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<a name="User-GC"></a>
<div class="header">
<p>
Next: <a href="GGC-Roots.html#GGC-Roots" accesskey="n" rel="next">GGC Roots</a>, Previous: <a href="Inheritance-and-GTY.html#Inheritance-and-GTY" accesskey="p" rel="prev">Inheritance and GTY</a>, Up: <a href="Type-Information.html#Type-Information" accesskey="u" rel="up">Type Information</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Support-for-user_002dprovided-GC-marking-routines"></a>
<h3 class="section">23.3 Support for user-provided GC marking routines</h3>
<a name="index-user-gc"></a>
<p>The garbage collector supports types for which no automatic marking
code is generated.  For these types, the user is required to provide
three functions: one to act as a marker for garbage collection, and
two functions to act as marker and pointer walker for pre-compiled
headers.
</p>
<p>Given a structure <code>struct GTY((user)) my_struct</code>, the following functions
should be defined to mark <code>my_struct</code>:
</p>
<div class="smallexample">
<pre class="smallexample">void gt_ggc_mx (my_struct *p)
{
  /* This marks field 'fld'.  */
  gt_ggc_mx (p-&gt;fld);
}

void gt_pch_nx (my_struct *p)
{
  /* This marks field 'fld'.  */
  gt_pch_nx (tp-&gt;fld);
}

void gt_pch_nx (my_struct *p, gt_pointer_operator op, void *cookie)
{
  /* For every field 'fld', call the given pointer operator.  */
  op (&amp;(tp-&gt;fld), cookie);
}
</pre></div>

<p>In general, each marker <code>M</code> should call <code>M</code> for every
pointer field in the structure.  Fields that are not allocated in GC
or are not pointers must be ignored.
</p>
<p>For embedded lists (e.g., structures with a <code>next</code> or <code>prev</code>
pointer), the marker must follow the chain and mark every element in
it.
</p>
<p>Note that the rules for the pointer walker <code>gt_pch_nx (my_struct
*, gt_pointer_operator, void *)</code> are slightly different.  In this
case, the operation <code>op</code> must be applied to the <em>address</em> of
every pointer field.
</p>
<a name="User_002dprovided-marking-routines-for-template-types"></a>
<h4 class="subsection">23.3.1 User-provided marking routines for template types</h4>
<p>When a template type <code>TP</code> is marked with <code>GTY</code>, all
instances of that type are considered user-provided types.  This means
that the individual instances of <code>TP</code> do not need to be marked
with <code>GTY</code>.  The user needs to provide template functions to mark
all the fields of the type.
</p>
<p>The following code snippets represent all the functions that need to
be provided. Note that type <code>TP</code> may reference to more than one
type. In these snippets, there is only one type <code>T</code>, but there
could be more.
</p>
<div class="smallexample">
<pre class="smallexample">template&lt;typename T&gt;
void gt_ggc_mx (TP&lt;T&gt; *tp)
{
  extern void gt_ggc_mx (T&amp;);

  /* This marks field 'fld' of type 'T'.  */
  gt_ggc_mx (tp-&gt;fld);
}

template&lt;typename T&gt;
void gt_pch_nx (TP&lt;T&gt; *tp)
{
  extern void gt_pch_nx (T&amp;);

  /* This marks field 'fld' of type 'T'.  */
  gt_pch_nx (tp-&gt;fld);
}

template&lt;typename T&gt;
void gt_pch_nx (TP&lt;T *&gt; *tp, gt_pointer_operator op, void *cookie)
{
  /* For every field 'fld' of 'tp' with type 'T *', call the given
     pointer operator.  */
  op (&amp;(tp-&gt;fld), cookie);
}

template&lt;typename T&gt;
void gt_pch_nx (TP&lt;T&gt; *tp, gt_pointer_operator, void *cookie)
{
  extern void gt_pch_nx (T *, gt_pointer_operator, void *);

  /* For every field 'fld' of 'tp' with type 'T', call the pointer
     walker for all the fields of T.  */
  gt_pch_nx (&amp;(tp-&gt;fld), op, cookie);
}
</pre></div>

<p>Support for user-defined types is currently limited. The following
restrictions apply:
</p>
<ol>
<li> Type <code>TP</code> and all the argument types <code>T</code> must be
marked with <code>GTY</code>.

</li><li> Type <code>TP</code> can only have type names in its argument list.

</li><li> The pointer walker functions are different for <code>TP&lt;T&gt;</code> and
<code>TP&lt;T *&gt;</code>. In the case of <code>TP&lt;T&gt;</code>, references to
<code>T</code> must be handled by calling <code>gt_pch_nx</code> (which
will, in turn, walk all the pointers inside fields of <code>T</code>).
In the case of <code>TP&lt;T *&gt;</code>, references to <code>T *</code> must be
handled by calling the <code>op</code> function on the address of the
pointer (see the code snippets above).
</li></ol>

<hr>
<div class="header">
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Next: <a href="GGC-Roots.html#GGC-Roots" accesskey="n" rel="next">GGC Roots</a>, Previous: <a href="Inheritance-and-GTY.html#Inheritance-and-GTY" accesskey="p" rel="prev">Inheritance and GTY</a>, Up: <a href="Type-Information.html#Type-Information" accesskey="u" rel="up">Type Information</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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