232 lines
8.7 KiB
HTML
232 lines
8.7 KiB
HTML
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<title>Writing an Xmethod - Debugging with GDB</title>
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Python-API.html#Python-API" title="Python API">
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<link rel="prev" href="Xmethod-API.html#Xmethod-API" title="Xmethod API">
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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</head>
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<body>
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<div class="node">
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<a name="Writing-an-Xmethod"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Inferiors-In-Python.html#Inferiors-In-Python">Inferiors In Python</a>,
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Previous: <a rel="previous" accesskey="p" href="Xmethod-API.html#Xmethod-API">Xmethod API</a>,
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Up: <a rel="up" accesskey="u" href="Python-API.html#Python-API">Python API</a>
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<hr>
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</div>
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<h5 class="subsubsection">23.2.2.15 Writing an Xmethod</h5>
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<p><a name="index-writing-xmethods-in-Python-2051"></a>
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Implementing xmethods in Python will require implementing xmethod
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matchers and xmethod workers (see <a href="Xmethods-In-Python.html#Xmethods-In-Python">Xmethods In Python</a>). Consider
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the following C<tt>++</tt> class:
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<pre class="smallexample"> class MyClass
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{
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public:
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MyClass (int a) : a_(a) { }
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int geta (void) { return a_; }
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int operator+ (int b);
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private:
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int a_;
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};
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int
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MyClass::operator+ (int b)
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{
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return a_ + b;
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}
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</pre>
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<p class="noindent">Let us define two xmethods for the class <code>MyClass</code>, one
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replacing the method <code>geta</code>, and another adding an overloaded
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flavor of <code>operator+</code> which takes a <code>MyClass</code> argument (the
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C<tt>++</tt> code above already has an overloaded <code>operator+</code>
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which takes an <code>int</code> argument). The xmethod matcher can be
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defined as follows:
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<pre class="smallexample"> class MyClass_geta(gdb.xmethod.XMethod):
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def __init__(self):
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gdb.xmethod.XMethod.__init__(self, 'geta')
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def get_worker(self, method_name):
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if method_name == 'geta':
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return MyClassWorker_geta()
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class MyClass_sum(gdb.xmethod.XMethod):
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def __init__(self):
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gdb.xmethod.XMethod.__init__(self, 'sum')
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def get_worker(self, method_name):
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if method_name == 'operator+':
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return MyClassWorker_plus()
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class MyClassMatcher(gdb.xmethod.XMethodMatcher):
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def __init__(self):
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gdb.xmethod.XMethodMatcher.__init__(self, 'MyClassMatcher')
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# List of methods 'managed' by this matcher
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self.methods = [MyClass_geta(), MyClass_sum()]
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def match(self, class_type, method_name):
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if class_type.tag != 'MyClass':
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return None
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workers = []
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for method in self.methods:
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if method.enabled:
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worker = method.get_worker(method_name)
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if worker:
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workers.append(worker)
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return workers
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</pre>
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<p class="noindent">Notice that the <code>match</code> method of <code>MyClassMatcher</code> returns
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a worker object of type <code>MyClassWorker_geta</code> for the <code>geta</code>
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method, and a worker object of type <code>MyClassWorker_plus</code> for the
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<code>operator+</code> method. This is done indirectly via helper classes
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derived from <code>gdb.xmethod.XMethod</code>. One does not need to use the
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<code>methods</code> attribute in a matcher as it is optional. However, if a
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matcher manages more than one xmethod, it is a good practice to list the
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xmethods in the <code>methods</code> attribute of the matcher. This will then
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facilitate enabling and disabling individual xmethods via the
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<code>enable/disable</code> commands. Notice also that a worker object is
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returned only if the corresponding entry in the <code>methods</code> attribute
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of the matcher is enabled.
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<p>The implementation of the worker classes returned by the matcher setup
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above is as follows:
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<pre class="smallexample"> class MyClassWorker_geta(gdb.xmethod.XMethodWorker):
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def get_arg_types(self):
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return None
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def get_result_type(self, obj):
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return gdb.lookup_type('int')
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def __call__(self, obj):
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return obj['a_']
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class MyClassWorker_plus(gdb.xmethod.XMethodWorker):
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def get_arg_types(self):
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return gdb.lookup_type('MyClass')
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def get_result_type(self, obj):
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return gdb.lookup_type('int')
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def __call__(self, obj, other):
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return obj['a_'] + other['a_']
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</pre>
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<p>For <span class="sc">gdb</span> to actually lookup a xmethod, it has to be
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registered with it. The matcher defined above is registered with
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<span class="sc">gdb</span> globally as follows:
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<pre class="smallexample"> gdb.xmethod.register_xmethod_matcher(None, MyClassMatcher())
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</pre>
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<p>If an object <code>obj</code> of type <code>MyClass</code> is initialized in C<tt>++</tt>
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code as follows:
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<pre class="smallexample"> MyClass obj(5);
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</pre>
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<p class="noindent">then, after loading the Python script defining the xmethod matchers
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and workers into <code>GDBN</code>, invoking the method <code>geta</code> or using
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the operator <code>+</code> on <code>obj</code> will invoke the xmethods
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defined above:
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<pre class="smallexample"> (gdb) p obj.geta()
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$1 = 5
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(gdb) p obj + obj
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$2 = 10
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</pre>
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<p>Consider another example with a C++ template class:
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<pre class="smallexample"> template <class T>
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class MyTemplate
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{
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public:
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MyTemplate () : dsize_(10), data_ (new T [10]) { }
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~MyTemplate () { delete [] data_; }
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int footprint (void)
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{
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return sizeof (T) * dsize_ + sizeof (MyTemplate<T>);
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}
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private:
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int dsize_;
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T *data_;
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};
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</pre>
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<p>Let us implement an xmethod for the above class which serves as a
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replacement for the <code>footprint</code> method. The full code listing
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of the xmethod workers and xmethod matchers is as follows:
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<pre class="smallexample"> class MyTemplateWorker_footprint(gdb.xmethod.XMethodWorker):
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def __init__(self, class_type):
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self.class_type = class_type
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def get_arg_types(self):
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return None
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def get_result_type(self):
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return gdb.lookup_type('int')
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def __call__(self, obj):
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return (self.class_type.sizeof +
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obj['dsize_'] *
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self.class_type.template_argument(0).sizeof)
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class MyTemplateMatcher_footprint(gdb.xmethod.XMethodMatcher):
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def __init__(self):
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gdb.xmethod.XMethodMatcher.__init__(self, 'MyTemplateMatcher')
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def match(self, class_type, method_name):
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if (re.match('MyTemplate<[ \t\n]*[_a-zA-Z][ _a-zA-Z0-9]*>',
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class_type.tag) and
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method_name == 'footprint'):
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return MyTemplateWorker_footprint(class_type)
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</pre>
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<p>Notice that, in this example, we have not used the <code>methods</code>
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attribute of the matcher as the matcher manages only one xmethod. The
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user can enable/disable this xmethod by enabling/disabling the matcher
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itself.
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</body></html>
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