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<title>Debugging with GDB: Type Printing API</title>
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<a name="Type-Printing-API"></a>
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Next: <a href="Frame-Filter-API.html#Frame-Filter-API" accesskey="n" rel="next">Frame Filter API</a>, Previous: <a href="Writing-a-Pretty_002dPrinter.html#Writing-a-Pretty_002dPrinter" accesskey="p" rel="previous">Writing a Pretty-Printer</a>, Up: <a href="Python-API.html#Python-API" accesskey="u" rel="up">Python API</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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<a name="Type-Printing-API-1"></a>
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<h4 class="subsubsection">23.2.2.8 Type Printing API</h4>
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<a name="index-type-printing-API-for-Python"></a>
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<p><small>GDB</small> provides a way for Python code to customize type display.
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This is mainly useful for substituting canonical typedef names for
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types.
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</p>
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<a name="index-type-printer"></a>
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<p>A <em>type printer</em> is just a Python object conforming to a certain
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protocol. A simple base class implementing the protocol is provided;
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see <a href="gdb_002etypes.html#gdb_002etypes">gdb.types</a>. A type printer must supply at least:
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</p>
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<dl>
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<dt><a name="index-enabled-of-type_005fprinter"></a>Instance Variable of type_printer: <strong>enabled</strong></dt>
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<dd><p>A boolean which is True if the printer is enabled, and False
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otherwise. This is manipulated by the <code>enable type-printer</code>
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and <code>disable type-printer</code> commands.
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</p></dd></dl>
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<dl>
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<dt><a name="index-name-of-type_005fprinter"></a>Instance Variable of type_printer: <strong>name</strong></dt>
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<dd><p>The name of the type printer. This must be a string. This is used by
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the <code>enable type-printer</code> and <code>disable type-printer</code>
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commands.
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</p></dd></dl>
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<dl>
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<dt><a name="index-instantiate-on-type_005fprinter"></a>Method on type_printer: <strong>instantiate</strong> <em>(self)</em></dt>
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<dd><p>This is called by <small>GDB</small> at the start of type-printing. It is
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only called if the type printer is enabled. This method must return a
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new object that supplies a <code>recognize</code> method, as described below.
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</p></dd></dl>
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<p>When displaying a type, say via the <code>ptype</code> command, <small>GDB</small>
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will compute a list of type recognizers. This is done by iterating
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first over the per-objfile type printers (see <a href="Objfiles-In-Python.html#Objfiles-In-Python">Objfiles In Python</a>),
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followed by the per-progspace type printers (see <a href="Progspaces-In-Python.html#Progspaces-In-Python">Progspaces In Python</a>), and finally the global type printers.
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</p>
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<p><small>GDB</small> will call the <code>instantiate</code> method of each enabled
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type printer. If this method returns <code>None</code>, then the result is
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ignored; otherwise, it is appended to the list of recognizers.
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</p>
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<p>Then, when <small>GDB</small> is going to display a type name, it iterates
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over the list of recognizers. For each one, it calls the recognition
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function, stopping if the function returns a non-<code>None</code> value.
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The recognition function is defined as:
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</p>
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<dl>
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<dt><a name="index-recognize-on-type_005frecognizer"></a>Method on type_recognizer: <strong>recognize</strong> <em>(self, type)</em></dt>
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<dd><p>If <var>type</var> is not recognized, return <code>None</code>. Otherwise,
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return a string which is to be printed as the name of <var>type</var>.
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The <var>type</var> argument will be an instance of <code>gdb.Type</code>
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(see <a href="Types-In-Python.html#Types-In-Python">Types In Python</a>).
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</p></dd></dl>
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<p><small>GDB</small> uses this two-pass approach so that type printers can
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efficiently cache information without holding on to it too long. For
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example, it can be convenient to look up type information in a type
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printer and hold it for a recognizer’s lifetime; if a single pass were
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done then type printers would have to make use of the event system in
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order to avoid holding information that could become stale as the
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inferior changed.
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</p>
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<div class="header">
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Next: <a href="Frame-Filter-API.html#Frame-Filter-API" accesskey="n" rel="next">Frame Filter API</a>, Previous: <a href="Writing-a-Pretty_002dPrinter.html#Writing-a-Pretty_002dPrinter" accesskey="p" rel="previous">Writing a Pretty-Printer</a>, Up: <a href="Python-API.html#Python-API" accesskey="u" rel="up">Python API</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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