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Copies published by the Free Software Foundation raise funds for GNU development. --> <!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ --> <head> <title>User GC (GNU Compiler Collection (GCC) Internals)</title> <meta name="description" content="User GC (GNU Compiler Collection (GCC) Internals)"> <meta name="keywords" content="User GC (GNU Compiler Collection (GCC) Internals)"> <meta name="resource-type" content="document"> <meta name="distribution" content="global"> <meta name="Generator" content="makeinfo"> <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> <link href="index.html#Top" rel="start" title="Top"> <link href="Option-Index.html#Option-Index" rel="index" title="Option Index"> <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> <link href="Type-Information.html#Type-Information" rel="up" title="Type Information"> <link href="GGC-Roots.html#GGC-Roots" rel="next" title="GGC Roots"> <link href="Inheritance-and-GTY.html#Inheritance-and-GTY" rel="prev" title="Inheritance and GTY"> <style type="text/css"> <!-- a.summary-letter {text-decoration: none} blockquote.indentedblock {margin-right: 0em} blockquote.smallindentedblock {margin-right: 0em; font-size: smaller} blockquote.smallquotation {font-size: smaller} div.display {margin-left: 3.2em} div.example {margin-left: 3.2em} div.lisp {margin-left: 3.2em} div.smalldisplay {margin-left: 3.2em} div.smallexample {margin-left: 3.2em} div.smalllisp {margin-left: 3.2em} kbd {font-style: oblique} pre.display {font-family: inherit} pre.format {font-family: inherit} pre.menu-comment {font-family: serif} pre.menu-preformatted {font-family: serif} pre.smalldisplay {font-family: inherit; font-size: smaller} pre.smallexample {font-size: smaller} pre.smallformat {font-family: inherit; font-size: smaller} pre.smalllisp {font-size: smaller} span.nolinebreak {white-space: nowrap} span.roman {font-family: initial; font-weight: normal} span.sansserif {font-family: sans-serif; font-weight: normal} ul.no-bullet {list-style: none} --> </style> </head> <body lang="en"> <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> [<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> </div> <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->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">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<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> <hr> <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> [<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> </div> </body> </html>