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| <title>GNU Compiler Collection (GCC) Internals: Type Information</title> |
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| <a name="Type-Information"></a> |
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| Next: <a href="Plugins.html#Plugins" accesskey="n" rel="next">Plugins</a>, Previous: <a href="Header-Dirs.html#Header-Dirs" accesskey="p" rel="prev">Header Dirs</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</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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| <a name="Memory-Management-and-Type-Information"></a> |
| <h2 class="chapter">22 Memory Management and Type Information</h2> |
| <a name="index-GGC"></a> |
| <a name="index-GTY"></a> |
| |
| <p>GCC uses some fairly sophisticated memory management techniques, which |
| involve determining information about GCC’s data structures from GCC’s |
| source code and using this information to perform garbage collection and |
| implement precompiled headers. |
| </p> |
| <p>A full C++ parser would be too complicated for this task, so a limited |
| subset of C++ is interpreted and special markers are used to determine |
| what parts of the source to look at. All <code>struct</code>, <code>union</code> |
| and <code>template</code> structure declarations that define data structures |
| that are allocated under control of the garbage collector must be |
| marked. All global variables that hold pointers to garbage-collected |
| memory must also be marked. Finally, all global variables that need |
| to be saved and restored by a precompiled header must be marked. (The |
| precompiled header mechanism can only save static variables if they’re |
| scalar. Complex data structures must be allocated in garbage-collected |
| memory to be saved in a precompiled header.) |
| </p> |
| <p>The full format of a marker is |
| </p><div class="smallexample"> |
| <pre class="smallexample">GTY (([<var>option</var>] [(<var>param</var>)], [<var>option</var>] [(<var>param</var>)] …)) |
| </pre></div> |
| <p>but in most cases no options are needed. The outer double parentheses |
| are still necessary, though: <code>GTY(())</code>. Markers can appear: |
| </p> |
| <ul> |
| <li> In a structure definition, before the open brace; |
| </li><li> In a global variable declaration, after the keyword <code>static</code> or |
| <code>extern</code>; and |
| </li><li> In a structure field definition, before the name of the field. |
| </li></ul> |
| |
| <p>Here are some examples of marking simple data structures and globals. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">struct GTY(()) <var>tag</var> |
| { |
| <var>fields</var>… |
| }; |
| |
| typedef struct GTY(()) <var>tag</var> |
| { |
| <var>fields</var>… |
| } *<var>typename</var>; |
| |
| static GTY(()) struct <var>tag</var> *<var>list</var>; /* <span class="roman">points to GC memory</span> */ |
| static GTY(()) int <var>counter</var>; /* <span class="roman">save counter in a PCH</span> */ |
| </pre></div> |
| |
| <p>The parser understands simple typedefs such as |
| <code>typedef struct <var>tag</var> *<var>name</var>;</code> and |
| <code>typedef int <var>name</var>;</code>. |
| These don’t need to be marked. |
| </p> |
| <p>Since <code>gengtype</code>’s understanding of C++ is limited, there are |
| several constructs and declarations that are not supported inside |
| classes/structures marked for automatic GC code generation. The |
| following C++ constructs produce a <code>gengtype</code> error on |
| structures/classes marked for automatic GC code generation: |
| </p> |
| <ul> |
| <li> Type definitions inside classes/structures are not supported. |
| </li><li> Enumerations inside classes/structures are not supported. |
| </li></ul> |
| |
| <p>If you have a class or structure using any of the above constructs, |
| you need to mark that class as <code>GTY ((user))</code> and provide your |
| own marking routines (see section <a href="User-GC.html#User-GC">User GC</a> for details). |
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| <p>It is always valid to include function definitions inside classes. |
| Those are always ignored by <code>gengtype</code>, as it only cares about |
| data members. |
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| <tr><td align="left" valign="top">• <a href="GTY-Options.html#GTY-Options" accesskey="1">GTY Options</a>:</td><td> </td><td align="left" valign="top">What goes inside a <code>GTY(())</code>. |
| </td></tr> |
| <tr><td align="left" valign="top">• <a href="Inheritance-and-GTY.html#Inheritance-and-GTY" accesskey="2">Inheritance and GTY</a>:</td><td> </td><td align="left" valign="top">Adding GTY to a class hierarchy. |
| </td></tr> |
| <tr><td align="left" valign="top">• <a href="User-GC.html#User-GC" accesskey="3">User GC</a>:</td><td> </td><td align="left" valign="top">Adding user-provided GC marking routines. |
| </td></tr> |
| <tr><td align="left" valign="top">• <a href="GGC-Roots.html#GGC-Roots" accesskey="4">GGC Roots</a>:</td><td> </td><td align="left" valign="top">Making global variables GGC roots. |
| </td></tr> |
| <tr><td align="left" valign="top">• <a href="Files.html#Files" accesskey="5">Files</a>:</td><td> </td><td align="left" valign="top">How the generated files work. |
| </td></tr> |
| <tr><td align="left" valign="top">• <a href="Invoking-the-garbage-collector.html#Invoking-the-garbage-collector" accesskey="6">Invoking the garbage collector</a>:</td><td> </td><td align="left" valign="top">How to invoke the garbage collector. |
| </td></tr> |
| <tr><td align="left" valign="top">• <a href="Troubleshooting.html#Troubleshooting" accesskey="7">Troubleshooting</a>:</td><td> </td><td align="left" valign="top">When something does not work as expected. |
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