Throwing an exception in C++ in a C callback, possibly crossing over dynamic library boundary... is it safe?

The other answer applies here. Nothing will get trashed when unwinding the stack. It doesn't even matter if the library uses some crazy calling convention internally, as long as it doesn't specifically mess with your C++ implementation's exception handling structures (which it wont as a C program). No C++ implementation that I know of finds the catch block by popping off stack frames (that would make optimization a nightmare), they all maintain internal structures for exception handling. As long as a call lower down in the call chain doesn't mess with those structures, stack unwinding will work perfectly fine for all of your personal code. Now, in general, it's quite possible that this would leave a library with a messed up internal state as you never return execution to the library for cleanup, but in the case of your error callback, libjpeg expects for control flow to not return and has presumably already cleaned up after itself.

In this case, I would go for it. In general, I would only throw fatal exceptions from a C callback.

Hope that helped.

It's not safe. Depending on how the relevant non-C++ library code was compiled, the necessary unwind tables may not exist. This is just a practical reason why it might fail; the conceptual reason is that it's simply undefined behavior.

You should follow the documentation and use setjmp/longjmp to get just outside the call to libjpeg code, then throw an exception immediately in the if (setjmp(...)) { ... } body if you want to use exceptions.