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101 lines
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<link rel="up" href="What-BFD-Version-2-Can-Do.html#What-BFD-Version-2-Can-Do" title="What BFD Version 2 Can Do">
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This file documents the BFD library.
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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 or
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any later version published by the Free Software Foundation; with the
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Free Software'', the Front-Cover texts being (a) (see below), and with
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<a name="BFD-information-loss"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Canonical-format.html#Canonical-format">Canonical format</a>,
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Up: <a rel="up" accesskey="u" href="What-BFD-Version-2-Can-Do.html#What-BFD-Version-2-Can-Do">What BFD Version 2 Can Do</a>
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</div>
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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>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><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>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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</body></html>
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