C++ ABI has some per-thread globals: the number that is returned from std::uncaught_exceptions(), and the chain of currently caught exceptions. For example in l

C++ ABI has some per-thread globals: the number that is returned from std::uncaught_exceptions(), and the chain of currently caught exceptions. For example in llvm this is available with a cxa_get_globals call: https://github.com/llvm/llvm-project/blob/b05f1d93469fbd645118739798e51f21a9e410de/libcxxabi/src/cxa_exception.h#L161 https://github.com/llvm/llvm-project/blob/b05f1d93469fbd6451... These need to be saved/restored when switching fibers, otherwise fiber switches from catch clauses (and destructors!) are unsafe, throw without argument may rethrow incorrect exception, code that commits/rollbacks based on uncaught exceptions counter will not work correctly, etc. One example I know where this save/restore is implemented is the userver framework, but it seems to be unexpectedly rare in fiber implementations last time I looked.