Symver.html 6.4 KB

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  63. <a name="g_t_002esymver"></a>
  64. <h3 class="section">7.90 <code>.symver</code></h3>
  65. <a name="index-symver-directive"></a>
  66. <a name="index-symbol-versioning"></a>
  67. <a name="index-versions-of-symbols"></a>
  68. <p>Use the <code>.symver</code> directive to bind symbols to specific version nodes
  69. within a source file. This is only supported on ELF platforms, and is
  70. typically used when assembling files to be linked into a shared library.
  71. There are cases where it may make sense to use this in objects to be bound
  72. into an application itself so as to override a versioned symbol from a
  73. shared library.
  74. </p>
  75. <p>For ELF targets, the <code>.symver</code> directive can be used like this:
  76. </p><div class="smallexample">
  77. <pre class="smallexample">.symver <var>name</var>, <var>name2@nodename</var>
  78. </pre></div>
  79. <p>If the symbol <var>name</var> is defined within the file
  80. being assembled, the <code>.symver</code> directive effectively creates a symbol
  81. alias with the name <var>name2@nodename</var>, and in fact the main reason that we
  82. just don&rsquo;t try and create a regular alias is that the <var>@</var> character isn&rsquo;t
  83. permitted in symbol names. The <var>name2</var> part of the name is the actual name
  84. of the symbol by which it will be externally referenced. The name <var>name</var>
  85. itself is merely a name of convenience that is used so that it is possible to
  86. have definitions for multiple versions of a function within a single source
  87. file, and so that the compiler can unambiguously know which version of a
  88. function is being mentioned. The <var>nodename</var> portion of the alias should be
  89. the name of a node specified in the version script supplied to the linker when
  90. building a shared library. If you are attempting to override a versioned
  91. symbol from a shared library, then <var>nodename</var> should correspond to the
  92. nodename of the symbol you are trying to override.
  93. </p>
  94. <p>If the symbol <var>name</var> is not defined within the file being assembled, all
  95. references to <var>name</var> will be changed to <var>name2@nodename</var>. If no
  96. reference to <var>name</var> is made, <var>name2@nodename</var> will be removed from the
  97. symbol table.
  98. </p>
  99. <p>Another usage of the <code>.symver</code> directive is:
  100. </p><div class="smallexample">
  101. <pre class="smallexample">.symver <var>name</var>, <var>name2@@nodename</var>
  102. </pre></div>
  103. <p>In this case, the symbol <var>name</var> must exist and be defined within
  104. the file being assembled. It is similar to <var>name2@nodename</var>. The
  105. difference is <var>name2@@nodename</var> will also be used to resolve
  106. references to <var>name2</var> by the linker.
  107. </p>
  108. <p>The third usage of the <code>.symver</code> directive is:
  109. </p><div class="smallexample">
  110. <pre class="smallexample">.symver <var>name</var>, <var>name2@@@nodename</var>
  111. </pre></div>
  112. <p>When <var>name</var> is not defined within the
  113. file being assembled, it is treated as <var>name2@nodename</var>. When
  114. <var>name</var> is defined within the file being assembled, the symbol
  115. name, <var>name</var>, will be changed to <var>name2@@nodename</var>.
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