It's not really about asynchronous callbacks or their equivalents. (In this case, the thread running it is otherwise meant to be blocked in a safe state, so tha

It's not really about asynchronous callbacks or their equivalents. (In this case, the thread running it is otherwise meant to be blocked in a safe state, so that there's none of the usual dangers of interrupting arbitrary code.) Instead, it's about any callbacks coming out of C code, even something as trivial as qsort(). If you pass a C library your C++ callback, and your callback runs back through it with an exception, then 9 times out of 10, the C library will leak some resources at best, or reach an unstable state at worst. C just doesn't have any portable 'try/finally' construct that can help deal with it. So I'd say it's more about the basic expectations of a function called from C, which includes a million other trivial things like "don't write beyond the bounds of buffers you're given" and "don't clobber your caller's stack frame" and "don't spawn another thread just to write to output pointers after your function returns" (not that any of these is the issue here).