| <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> |
| <html> |
| <!-- Copyright (C) 1988-2015 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. Buying copies from GNU Press supports the FSF in |
| developing GNU and promoting software freedom." --> |
| <!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ --> |
| <head> |
| <title>Debugging with GDB: Writing an Xmethod</title> |
| |
| <meta name="description" content="Debugging with GDB: Writing an Xmethod"> |
| <meta name="keywords" content="Debugging with GDB: Writing an Xmethod"> |
| <meta name="resource-type" content="document"> |
| <meta name="distribution" content="global"> |
| <meta name="Generator" content="makeinfo"> |
| <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> |
| <link href="index.html#Top" rel="start" title="Top"> |
| <link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index"> |
| <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> |
| <link href="Python-API.html#Python-API" rel="up" title="Python API"> |
| <link href="Inferiors-In-Python.html#Inferiors-In-Python" rel="next" title="Inferiors In Python"> |
| <link href="Xmethod-API.html#Xmethod-API" rel="prev" title="Xmethod API"> |
| <style type="text/css"> |
| <!-- |
| a.summary-letter {text-decoration: none} |
| blockquote.smallquotation {font-size: smaller} |
| div.display {margin-left: 3.2em} |
| div.example {margin-left: 3.2em} |
| div.indentedblock {margin-left: 3.2em} |
| div.lisp {margin-left: 3.2em} |
| div.smalldisplay {margin-left: 3.2em} |
| div.smallexample {margin-left: 3.2em} |
| div.smallindentedblock {margin-left: 3.2em; font-size: smaller} |
| div.smalllisp {margin-left: 3.2em} |
| kbd {font-style:oblique} |
| pre.display {font-family: inherit} |
| pre.format {font-family: inherit} |
| pre.menu-comment {font-family: serif} |
| pre.menu-preformatted {font-family: serif} |
| pre.smalldisplay {font-family: inherit; font-size: smaller} |
| pre.smallexample {font-size: smaller} |
| pre.smallformat {font-family: inherit; font-size: smaller} |
| pre.smalllisp {font-size: smaller} |
| span.nocodebreak {white-space:nowrap} |
| span.nolinebreak {white-space:nowrap} |
| span.roman {font-family:serif; font-weight:normal} |
| span.sansserif {font-family:sans-serif; font-weight:normal} |
| ul.no-bullet {list-style: none} |
| --> |
| </style> |
| |
| |
| </head> |
| |
| <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> |
| <a name="Writing-an-Xmethod"></a> |
| <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="prev">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> |
| <hr> |
| <a name="Writing-an-Xmethod-1"></a> |
| <h4 class="subsubsection">23.2.2.15 Writing an Xmethod</h4> |
| <a name="index-writing-xmethods-in-Python"></a> |
| |
| <p>Implementing xmethods in Python will require implementing xmethod |
| matchers and xmethod workers (see <a href="Xmethods-In-Python.html#Xmethods-In-Python">Xmethods In Python</a>). 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="prev">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> |
