Re: Throwing exception accross C callback

From:

"Igor Tandetnik" <itandetnik@mvps.org>

Newsgroups:

microsoft.public.vc.language

Date:

Fri, 31 Oct 2008 16:31:43 -0400

Message-ID:

<#xY8pi5OJHA.1148@TK2MSFTNGP05.phx.gbl>

Alexander Lamaison <newsgroups@lammy.co.uk> wrote:

The problem is, I would like my callback function to be able to throw
exceptions. The C library doesn't need to understand them; just to
pass them up to the calling C++ code. So the execution chain looks
like this:

C++ class -------> C library ---(callback)---> C++ c/b method

The callback method would throw an exception which would be called in
the C++ code that invoked the library function.

Is this possible? I've tried it and the exception is caught but
various stack objects are not destructed correctly. I'm not willing
to use this unless it's safe. Is there a correct way to do this.

C code doesn't expect C++ exceptions ripping through it, and can't do
much to prevent leaks anyway. Consider:

void* memory = malloc(...);
yourCallback();
free(memory);

Lacking destructors and try/catch clauses, I don't quite see how C code
can possibly prevent a leak in this situation. Your only hope is that
it's very carefully written so as not to hold any dynamically allocated
resources around the callback call. This sounds like a pretty poor bet.
--
With best wishes,
Igor Tandetnik

With sufficient thrust, pigs fly just fine. However, this is not
necessarily a good idea. It is hard to be sure where they are going to
land, and it could be dangerous sitting under them as they fly
overhead. -- RFC 1925