Clozure CL supports the definition of static variables, whose values are the same across threads, and which may not be dynamically bound. The value of a static variable is thus the same across all threads; changing the value in one thread changes it for all threads.

Attempting to dynamically rebind a static variable (for instance, by using LET, or using the variable name as a parameter in a LAMBDA form) signals an error. Static variables are shared global resources; a dynamic binding is private to a single thread.

Static variables therefore provide a simple way to share mutable state across threads. They also provide a simple way to introduce race conditions and obscure bugs into your code, since every thread reads and writes the same instance of a given static variable. You must take care, therefore, in how you change the values of static variables, and use normal multithreaded programming techniques, such as locks or semaphores, to protect against race conditions.

In Clozure CL, access to a static variable is usually faster than access to a special variable that has not been declared static.

DEFSTATIC var value &key doc-string [Macro]

Proclaims the variable special, assigns the variable the supplied value, and assigns the doc-string to the variable's VARIABLE documentation. Marks the variable static, preventing any attempt to dynamically rebind it. Any attempt to dynamically rebind var signals an error.