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  1. <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

  2. <html>

  3. <!-- This file documents the BFD library.

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  17. A GNU Manual

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  27. <title>BFD information loss (Untitled Document)</title>

  28. 

  29. <meta name="description" content="BFD information loss (Untitled Document)">

  30. <meta name="keywords" content="BFD information loss (Untitled Document)">

  31. <meta name="resource-type" content="document">

  32. <meta name="distribution" content="global">

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  34. <link href="index.html#Top" rel="start" title="Top">

  35. <link href="BFD-Index.html#BFD-Index" rel="index" title="BFD Index">

  36. <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">

  37. <link href="What-BFD-Version-2-Can-Do.html#What-BFD-Version-2-Can-Do" rel="up" title="What BFD Version 2 Can Do">

  38. <link href="Canonical-format.html#Canonical-format" rel="next" title="Canonical format">

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  71. <body lang="en">

  72. <a name="BFD-information-loss"></a>

  73. <div class="header">

  74. <p>

  75. 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> &nbsp; [<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>

  76. </div>

  77. <hr>

  78. <a name="Information-Loss"></a>

  79. <h4 class="subsection">1.3.1 Information Loss</h4>

  80. 

  81. <p><em>Information can be lost during output.</em> The output formats

  82. supported by BFD do not provide identical facilities, and

  83. information which can be described in one form has nowhere to go in

  84. another format. One example of this is alignment information in

  85. <code>b.out</code>. There is nowhere in an <code>a.out</code> format file to store

  86. alignment information on the contained data, so when a file is linked

  87. from <code>b.out</code> and an <code>a.out</code> image is produced, alignment

  88. information will not propagate to the output file. (The linker will

  89. still use the alignment information internally, so the link is performed

  90. correctly).

  91. </p>

  92. <p>Another example is COFF section names. COFF files may contain an

  93. unlimited number of sections, each one with a textual section name. If

  94. the target of the link is a format which does not have many sections (e.g.,

  95. <code>a.out</code>) or has sections without names (e.g., the Oasys format), the

  96. link cannot be done simply. You can circumvent this problem by

  97. describing the desired input-to-output section mapping with the linker command

  98. language.

  99. </p>

  100. <p><em>Information can be lost during canonicalization.</em> The BFD

  101. internal canonical form of the external formats is not exhaustive; there

  102. are structures in input formats for which there is no direct

  103. representation internally.  This means that the BFD back ends

  104. cannot maintain all possible data richness through the transformation

  105. between external to internal and back to external formats.

  106. </p>

  107. <p>This limitation is only a problem when an application reads one

  108. format and writes another.  Each BFD back end is responsible for

  109. maintaining as much data as possible, and the internal BFD

  110. canonical form has structures which are opaque to the BFD core,

  111. and exported only to the back ends. When a file is read in one format,

  112. the canonical form is generated for BFD and the application. At the

  113. same time, the back end saves away any information which may otherwise

  114. be lost. If the data is then written back in the same format, the back

  115. end routine will be able to use the canonical form provided by the

  116. BFD core as well as the information it prepared earlier.  Since

  117. there is a great deal of commonality between back ends,

  118. there is no information lost when

  119. linking or copying big endian COFF to little endian COFF, or <code>a.out</code> to

  120. <code>b.out</code>.  When a mixture of formats is linked, the information is

  121. only lost from the files whose format differs from the destination.

  122. </p>

  123. <hr>

  124. <div class="header">

  125. <p>

  126. 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> &nbsp; [<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>

  127. </div>

  128. 

  129. 

  130. 

  131. </body>

  132. </html>