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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.

  4. 

  5. Copyright (C) 1991-2020 Free Software Foundation, Inc.

  6. 

  7. Permission is granted to copy, distribute and/or modify this document

  8. under the terms of the GNU Free Documentation License, Version 1.3 or

  9. any later version published by the Free Software Foundation; with the

  10. Invariant Sections being "GNU General Public License" and "Funding

  11. Free Software", the Front-Cover texts being (a) (see below), and with

  12. the Back-Cover Texts being (b) (see below).  A copy of the license is

  13. included in the section entitled "GNU Free Documentation License".

  14. 

  15. (a) The FSF's Front-Cover Text is:

  16. 

  17. A GNU Manual

  18. 

  19. (b) The FSF's Back-Cover Text is:

  20. 

  21. You have freedom to copy and modify this GNU Manual, like GNU

  22.      software.  Copies published by the Free Software Foundation raise

  23.      funds for GNU development. -->

  24. <!-- Created by GNU Texinfo 6.5, http://www.gnu.org/software/texinfo/ -->

  25. <head>

  26. <meta http-equiv="Content-Type" content="text/html; charset=utf-8">

  27. <title>Section Output (Untitled Document)</title>

  28. 

  29. <meta name="description" content="Section Output (Untitled Document)">

  30. <meta name="keywords" content="Section Output (Untitled Document)">

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

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

  33. <meta name="Generator" content="makeinfo">

  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="Sections.html#Sections" rel="up" title="Sections">

  38. <link href="typedef-asection.html#typedef-asection" rel="next" title="typedef asection">

  39. <link href="Section-Input.html#Section-Input" rel="prev" title="Section Input">

  40. <style type="text/css">

  41. <!--

  42. a.summary-letter {text-decoration: none}

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  66. </style>

  67. 

  68. 

  69. </head>

  70. 

  71. <body lang="en">

  72. <a name="Section-Output"></a>

  73. <div class="header">

  74. <p>

  75. Next: <a href="typedef-asection.html#typedef-asection" accesskey="n" rel="next">typedef asection</a>, Previous: <a href="Section-Input.html#Section-Input" accesskey="p" rel="prev">Section Input</a>, Up: <a href="Sections.html#Sections" accesskey="u" rel="up">Sections</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="Section-output"></a>

  79. <h4 class="subsection">2.6.2 Section output</h4>

  80. <p>To write a new object style BFD, the various sections to be

  81. written have to be created. They are attached to the BFD in

  82. the same way as input sections; data is written to the

  83. sections using <code>bfd_set_section_contents</code>.

  84. </p>

  85. <p>Any program that creates or combines sections (e.g., the assembler

  86. and linker) must use the <code>asection</code> fields <code>output_section</code> and

  87. <code>output_offset</code> to indicate the file sections to which each

  88. section must be written.  (If the section is being created from

  89. scratch, <code>output_section</code> should probably point to the section

  90. itself and <code>output_offset</code> should probably be zero.)

  91. </p>

  92. <p>The data to be written comes from input sections attached

  93. (via <code>output_section</code> pointers) to

  94. the output sections.  The output section structure can be

  95. considered a filter for the input section: the output section

  96. determines the vma of the output data and the name, but the

  97. input section determines the offset into the output section of

  98. the data to be written.

  99. </p>

  100. <p>E.g., to create a section &quot;O&quot;, starting at 0x100, 0x123 long,

  101. containing two subsections, &quot;A&quot; at offset 0x0 (i.e., at vma

  102. 0x100) and &quot;B&quot; at offset 0x20 (i.e., at vma 0x120) the <code>asection</code>

  103. structures would look like:

  104. </p>

  105. <div class="example">

  106. <pre class="example">   section name          &quot;A&quot;

  107.      output_offset   0x00

  108.      size            0x20

  109.      output_section -----------&gt;  section name    &quot;O&quot;

  110.                              |    vma             0x100

  111.    section name          &quot;B&quot; |    size            0x123

  112.      output_offset   0x20    |

  113.      size            0x103   |

  114.      output_section  --------|

  115. </pre></div>

  116. 

  117. <a name="Link-orders"></a>

  118. <h4 class="subsection">2.6.3 Link orders</h4>

  119. <p>The data within a section is stored in a <em>link_order</em>.

  120. These are much like the fixups in <code>gas</code>.  The link_order

  121. abstraction allows a section to grow and shrink within itself.

  122. </p>

  123. <p>A link_order knows how big it is, and which is the next

  124. link_order and where the raw data for it is; it also points to

  125. a list of relocations which apply to it.

  126. </p>

  127. <p>The link_order is used by the linker to perform relaxing on

  128. final code.  The compiler creates code which is as big as

  129. necessary to make it work without relaxing, and the user can

  130. select whether to relax.  Sometimes relaxing takes a lot of

  131. time.  The linker runs around the relocations to see if any

  132. are attached to data which can be shrunk, if so it does it on

  133. a link_order by link_order basis.

  134. </p>

  135. 

  136. <hr>

  137. <div class="header">

  138. <p>

  139. Next: <a href="typedef-asection.html#typedef-asection" accesskey="n" rel="next">typedef asection</a>, Previous: <a href="Section-Input.html#Section-Input" accesskey="p" rel="prev">Section Input</a>, Up: <a href="Sections.html#Sections" accesskey="u" rel="up">Sections</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>

  140. </div>

  141. 

  142. 

  143. 

  144. </body>

  145. </html>