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

[Holger Joukl]

Hi Giuseppe,

thanks for answering and sorry for the delayed response. Easter
holidays :-)

Giuseppe Corbelli <giuseppe.corbelli at copanitalia.com> wrote on 28.03.2013
09:37:39:

> On 26/03/2013 18:51, Holger Joukl wrote:
> >
> > Hi,
> >
> > I'm wrapping a C++ library that's actually just a thin wrapper around a
C
> > lib.
> >
> > Through a dispatch() method of a Dispatcher class there's an invocation
of
> > callbacks which get implemented on the Python side, by subclassing
callback
> > classes exposed through Boost.Python.
> >
> > Now, for performing the actual dispatch the C++ library calls into the
> > underlying C library.
> >
> > This hasn't been a problem so far as we've been targetting Solaris
using
> > Sun/Oracle studio
> > compilers.
> >
> > 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)

> 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 with exception
support (-fexceptions)
  - properly propagate to the main caller if the C compiled function has
been compiled with exception support

Holger

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