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<h3 class="section">22.3 Support for user-provided GC marking routines</h3>
<p><a name="index-user-gc-5094"></a>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>Given a structure <code>struct GTY((user)) my_struct</code>, the following functions
should be defined to mark <code>my_struct</code>:
<pre class="smallexample"> void gt_ggc_mx (my_struct *p)
{
/* This marks field 'fld'. */
gt_ggc_mx (p->fld);
}
void gt_pch_nx (my_struct *p)
{
/* This marks field 'fld'. */
gt_pch_nx (tp->fld);
}
void gt_pch_nx (my_struct *p, gt_pointer_operator op, void *cookie)
{
/* For every field 'fld', call the given pointer operator. */
op (&(tp->fld), cookie);
}
</pre>
<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>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>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.
<h4 class="subsection">22.3.1 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>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.
<pre class="smallexample"> template<typename T>
void gt_ggc_mx (TP<T> *tp)
{
extern void gt_ggc_mx (T&);
/* This marks field 'fld' of type 'T'. */
gt_ggc_mx (tp->fld);
}
template<typename T>
void gt_pch_nx (TP<T> *tp)
{
extern void gt_pch_nx (T&);
/* This marks field 'fld' of type 'T'. */
gt_pch_nx (tp->fld);
}
template<typename T>
void gt_pch_nx (TP<T *> *tp, gt_pointer_operator op, void *cookie)
{
/* For every field 'fld' of 'tp' with type 'T *', call the given
pointer operator. */
op (&(tp->fld), cookie);
}
template<typename T>
void gt_pch_nx (TP<T> *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 (&(tp->fld), op, cookie);
}
</pre>
<p>Support for user-defined types is currently limited. The following
restrictions apply:
<ol type=1 start=1>
<li>Type <code>TP</code> and all the argument types <code>T</code> must be
marked with <code>GTY</code>.
<li>Type <code>TP</code> can only have type names in its argument list.
<li>The pointer walker functions are different for <code>TP<T></code> and
<code>TP<T *></code>. In the case of <code>TP<T></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<T *></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).
</ol>
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