Writing an Xmethod (Debugging with GDB)

Implementing xmethods in Python will require implementing xmethod matchers and xmethod workers (see Xmethods In Python). Consider the following C++ class:

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;
}

Let us define two xmethods for the class MyClass, one replacing the method geta, and another adding an overloaded flavor of operator+ which takes a MyClass argument (the C++ code above already has an overloaded operator+ which takes an int argument). The xmethod matcher can be defined as follows:

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

Notice that the match method of MyClassMatcher returns a worker object of type MyClassWorker_geta for the geta method, and a worker object of type MyClassWorker_plus for the operator+ method. This is done indirectly via helper classes derived from gdb.xmethod.XMethod. One does not need to use the methods 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 methods attribute of the matcher. This will then facilitate enabling and disabling individual xmethods via the enable/disable commands. Notice also that a worker object is returned only if the corresponding entry in the methods attribute of the matcher is enabled.

The implementation of the worker classes returned by the matcher setup above is as follows:

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_']

For GDB to actually lookup a xmethod, it has to be registered with it. The matcher defined above is registered with GDB globally as follows:

gdb.xmethod.register_xmethod_matcher(None, MyClassMatcher())

then, after loading the Python script defining the xmethod matchers and workers into GDB, invoking the method geta or using the operator + on obj will invoke the xmethods defined above:

(gdb) p obj.geta()
$1 = 5

(gdb) p obj + obj
$2 = 10

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_;
};

Let us implement an xmethod for the above class which serves as a replacement for the footprint method. The full code listing of the xmethod workers and xmethod matchers is as follows:

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)

Notice that, in this example, we have not used the methods attribute of the matcher as the matcher manages only one xmethod. The user can enable/disable this xmethod by enabling/disabling the matcher itself.

23.3.2.16 Writing an Xmethod