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<title>GNU Compiler Collection (GCC) Internals: User GC</title>
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<a name="User-GC"></a>
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<div class="header">
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<p>
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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> [<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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</div>
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<hr>
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<a name="Support-for-user_002dprovided-GC-marking-routines"></a>
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<h3 class="section">22.3 Support for user-provided GC marking routines</h3>
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<a name="index-user-gc"></a>
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<p>The garbage collector supports types for which no automatic marking
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code is generated. For these types, the user is required to provide
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three functions: one to act as a marker for garbage collection, and
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two functions to act as marker and pointer walker for pre-compiled
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headers.
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</p>
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<p>Given a structure <code>struct GTY((user)) my_struct</code>, the following functions
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should be defined to mark <code>my_struct</code>:
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</p>
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<div class="smallexample">
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<pre class="smallexample">void gt_ggc_mx (my_struct *p)
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{
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/* This marks field 'fld'. */
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gt_ggc_mx (p->fld);
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}
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void gt_pch_nx (my_struct *p)
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{
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/* This marks field 'fld'. */
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gt_pch_nx (tp->fld);
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}
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void gt_pch_nx (my_struct *p, gt_pointer_operator op, void *cookie)
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{
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/* For every field 'fld', call the given pointer operator. */
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op (&(tp->fld), cookie);
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}
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</pre></div>
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<p>In general, each marker <code>M</code> should call <code>M</code> for every
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pointer field in the structure. Fields that are not allocated in GC
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or are not pointers must be ignored.
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</p>
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<p>For embedded lists (e.g., structures with a <code>next</code> or <code>prev</code>
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pointer), the marker must follow the chain and mark every element in
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it.
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</p>
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<p>Note that the rules for the pointer walker <code>gt_pch_nx (my_struct
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*, gt_pointer_operator, void *)</code> are slightly different. In this
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case, the operation <code>op</code> must be applied to the <em>address</em> of
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every pointer field.
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</p>
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<a name="User_002dprovided-marking-routines-for-template-types"></a>
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<h4 class="subsection">22.3.1 User-provided marking routines for template types</h4>
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<p>When a template type <code>TP</code> is marked with <code>GTY</code>, all
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instances of that type are considered user-provided types. This means
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that the individual instances of <code>TP</code> do not need to be marked
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with <code>GTY</code>. The user needs to provide template functions to mark
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all the fields of the type.
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</p>
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<p>The following code snippets represent all the functions that need to
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be provided. Note that type <code>TP</code> may reference to more than one
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type. In these snippets, there is only one type <code>T</code>, but there
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could be more.
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</p>
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<div class="smallexample">
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<pre class="smallexample">template<typename T>
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void gt_ggc_mx (TP<T> *tp)
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{
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extern void gt_ggc_mx (T&);
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/* This marks field 'fld' of type 'T'. */
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gt_ggc_mx (tp->fld);
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}
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template<typename T>
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void gt_pch_nx (TP<T> *tp)
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{
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extern void gt_pch_nx (T&);
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/* This marks field 'fld' of type 'T'. */
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gt_pch_nx (tp->fld);
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}
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template<typename T>
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void gt_pch_nx (TP<T *> *tp, gt_pointer_operator op, void *cookie)
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{
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/* For every field 'fld' of 'tp' with type 'T *', call the given
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pointer operator. */
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op (&(tp->fld), cookie);
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}
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template<typename T>
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void gt_pch_nx (TP<T> *tp, gt_pointer_operator, void *cookie)
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{
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extern void gt_pch_nx (T *, gt_pointer_operator, void *);
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/* For every field 'fld' of 'tp' with type 'T', call the pointer
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walker for all the fields of T. */
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gt_pch_nx (&(tp->fld), op, cookie);
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}
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</pre></div>
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<p>Support for user-defined types is currently limited. The following
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restrictions apply:
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</p>
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<ol>
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<li> Type <code>TP</code> and all the argument types <code>T</code> must be
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marked with <code>GTY</code>.
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</li><li> Type <code>TP</code> can only have type names in its argument list.
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</li><li> The pointer walker functions are different for <code>TP<T></code> and
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<code>TP<T *></code>. In the case of <code>TP<T></code>, references to
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<code>T</code> must be handled by calling <code>gt_pch_nx</code> (which
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will, in turn, walk all the pointers inside fields of <code>T</code>).
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In the case of <code>TP<T *></code>, references to <code>T *</code> must be
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handled by calling the <code>op</code> function on the address of the
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pointer (see the code snippets above).
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</li></ol>
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<hr>
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<div class="header">
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<p>
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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> [<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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