What happens if exception gets thrown "through" c code? [duplicate]

Question

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Will C++ exceptions safely propagate through C code?

If you have c code, for example the png lib, with your own io handlers which are written in c++ and an exception gets thrown due to some io error. is it ok to let it go through the c code and catch it outside of the c code? i know that care has to be taken with memory leaks but typically all structures get pre-allocated.

Answer 1

It is entirely up to the compiler if this will work or not. None of the language standards can obviously say anything about what the other language should do.

In the best case, the exception will pass the C code and get back to the next C++ level while possibly leaking any dynamically allocated C structures. In the less good case, it will crash and burn!

One possibilty is of course to compile the C code as C++, while also checking that it is exception neutral.

Answer 2

The answer to your question is implementation-defined.

For GCC, as @Kerrek says, compiling the C code with -fexceptions will ensure the run-time remains consistent when an exception is thrown through C code. (Note that modern exception mechanisms are not just implemented via setjmp/longjmp; they actually unwind the stack frame by frame in order to handle destructors and try/catch blocks properly.)

However, the C code itself may not be expecting to have exceptions thrown through it. Lots of C code is written like this:

acquire a resource do some stuff release resource 

Here "acquire a resource" could mean malloc, or pthread_mutex_lock, or fopen. If your C++ code is part of the "do some stuff", and it throws an exception, well... Whoops.

Resource leaks are not the only problem; so is correctness is general. Imagine:

subtract $100 from savings account add $100 to checking account 

Now imagine an exception gets thrown after the first step finishes but before the second one does.

In short, although your implementation may provide a way to let you throw exceptions through C code, it is a bad idea unless you also wrote that C code knowing exactly where an exception might be thrown.