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### 12.7 Foreign threads

Direct calls to `pthread_create` (instead of `MAKE-THREAD`) create threads that SBCL is not aware of, these are called foreign threads. Currently, it is not possible to run Lisp code in such threads. This means that the Lisp side signal handlers cannot work. The best solution is to start foreign threads with signals blocked, but since third party libraries may create threads, it is not always feasible to do so. As a workaround, upon receiving a signal in a foreign thread, SBCL changes the thread's sigmask to block all signals that it wants to handle and resends the signal to the current process which should land in a thread that does not block it, that is, a Lisp thread.

The resignalling trick cannot work for synchronously triggered signals (SIGSEGV and co), take care not to trigger any. Resignalling for synchronously triggered signals in foreign threads is subject to `--lose-on-corruption`, see Runtime Options.