@@ -94,6 +94,11 @@ qXfer:btrace:read's annex
** Valid Python operations on gdb.Value objects representing
structs/classes invoke the corresponding overloaded operators if
available.
+ ** New `Debug Methods' feature in the Python API. Debug methods are
+ additional methods or replacements for existing methods of a C++
+ class. This feature is useful for those cases where a method
+ defined in C++ source code could be inlined or optimized out by
+ the compiler, making it unavailable to GDB.
* New targets
PowerPC64 GNU/Linux little-endian powerpc64le-*-linux*
@@ -144,6 +144,9 @@ optional arguments while skipping others. Example:
* Frame Filter API:: Filtering Frames.
* Frame Decorator API:: Decorating Frames.
* Writing a Frame Filter:: Writing a Frame Filter.
+* Debug Methods In Python:: Adding and replacing methods of C++ classes.
+* Debug Method API:: Debug method types.
+* Writing a Debug Method:: Writing a debug method.
* Inferiors In Python:: Python representation of inferiors (processes)
* Events In Python:: Listening for events from @value{GDBN}.
* Threads In Python:: Accessing inferior threads from Python.
@@ -2195,6 +2198,266 @@ printed hierarchically. Another approach would be to combine the
marker in the inlined frame, and also show the hierarchical
relationship.
+@node Debug Methods In Python
+@subsubsection Debug Methods In Python
+@cindex debug methods in Python
+
+@dfn{Debug methods} are additional methods or replacements for existing
+methods of a C@t{++} class. This feature is useful for those cases
+where a method defined in C@t{++} source code could be inlined or
+optimized out by the compiler, making it unavailable to @value{GDBN}.
+For such cases, one can define a debug method to serve as a replacement
+for the method defined in the C@t{++} source code. @value{GDBN} will
+then invoke the debug method, instead of the C@t{++} method, to
+evaluate expressions. One can also use debug methods when debugging
+with core files. Moreover, when debugging live programs, invoking a
+debug method need not involve running the inferior (which can potentially
+perturb its state). Hence, even if the C@t{++} method is available, it
+is better to use its replacement debug method if one is defined.
+
+The debug methods feature in Python is available via the concepts of a
+@dfn{debug method matcher} and a @dfn{debug method worker}. To
+implement a debug method, one has to implement a matcher and a
+corresponding worker for it (more than one worker can be
+implemented, each catering to a different overloaded instance of the
+method). Internally, @value{GDBN} invokes the @code{match} method of a
+matcher to match the class type and method name. On a match, the
+@code{match} method returns a list of matching @emph{worker} objects.
+Each worker object typically corresponds to an overloaded instance of
+the debug method. They implement a @code{get_arg_types} method which
+returns a sequence of types corresponding to the arguments the debug
+method requires. @value{GDBN} uses this sequence of types to perform
+overload resolution and picks a winning debug method worker. A winner
+is also selected from among the methods @value{GDBN} finds in the
+C@t{++} source code. Next, the winning debug method worker and the
+winning C@t{++} method are compared to select an overall winner. In
+case of a tie between a debug method worker and a C@t{++} method, the
+debug method worker is selected as the winner. That is, if a winning
+debug method worker is found to be equivalent to the winning C@t{++}
+method, then the debug method worker is treated as a replacement for
+the C@t{++} method. @value{GDBN} uses the overall winner to invoke the
+method. If the winning debug method worker is the overall winner, then
+the corresponding debug method is invoked via the @code{invoke} method
+of the worker object.
+
+If one wants to implement a debug method as a replacement for an
+existing C@t{++} method, then they have to implement an equivalent
+debug method which has exactly the same name and takes arguments of
+exactly the same type as the C@t{++} method. If the user wants to
+invoke the C@t{++} method even though a replacement debug method is
+available for that method, then they can disable the debug method.
+
+@xref{Debug Method API}, for API to implement debug methods in Python.
+@xref{Writing a Debug Method}, for implementing debug methods in Python.
+
+@node Debug Method API
+@subsubsection Debug Method API
+@cindex debug method API
+
+The @value{GDBN} Python API provides classes, interfaces and functions
+to implement, register and manipulate debug methods.
+@xref{Debug Methods In Python}.
+
+A debug method matcher should be an instance of a class derived from
+@code{DebugMethodMatcher} defined in the module
+@code{gdb.debug_method}. An instance of @code{DebugMethodMatcher} has
+the following attributes:
+
+@defvar name
+The name of the matcher.
+@end defvar
+
+@defvar enabled
+A boolean value indicating whether the matcher is enabled or disabled.
+@end defvar
+
+@defvar methods
+A list of named methods managed by the matcher. Each object in the list
+is an instance of the class @code{DebugMethod} defined in the module
+@code{gdb.debug_method}, or any object with the following attributes:
+
+@table @code
+
+@item name
+Name of the debug method which should be unique for each debug method
+managed by the matcher.
+
+@item enabled
+A boolean value indicating whether the debug method is enabled or
+disabled.
+
+@end table
+
+The class @code{DebugMethod} is a convenience class with same
+attributes as above along with the following constructor:
+
+@defun DebugMethod.__init__(self, name)
+Constructs an enabled debug method with name @var{name}.
+@end defun
+@end defvar
+
+@noindent
+The @code{DebugMethodMatcher} class has the following methods:
+
+@defun DebugMethodMatcher.__init__(self, name)
+Constructs an enabled debug method matcher with name @var{name}. The
+@code{methods} attribute is initialized to @code{None}.
+@end defun
+
+@defun DebugMethodMatcher.match(self, class_type, method_name)
+Derived classes should override this method. It should return a
+debug method worker object (or a sequence of debug method worker
+objects) matching the @var{class_type} and @var{method_name}.
+@var{class_type} is a @code{gdb.Type} object, and @var{method_name}
+is a string value. If the matcher manages named methods as listed in
+its @code{methods} attribute, then only those worker objects whose
+corresponding entries in the @code{methods} list are enabled should be
+returned.
+@end defun
+
+A debug method worker should be an instance of a class derived from
+@code{DebugMethodWorker} defined in the module @code{gdb.debug_method},
+or support the following interface:
+
+@defun DebugMethodWorker.get_arg_types(self)
+This method returns a sequence of @code{gdb.Type} objects corresponding
+to the arguments that the debug method takes. It can return an empty
+sequence or @code{None} if the debug method does not take any arguments.
+If the debug method takes a single argument, then a single
+@code{gdb.Type} object corresponding to it can be returned.
+@end defun
+
+@defun DebugMethodWorker.invoke(self, obj, args)
+This is the method which does the @emph{work} of the debug method.
+@var{obj} is the object on which the method is being invoked, and
+@var{args} is the tuple of arguments to the method. @var{obj} and the
+elements of @var{args} are @code{gdb.Value} objects.
+@end defun
+
+For @value{GDBN} to lookup debug methods, the debug method matchers
+should be registered using the following function defined in the module
+@code{gdb.debug_method}:
+
+@defun register_debug_method_matcher(locus, matcher, replace=False)
+The @code{matcher} is registered with @code{locus}, replacing an
+existing matcher with the same name as @code{matcher} if
+@code{replace} is @code{True}. @code{locus} can be a
+@code{gdb.Objfile} object (@pxref{Objfiles In Python}), or a
+@code{gdb.Progspace} object (@pxref{Program Spaces In Python}), or
+@code{None}. If it is @code{None}, then @code{matcher} is registered
+globally.
+@end defun
+
+@node Writing a Debug Method
+@subsubsection Writing a Debug Method
+@cindex writing debug methods in Python
+
+Implementing debug methods in Python will require implementing debug
+method matchers and debug method workers
+(@pxref{Debug Methods In Python}). Consider the following C@t{++}
+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;
+@}
+@end smallexample
+
+@noindent
+Let us define two debug methods for the class @code{MyClass}, one
+replacing the method @code{geta}, and another adding an overloaded
+flavor of @code{operator+} which takes a @code{MyClass} argument. The
+debug method matcher can be defined as follows:
+
+@smallexample
+class MyClassMatcher(gdb.debug_method.DebugMethodMatcher):
+ def __init__(self):
+ gdb.debug_method.DebugMethodMatcher.__init__(self, 'MyMatcher')
+ # List of methods 'managed' by this matcher
+ self.methods = [gdb.debug_method.DebugMethod('geta'),
+ gdb.debug_method.DebugMethod('sum')]
+
+ def match(self, class_type, method_name):
+ if class_type.tag != 'MyClass':
+ return None
+ if method_name == 'geta' and self.methods[0].enabled:
+ return MyClassWorker_geta()
+ elif method_name == 'operator+' and self.methods[1].enabled:
+ return MyClassWorker_plus()
+ else:
+ return None
+@end smallexample
+
+@noindent
+Notice that the @code{match} method of @code{MyClassMatcher} returns
+a worker object of type @code{MyClassWorker_geta} for the @code{geta}
+method, and a worker object of type @code{MyClassWorker_plus} for the
+@code{operator+} method. Also, a worker object is returned only if the
+corresponding entry in the @code{methods} attribute is enabled.
+
+The implementation of the worker classes returned by the matcher above
+is as follows:
+
+@smallexample
+class MyClassWorker_geta(gdb.debug_method.DebugMethodWorker):
+ def get_arg_types(self):
+ return None
+
+ def invoke(self, obj, args):
+ return obj['a_']
+
+
+class MyClassWorker_plus(gdb.debug_method.DebugMethodWorker):
+ def get_arg_types(self):
+ return gdb.lookup_type('MyClass')
+
+ def invoke(self, obj, args):
+ return obj['a_'] + args[0]['a_']
+@end smallexample
+
+For @value{GDBN} to actually lookup a debug method, it has to be
+registered with it. The matcher defined above is registered with
+@value{GDBN} globally as follows:
+
+@smallexample
+gdb.debug_method.register_debug_method_matcher(None, MyClassMatcher())
+@end smallexample
+
+If an object @code{obj} of type @code{MyClass} is initialized in C@t{++}
+code as follows:
+
+@smallexample
+MyClass obj(5);
+@end smallexample
+
+@noindent
+then, after loading the Python script defining the debug method matchers
+and workers into @code{GDBN}, invoking the method @code{geta} or using
+the operator @code{+} on @code{obj} will invoke the debug methods
+defined above:
+
+@smallexample
+(gdb) p obj.geta()
+$1 = 5
+
+(gdb) p obj + obj
+$2 = 10
+@end smallexample
+
@node Inferiors In Python
@subsubsection Inferiors In Python
@cindex inferiors in Python