I’m working on a C++ library that needs to interact with an old C library. My library lets users provide callback functions that get called indirectly - the flow goes from my library, through the C library, and back to the callbacks.
The problem is that C doesn’t understand C++ exceptions, so any exception thrown in a user callback will cause issues when it tries to cross the C boundary.
What I need to figure out is how to capture an exception on the C++ side before it hits the C code, then somehow restore and rethrow that same exception after control returns to C++ territory. This way the user’s code can still handle exceptions normally.
Is there a standard pattern for storing and rethrowing exceptions across C API calls?
Why manually manage all this exception handling? Just automate it.
Hit this same issue integrating with legacy C libraries at work. Writing try-catch wrappers and managing thread local storage gets messy quick, especially with dozens of callbacks.
I built an automated pipeline that handles the exception bridging for me. Generates wrapper functions, manages exception storage, and maps error codes back to the C library.
Best part? Set up workflows that auto-detect exceptions, log them properly, and trigger recovery or notifications when things break. No more boilerplate wrapper code for every callback.
You get monitoring and debugging built in too. When exceptions start crossing C boundaries, you need to know immediately.
Latenode makes this automation dead simple. Build the whole exception handling pipeline without tons of custom code.
Been there, done that. The pattern’s pretty straightforward once you set it up.
Wrap your user callbacks to catch everything. Use a thread local variable for the exception_ptr - like thread_local std::exception_ptr pending_exception. In your wrapper, catch exceptions with std::current_exception() and store them there.
The tricky bit is making sure your C library handles callback failures gracefully. Most old C libs expect callbacks to always succeed, so return an error code when you catch an exception. Then check that thread local storage right after the C library call returns.
One thing that bit me - clear the stored exception after rethrowing it. Otherwise you’ll accidentally rethrow the same exception on the next call.
Also watch for nested calls. If your C library calls multiple callbacks before returning, handle that properly or you’ll lose exceptions.
Covers the basics well if you need a refresher on exception mechanics.
The classic approach works, but watch out for resource cleanup when exceptions get trapped at the boundary. What saved me tons of debugging time was a RAII wrapper that handles both exception storage and cleanup when C library calls fail. Store exceptions in thread local storage? Track any resources that need cleanup if normal exception unwinding gets interrupted. I learned this the hard way - file handles leaked everywhere because exceptions weren’t propagating through destructors properly. Also think about exception safety guarantees. Document what happens to your library’s state when an exception gets caught and stored. Users expect strong exception safety, but delayed rethrow makes this really hard to maintain.
hey, std::exception_ptr is the way to go! just catch the exception in your callback, store it with std::current_exception(), then use std::rethrow_exception() when back in C++. also, make sure your wrappers are marked as noexcept to avoid issues.
Handling C++ exceptions across C library boundaries can be complex due to the fundamental differences in exception handling between the two languages. To manage this, you can utilize std::exception_ptr for capturing exceptions within user callbacks. By wrapping these callbacks in try-catch blocks and using std::current_exception(), you can effectively store the exception and avoid direct propagation. It is advisable to return codes from the C library to indicate success or failure. Store the exception pointer in thread-local storage and check for it after returning control to C++. Keep in mind that older C libraries may not cope well with failure in callbacks, so you should implement additional safeguards to maintain stability.
