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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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| <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> [<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">22.3 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->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></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">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> |
| <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<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></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<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). |
| </li></ol> |
| |
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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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