gdb: Xmethods In Python
23.2.2.13 Xmethods In Python
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"Xmethods" 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. For such cases, one can define
an xmethod to serve as a replacement for the method defined in the C++
source code. GDB will then invoke the xmethod, instead of the C++
method, to evaluate expressions. One can also use xmethods when
debugging with core files. Moreover, when debugging live programs,
invoking an xmethod need not involve running the inferior (which can
potentially perturb its state). Hence, even if the C++ method is
available, it is better to use its replacement xmethod if one is
defined.
The xmethods feature in Python is available via the concepts of an
"xmethod matcher" and an "xmethod worker". To implement an xmethod, 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, GDB invokes the 'match' method of
a matcher to match the class type and method name. On a match, the
'match' method returns a list of matching _worker_ objects. Each worker
object typically corresponds to an overloaded instance of the xmethod.
They implement a 'get_arg_types' method which returns a sequence of
types corresponding to the arguments the xmethod requires. GDB uses
this sequence of types to perform overload resolution and picks a
winning xmethod worker. A winner is also selected from among the
methods GDB finds in the C++ source code. Next, the winning xmethod
worker and the winning C++ method are compared to select an overall
winner. In case of a tie between a xmethod worker and a C++ method, the
xmethod worker is selected as the winner. That is, if a winning xmethod
worker is found to be equivalent to the winning C++ method, then the
xmethod worker is treated as a replacement for the C++ method. GDB uses
the overall winner to invoke the method. If the winning xmethod worker
is the overall winner, then the corresponding xmethod is invoked via the
'__call__' method of the worker object.
If one wants to implement an xmethod as a replacement for an existing
C++ method, then they have to implement an equivalent xmethod which has
exactly the same name and takes arguments of exactly the same type as
the C++ method. If the user wants to invoke the C++ method even though
a replacement xmethod is available for that method, then they can
disable the xmethod.
DONTPRINTYET 
Xmethod API, for API to implement xmethods in Python. *NoteDONTPRINTYET Xmethod API, for API to implement xmethods in Python.
Writing an Xmethod, for implementing xmethods in Python.
Info Catalog
gdb: Unwinding Frames in Python
gdb: Python API
gdb: Xmethod API