves away any information which may otherwise
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be lost. If the data is then written back in the same format, the back
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end routine will be able to use the canonical form provided by the
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BFD core as well as the information it prepared earlier. Since
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there is a great deal of commonality between back ends,
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there is no information lost when
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linking or copying big endian COFF to little endian COFF, or <code>a.out</code> to
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<code>b.out</code>. When a mixture of formats is linked, the information is
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only lost from the files whose format differs from the destination.
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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="Canonical-format.html#Canonical-format" accesskey="n" rel="next">Canonical format</a>, Up: <a href="What-BFD-Version-2-Can-Do.html#What-BFD-Version-2-Can-Do" accesskey="u" rel="up">What BFD Version 2 Can Do</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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