[Boost-users] Boost.Context: SIGSEGV when switch a thread within a catch block

[Александр Пронченков]

Hi,

I get SIGSEGV on macOS when I switch an execution_context from a catch
block with one thread and than switch back with another thread.
Segmentation fault happens in libc++abi.dylib`__cxa_end_catch

Does anyone have any ideas, what it could be? Is it legal to switch
thread under an excecution_context while it's within a catch block?

A minimal example is here:

#include <boost/context/all.hpp>
#include <future>

using Context = boost::context::execution_context<void>;

Context Main(Context context) {
try {
throw std::runtime_error("123");
} catch (const std::exception& ex) {
context = context();
}
return context();
}

int main() {
Context context(&Main);
context = context();
std::thread([&] { context = context(); }).join();
return 0;
}


I see the problem on macOS with clang++ but can't reproduce on a linux with g++.

$ uname -ap
Darwin air.local 16.3.0 Darwin Kernel Version 16.3.0: Thu Nov 17
20:23:58 PST 2016; root:xnu-3789.31.2~1/RELEASE_X86_64 x86_64 i386

$ clang++ --version
Apple LLVM version 8.0.0 (clang-800.0.42.1)
Target: x86_64-apple-darwin16.3.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

--
Thanks,
Alexander Pronchenkov

[Oliver Kowalke]

not permitted - see
http://www.boost.org/doc/libs/1_63_0/libs/context/doc/html/context/ecv2.html

[Александр Пронченков]

Hi Oliver,

Thanks for your relay. Do you mean this statement in the documentation?

[image: [Important]]Important

Do not jump from inside a catch block and then re-throw the exception in
another execution context.

It's a bit unclear for me because I don't re-throw the exception neither in
one execution context nor in another.

Do you know how to address this issue? Is it addressed in Boost.Coroutines2?

In my case I play with a green-threads runtime and this limitation means
that I can't use asynchronous operations within exception handlers. It
looks artificial and also requires checks/modifications of the existing
code to make it compatible. :(

--
Thanks,
Alexander Pronchenkov

Post by Oliver Kowalke
not permitted - see
http://www.boost.org/doc/libs/1_63_0/libs/context/doc/html/context/ecv2.html
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[Oliver Kowalke]

you must no call jump-op inside a catch block - that means you can't use
async-op handlers inside exception handler
exceptions might be implemented using sj/lj - in general, exception
handlers are handled/implemented in a special way.

[Alexander G. Pronchenkov]

Hi Oliver,

I think it would make sense to slightly clarify the documentation:

- Do not jump from inside a catch block and then re-throw the
exception in another execution context.
+ Do not jump from inside a catch block.

Otherwise it's a bit confusing because I don't re-throw exceptions in
my case but the problem is here.

Also, do you known whether it's safe to jump from inside of a try
block? I'm asking because there is a not at
https://docs.oracle.com/cd/E24457_01/html/E21991/bkahg.html that

... you must not longjmp into or out of a try-block or catch-block (directly or indirectly) ...

--
Thanks,
Alexander Pronchenkov

you must no call jump-op inside a catch block - that means you can't use
async-op handlers inside exception handler
exceptions might be implemented using sj/lj - in general, exception handlers
are handled/implemented in a special way.
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