Emacs Lisp provides a limited form of concurrency, called threads. All the threads in a given instance of Emacs share the same memory. Concurrency in Emacs Lisp is “mostly cooperative”, meaning that Emacs will only switch execution between threads at well-defined times. However, the Emacs thread support has been designed in a way to later allow more fine-grained concurrency, and correct programs should not rely on cooperative threading.
Currently, thread switching will occur upon explicit request via
thread-yield, when waiting for keyboard input or for process
output from asynchronous processes (e.g., during
accept-process-output), or during blocking operations relating
to threads, such as mutex locking or thread-join.
Emacs Lisp provides primitives to create and control threads, and also to create and control mutexes and condition variables, useful for thread synchronization.
While global variables are shared among all Emacs Lisp threads,
local variables are not—a dynamic let binding is local. Each
thread also has its own current buffer (see The Current Buffer) and
its own match data (see The Match Data).
Note that let bindings are treated specially by the Emacs
Lisp implementation. There is no way to duplicate this unwinding and
rewinding behavior other than by using let. For example, a
manual implementation of let written using
unwind-protect cannot arrange for variable values to be
thread-specific.
In the case of lexical bindings (see Scoping Rules for Variable Bindings), a closure is an object like any other in Emacs Lisp, and bindings in a closure are shared by any threads invoking the closure.