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<a name="Marking-Roots-for-the-Garbage-Collector"></a>
<h3 class="section">23.4 Marking Roots for the Garbage Collector</h3>
<a name="index-roots_002c-marking"></a>
<a name="index-marking-roots"></a>
<p>In addition to keeping track of types, the type machinery also locates
the global variables (<em>roots</em>) that the garbage collector starts
at. Roots must be declared using one of the following syntaxes:
</p>
<ul>
<li> <code>extern GTY(([<var>options</var>])) <var>type</var> <var>name</var>;</code>
</li><li> <code>static GTY(([<var>options</var>])) <var>type</var> <var>name</var>;</code>
</li></ul>
<p>The syntax
</p><ul>
<li> <code>GTY(([<var>options</var>])) <var>type</var> <var>name</var>;</code>
</li></ul>
<p>is <em>not</em> accepted. There should be an <code>extern</code> declaration
of such a variable in a header somewhere—mark that, not the
definition. Or, if the variable is only used in one file, make it
<code>static</code>.
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