Re: [boost] [Python] null pointer access violation when calling Python function

From: David Abrahams (dave_at_[hidden])
Date: 2006-08-30 15:46:14

Tanguy Fautre <tanguy.fautre_at_[hidden]> writes:

> Hi,
>
> I have an object "m_RegisterEvents" which holds a reference to a Python 
> function. This Python function is a callback given by a Python script.
>
> To call this function, I simply do:
>
> void subsystem_script::register_events()
> {
> 	m_RegisterEvents();
> }
>
>
> And it works.
>
> However, if a Python exception is thrown (and converted to the C++ 
> exception boost::python::error_already_set), I get a segfault in the 
> following function (m_ptr is null, hence the segfault):
>
> inline api::object_base::~object_base()
> {
>      Py_DECREF(m_ptr);
> }
>
>
> This destructor is directly called at the end of the 
> subsystem_script::register_events() function, leading me to believe that 
> it is a temporary object being destructed.

Well, are you sure you're not doing something illegal somewhere?  It's
supposed to be an invariant that object (and object_base) never has a
null m_ptr.  That's why we're not using Py_XDECREF.

So, either Boost.Python is doing something to break that invariant, or
some code (maybe yours, maybe mine) is incorrectly initializing that
object, or there's a bug in the compiler's exception unwinding.

> The funny thing is that if I do the following, then it works again.
>
>
> void subsystem_script::register_events()
> {
> 	try {
> 		m_RegisterEvents();
> 	}
>
> 	catch (const error_already_set &)
> 	{
>
> 	}
> }
>
>
> However, doing a "throw;" in the catch to re-throw the exception does 
> the same segfault. Converting the exception by throwing another object 
> (e.g. std::runtime_error()) works.
>
>
> I must admit that I'm totally puzzled.

Very strange.  All I can say is what I always tell people: reduce your
example to a _truly_ minimal one that reproduces the behavior (one C++
file and one python file) and either you'll discover what you did
wrong or you'll give me enough to fix the problem.  And please bring
your Boost.Python questions to the C++-sig
<http://boost.org/more/mailing_lists.htm#cplussig>.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com