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diff --git a/share/doc/gdb/Writing-an-Xmethod.html b/share/doc/gdb/Writing-an-Xmethod.html new file mode 100644 index 0000000..80d838c --- /dev/null +++ b/share/doc/gdb/Writing-an-Xmethod.html @@ -0,0 +1,277 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- Copyright (C) 1988-2023 Free Software Foundation, Inc. + +Permission is granted to copy, distribute and/or modify this document +under the terms of the GNU Free Documentation License, Version 1.3 or +any later version published by the Free Software Foundation; with the +Invariant Sections being "Free Software" and "Free Software Needs +Free Documentation", with the Front-Cover Texts being "A GNU Manual," +and with the Back-Cover Texts as in (a) below. + +(a) The FSF's Back-Cover Text is: "You are free to copy and modify +this GNU Manual. 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Consider +the following C<tt>++</tt> class: +</p> +<div class="smallexample"> +<pre class="smallexample">class MyClass +{ +public: + MyClass (int a) : a_(a) { } + + int geta (void) { return a_; } + int operator+ (int b); + +private: + int a_; +}; + +int +MyClass::operator+ (int b) +{ + return a_ + b; +} +</pre></div> + +<p>Let us define two xmethods for the class <code>MyClass</code>, one +replacing the method <code>geta</code>, and another adding an overloaded +flavor of <code>operator+</code> which takes a <code>MyClass</code> argument (the +C<tt>++</tt> code above already has an overloaded <code>operator+</code> +which takes an <code>int</code> argument). The xmethod matcher can be +defined as follows: +</p> +<div class="smallexample"> +<pre class="smallexample">class MyClass_geta(gdb.xmethod.XMethod): + def __init__(self): + gdb.xmethod.XMethod.__init__(self, 'geta') + + def get_worker(self, method_name): + if method_name == 'geta': + return MyClassWorker_geta() + + +class MyClass_sum(gdb.xmethod.XMethod): + def __init__(self): + gdb.xmethod.XMethod.__init__(self, 'sum') + + def get_worker(self, method_name): + if method_name == 'operator+': + return MyClassWorker_plus() + + +class MyClassMatcher(gdb.xmethod.XMethodMatcher): + def __init__(self): + gdb.xmethod.XMethodMatcher.__init__(self, 'MyClassMatcher') + # List of methods 'managed' by this matcher + self.methods = [MyClass_geta(), MyClass_sum()] + + def match(self, class_type, method_name): + if class_type.tag != 'MyClass': + return None + workers = [] + for method in self.methods: + if method.enabled: + worker = method.get_worker(method_name) + if worker: + workers.append(worker) + + return workers +</pre></div> + +<p>Notice that the <code>match</code> method of <code>MyClassMatcher</code> returns +a worker object of type <code>MyClassWorker_geta</code> for the <code>geta</code> +method, and a worker object of type <code>MyClassWorker_plus</code> for the +<code>operator+</code> method. This is done indirectly via helper classes +derived from <code>gdb.xmethod.XMethod</code>. One does not need to use the +<code>methods</code> attribute in a matcher as it is optional. However, if a +matcher manages more than one xmethod, it is a good practice to list the +xmethods in the <code>methods</code> attribute of the matcher. This will then +facilitate enabling and disabling individual xmethods via the +<code>enable/disable</code> commands. Notice also that a worker object is +returned only if the corresponding entry in the <code>methods</code> attribute +of the matcher is enabled. +</p> +<p>The implementation of the worker classes returned by the matcher setup +above is as follows: +</p> +<div class="smallexample"> +<pre class="smallexample">class MyClassWorker_geta(gdb.xmethod.XMethodWorker): + def get_arg_types(self): + return None + + def get_result_type(self, obj): + return gdb.lookup_type('int') + + def __call__(self, obj): + return obj['a_'] + + +class MyClassWorker_plus(gdb.xmethod.XMethodWorker): + def get_arg_types(self): + return gdb.lookup_type('MyClass') + + def get_result_type(self, obj): + return gdb.lookup_type('int') + + def __call__(self, obj, other): + return obj['a_'] + other['a_'] +</pre></div> + +<p>For <small>GDB</small> to actually lookup a xmethod, it has to be +registered with it. The matcher defined above is registered with +<small>GDB</small> globally as follows: +</p> +<div class="smallexample"> +<pre class="smallexample">gdb.xmethod.register_xmethod_matcher(None, MyClassMatcher()) +</pre></div> + +<p>If an object <code>obj</code> of type <code>MyClass</code> is initialized in C<tt>++</tt> +code as follows: +</p> +<div class="smallexample"> +<pre class="smallexample">MyClass obj(5); +</pre></div> + +<p>then, after loading the Python script defining the xmethod matchers +and workers into <code>GDBN</code>, invoking the method <code>geta</code> or using +the operator <code>+</code> on <code>obj</code> will invoke the xmethods +defined above: +</p> +<div class="smallexample"> +<pre class="smallexample">(gdb) p obj.geta() +$1 = 5 + +(gdb) p obj + obj +$2 = 10 +</pre></div> + +<p>Consider another example with a C++ template class: +</p> +<div class="smallexample"> +<pre class="smallexample">template <class T> +class MyTemplate +{ +public: + MyTemplate () : dsize_(10), data_ (new T [10]) { } + ~MyTemplate () { delete [] data_; } + + int footprint (void) + { + return sizeof (T) * dsize_ + sizeof (MyTemplate<T>); + } + +private: + int dsize_; + T *data_; +}; +</pre></div> + +<p>Let us implement an xmethod for the above class which serves as a +replacement for the <code>footprint</code> method. The full code listing +of the xmethod workers and xmethod matchers is as follows: +</p> +<div class="smallexample"> +<pre class="smallexample">class MyTemplateWorker_footprint(gdb.xmethod.XMethodWorker): + def __init__(self, class_type): + self.class_type = class_type + + def get_arg_types(self): + return None + + def get_result_type(self): + return gdb.lookup_type('int') + + def __call__(self, obj): + return (self.class_type.sizeof + + obj['dsize_'] * + self.class_type.template_argument(0).sizeof) + + +class MyTemplateMatcher_footprint(gdb.xmethod.XMethodMatcher): + def __init__(self): + gdb.xmethod.XMethodMatcher.__init__(self, 'MyTemplateMatcher') + + def match(self, class_type, method_name): + if (re.match('MyTemplate<[ \t\n]*[_a-zA-Z][ _a-zA-Z0-9]*>', + class_type.tag) and + method_name == 'footprint'): + return MyTemplateWorker_footprint(class_type) +</pre></div> + +<p>Notice that, in this example, we have not used the <code>methods</code> +attribute of the matcher as the matcher manages only one xmethod. The +user can enable/disable this xmethod by enabling/disabling the matcher +itself. +</p> +<hr> +<div class="header"> +<p> +Next: <a href="Inferiors-In-Python.html#Inferiors-In-Python" accesskey="n" rel="next">Inferiors In Python</a>, Previous: <a href="Xmethod-API.html#Xmethod-API" accesskey="p" rel="previous">Xmethod API</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> +</div> + + + +</body> +</html> |