I don’t believe the Common Lisp specification says anything about threads. For example here[1] is the relevant section of the SBCL manual. As you can see it’s q

I don’t believe the Common Lisp specification says anything about threads. For example here[1] is the relevant section of the SBCL manual. As you can see it’s quite low level. If you’re using those primitives then yes you’ll have to manage barriers, synchronization, use atomic operations, or otherwise maintain sanity. The REPL (or SWANK) thread is just another thread. Edit: I should add that the usual pattern is Bordeaux threads or some other higher level thing. [1] http://www.sbcl.org/manual/index.html#Threading http://www.sbcl.org/manual/index.html#Threading