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| <title>LD: BFD information loss</title> |
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| <a name="BFD-information-loss"></a> |
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| Next: <a href="Canonical-format.html#Canonical-format" accesskey="n" rel="next">Canonical format</a>, Up: <a href="BFD-outline.html#BFD-outline" accesskey="u" rel="up">BFD outline</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="LD-Index.html#LD-Index" title="Index" rel="index">Index</a>]</p> |
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| <a name="Information-Loss"></a> |
| <h4 class="subsection">5.1.1 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> |
| <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> |
| <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> |
| <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. |
| </p> |
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| Next: <a href="Canonical-format.html#Canonical-format" accesskey="n" rel="next">Canonical format</a>, Up: <a href="BFD-outline.html#BFD-outline" accesskey="u" rel="up">BFD outline</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="LD-Index.html#LD-Index" title="Index" rel="index">Index</a>]</p> |
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