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