10.2 Special Variables

The interaction of special variables with multiple threads is mostly as one would expect, but users of other Lisps are warned that the behaviour of locally bound specials differs in places from what they may expect.

• global special values are visible across all threads;
• bindings (e.g. using LET) are local to the thread;
• initial values in a new thread are taken from the thread that created it.

The last point means that

     (defparameter *x* 0)
     (let ((*x* 1))
       (sb-thread:make-thread (lambda () (print *x*))))

prints 1.