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<a name="BFD-information-loss"></a>Next: <a rel="next" accesskey="n" href="Canonical-format.html#Canonical-format">Canonical format</a>,
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<h4 class="subsection">1.3.1 Information Loss</h4>
<p><em>Information can be lost during output.</em> The output formats
supported by BFD do not provide identical facilities, and
information which can be described in one form has nowhere to go in
another format. One example of this is alignment information in
<code>b.out</code>. There is nowhere in an <code>a.out</code> format file to store
alignment information on the contained data, so when a file is linked
from <code>b.out</code> and an <code>a.out</code> image is produced, alignment
information will not propagate to the output file. (The linker will
still use the alignment information internally, so the link is performed
correctly).
<p>Another example is COFF section names. COFF files may contain an
unlimited number of sections, each one with a textual section name. If
the target of the link is a format which does not have many sections (e.g.,
<code>a.out</code>) or has sections without names (e.g., the Oasys format), the
link cannot be done simply. You can circumvent this problem by
describing the desired input-to-output section mapping with the linker command
language.
<p><em>Information can be lost during canonicalization.</em> The BFD
internal canonical form of the external formats is not exhaustive; there
are structures in input formats for which there is no direct
representation internally. This means that the BFD back ends
cannot maintain all possible data richness through the transformation
between external to internal and back to external formats.
<p>This limitation is only a problem when an application reads one
format and writes another. Each BFD back end is responsible for
maintaining as much data as possible, and the internal BFD
canonical form has structures which are opaque to the BFD core,
and exported only to the back ends. When a file is read in one format,
the canonical form is generated for BFD and the application. At the
same time, the back end saves away any information which may otherwise
be lost. If the data is then written back in the same format, the back
end routine will be able to use the canonical form provided by the
BFD core as well as the information it prepared earlier. Since
there is a great deal of commonality between back ends,
there is no information lost when
linking or copying big endian COFF to little endian COFF, or <code>a.out</code> to
<code>b.out</code>. When a mixture of formats is linked, the information is
only lost from the files whose format differs from the destination.
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