[C++-sig] question on boost.python exception mechanisms

[Holger Joukl]

Hi,

Giuseppe Corbelli <giuseppe.corbelli at copanitalia.com> wrote on 08.04.2013
11:29:10:

> On 03/04/2013 10:08, Holger Joukl wrote:
> > Hi Giuseppe,
> >
> > thanks for answering and sorry for the delayed response. Easter
> > holidays :-)
>
> To punish you here's another late reply.

;-)

> >>> However, on Linux compiled with GCC, if an exception gets raised in
the
> >>> Python callback the
> >>> program segfaults. Obviously the C part of the vendor library has not
> >>> been compiled with
> >>> (GCC-) exception support for Linux.
> >>>
> >>> Unfortunately this isn't under my control and the library provider
> >>> seems not to be willing
> >>> to make the library robust against exceptions in user callback code.
> >>>
> >>> So I need to keep the Boost C++ exception that "signals" the Python
> >>> exception from passing
> >>> through the C parts.
> >>
> >> How does the library handle callbacks? An event loop in a separate
> >> thread? Do you have to explicitly call some blocking function to run
the
> >> event loop?
> >
> > Yes. There's a separate thread that schedules events to a queue being
> > dispatch in the main loop, in the main thread.
> > And yes, there's a blocking call to a dispatch method with the
possibility
> > of a timeout or alternatively a non-blocking poll call (we make
> sure to properly
> > release&  reacquire the Python GIL during blocking).
> >
> >> If it's a C lib likely the callbacks are just function pointers. Being
> >> called inside a C object I'd say you assumption is correct: no C++
> >> exceptions to be
> >> raised inside callbacks (the lib is gcc-compiled, it has no knowledge
of
> >> exceptions).
> >
> > Correct, the callbacks are function pointers from the C libs point of
view.
> >
> >> If you could recompile with g++ the exception would raise from the
> >> event loop,
> >> unless catched while calling the callback function pointer.
> >
> > While I do have the C++ sources (to be able to recompile for different
C++
> > compilers)
> > I don't have the C sources so I unfortunately I can't recompile the
C-lib
> > parts with
> > exceptions support.
> > Sidenote: For Oracle/Sun Solaris Studio compilers this isn't a problem
as
> > you basically can't
> > and don't need to switch the exception support of the compiler for
mixing C
> > and C++ code
> > (http://www.oracle.com/technetwork/articles/servers-storage-dev/
> mixingcandcpluspluscode-305840.html)
>
> I have found a couple of references.
> http://gcc.gnu.org/onlinedocs/gcc/Link-Options.html (see static-libgcc)
> http://gcc.gnu.org/wiki/Visibility

Thanks, I'll need to look into these.

> The proprietary lib is shared, right? linked to? shared? static?

Shared C lib, we compile a shared C++ lib linked to the C lib from the
vendor
C++ sources, which we shared-link the (shared) Boost.Python wrapper to.

> >> Sidenote: how does the C++ exception mechanism work under the hood?
What
> >> happens if it's called inside a C compiled function?
> >
> > Not sure if I understand your question correctly, but:
> > - Solaris Studio compilers: An exception raised in a C++ compiled
function
> > that got called
> > from a C compiled function called from a C++ main will properly
propagate
> > to the main caller
> > - GCC: An exception raised in a C++ compiled function that got called
> > from a C compiled function called from a C++ main will
> >    - segfault if the C compiled function hasn't been compiled
withexception
> > support (-fexceptions)
> >    - properly propagate to the main caller if the C compiled function
has
> > been compiled with exception support
>
> Well, I was thinking about ASM level :-)

No clue whatsoever.

Regards
Holger

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