to be clear, the single-threaded behaviour of some implementations is, well, implementation specific. There is nothing in the C++ standard that requires that. I

to be clear, the single-threaded behaviour of some implementations is, well, implementation specific. There is nothing in the C++ standard that requires that. In particular, for GCC under linux, the issue is that the unwinder need to parse the unwind tables and needs to protect itself from a concurrent ldclose yanking them under it. There is some work currently on GCC to move this cost from unwinding to ldclose and allow concurrent unwinding without breaking the ABI (but it is hard).