Direct calls to
pthread_create (instead of
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
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.