Can you send the C++ code of MyObj? Kind of hard to diagnose a
C++-side crash without any C++ code.
On 3/20/2013 9:46 AM, Alex Leach wrote:
Dear list,
I've started using Boost.Python to wrap a 3rd party C++ library,
and whilst running my unit tests, the Python interpreter
segfault's during garbage collection.
With explicit object deletion:
$ gdb -q --args python -c 'import mylib; obj = mylib.MyObj();
del(obj)'
...
*** Error in `/usr/bin/python': free(): invalid next size (fast):
0x0000000000806fd0 ***
======= Backtrace: =========
/usr/lib/libc.so.6(+0x7ab06)[0x7ffff74d3b06]
/usr/lib/libc.so.6(+0x7b883)[0x7ffff74d4883]
/usr/lib/libboost_python.so.1.53.0(_ZN5boost6python15instance_holder10deallocateEP7_objectPv+0x15)[0x7ffff004f555]
/usr/lib/libboost_python.so.1.53.0(+0x265a1)[0x7ffff004f5a1]
...
======= Memory map: ========
...
or, leaving it to the garbage collector:-
$ gdb -q --args python -c 'import mylib; obj = mylib.obj() '
...
Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7b2b0b0 in visit_decref () from
/usr/lib/libpython2.7.so.1.0
(gdb)
Quick question: What do I need to do to fix this?
==========================
--- The wrapper class ---
The wrapper class I've written is fairly simple; I haven't
explicitly added a 'PyObject* _self' attribute, nor done any of
the wiki recommendations[3] for managing ownership, as I don't see
anywhere in the tutorial that recommends any of these methods.
------------------------------------
--- Using boost::python::wrapper ---
The wrapped class (in 3rd party library) does have one virtual
method (that is protected and I haven't exposed to Python) and a
virtual destructor. I gather the right thing to do with virtual
classes is to also inherit from
'boost::python::wrapper<Base>', but this doesn't affect the
seg-fault. I guess I don't need to inherit from
wrapper<Base>, as I am not exposing any virtual methods, nor
using 'this->get_override(...)' anywhere in the class
definition.
Another very similar class is in the same source file though
(virtual destructor and one unexposed virtual method), but this
one has a public copy constructor. If I inherit from
'wrapper<Base>', I get a compilation error regarding an
argument mismatch when Boost.Python passes the derived class to
the copy constructor of the base class. Of course I can add
noncopyable, to the class_<..> registration, but I'd like
the exposed class to stay copyable, and I might like to use some
of the wrapper template class' functionality. Should I wrap the
copy constructor in the wrapper class too? Would it be possible to
add this functionality to the wrapper<> template, when the
class is not noncopyable?
------------------------------------
--- Calling thread-safe methods ---
I don't think this is directly relevant, but the classes in
question use the 3rd party library's own mutex implementations to
ensure thread-safety. I've just got these working by writing a
simple utility class that releases the GIL in the constructor and
reacquires it in its destructor, using the C-Python API
PyGILState_* functions[1].
The general tactic was described on the boost.python wiki[2], but
the PyEval_(Save|Restore)Thread functions described caused the
same assertion errors that I was getting earlier, with the
pthreads mutex. This happened when the 3rd party library tried to
lock the global mutex.
Although this is mentioned in the C-Python manual, I would have
found it useful if thread-safety was specifically mentioned in the
tutorial or reference manual somewhere... I attach the solution I
wrote to this problem, in case someone would want to add it and
document it in Boost.Python somewhere.
Usage:
{
MakeThreadsafe scope; //< GIL acquired
// call function that acquires pthreads mutex.
} //< GIL released when leave scope
------------------------------------
--- Garbage collection segfaul ---
Any help with this would be really appreciated! I thought that
registering a wrapped class with class_<...> was enough, to
ensure an object's ref count was incremented and decremented
properly. I guess that's the fundamentals of the problem, but am
not sure the easiest / best / most efficient way of solving the
problem.. Again, some specific documentation, on when and why this
problem occurs and how to solve it, in the tutorial would be
great!
Looking forward to hearing back...
Kind regards,
Alex
[1] -
http://docs.python.org/2.7/c-api/init.html#non-python-created-threads
[2] -
http://wiki.python.org/moin/boost.python/HowTo#Multithreading_Support_for_my_function
[3] -
http://wiki.python.org/moin/boost.python/HowTo#ownership_of_C.2B-.2B-_object
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