[Swig-user] Problem passing C++ exceptions between modules

[Ben W. <be...@sa...>]

I have run into a problem wrapping two C++ dynamic libraries for Python
with SWIG. Not sure this is a SWIG-specific problem, but it seems likely
that others have encountered something similar:

I have a dynamic library, libbase.so, which declares and throws a custom
exception. A second library, libmodule.so, calls into libbase and
catches that exception. This works just fine if I link libmodule and
libbase into a C++ program, or if I wrap just libmodule with SWIG. But
if I also wrap libbase, and "import base" before "import module" then
the C++ function in libmodule fails to catch the exception from libbase.

I think this is a problem with typeinfo information not being shared
(see also
http://wiki.python.org/moin/boost.python/CrossExtensionModuleDependencies)
and I can "fix" the problem by setting RTLD_GLOBAL with
sys.setdlopenflags, but this seems a) inelegant and b) like it should be
unnecessary since my custom exception is declared in a shared base
library. Any better ideas what is going on? Defining a virtual
destructor for the exception class seems to fix things, but then I have
to be very careful to do this for each exception I define.

More specifically, all of the files can be found in the attachment; the
mk.sh script builds everything (at least on 32-bit Linux with Python
2.5) and demonstrates the problem.

libbase simply looks like:
base.h:
class Exception {};
void basefunc();

base.cpp:
#include <base.h>
void basefunc() { throw Exception(); }

base.i (notice that I don't actually wrap anything in base):
%module base;
%{
#include "base.h"
%}

libmodule looks like:
module.h:
void modulefunc();

module.cpp:
#include <base.h>
#include <module.h>
#include <iostream>

void modulefunc()
{
  try {
    basefunc();
  } catch (Exception &e) {
    std::cerr << "exception caught" << std::endl;
  }
}

module.i:
%module module;

%{
#include <iostream>
#include "base.h"
#include "module.h"
%}

%exception {
  try {
    $action;
  } catch(Exception &e) {
    std::cerr << "SWIG module caught an exception" << std::endl;
  }
}

%include "module.h"

Everything can be compiled with:
g++ -Wall -shared base.cpp -I. -o libbase.so
g++ -Wall -shared module.cpp -I. -o libmodule.so
swig -c++ -python -I. base.i
swig -c++ -python -I. module.i
g++ -shared -o _base.so base_wrap.cxx -I/usr/include/python2.5 -lbase -L.
g++ -shared -o _module.so module_wrap.cxx -I/usr/include/python2.5
-lbase -lmodule -L.

Then a simple Python script that works (i.e. it prints "exception caught"):
import module
module.modulefunc()

...and one that fails (the exception is not caught within libmodule *or*
in the %exception handler, but terminates the interpreter):
import base
import module
module.modulefunc()

	Ben
-- 
be...@sa...                      http://salilab.org/~ben/
"It is a capital mistake to theorize before one has data."
	- Sir Arthur Conan Doyle