37.2 Mutexes

A mutex is an exclusive lock. At any moment, zero or one threads may own a mutex. If a thread attempts to acquire a mutex, and the mutex is already owned by some other thread, then the acquiring thread will block until the mutex becomes available.

Emacs Lisp mutexes are of a type called recursive, which means that a thread can re-acquire a mutex it owns any number of times. A mutex keeps a count of how many times it has been acquired, and each acquisition of a mutex must be paired with a release. The last release by a thread of a mutex reverts it to the unowned state, potentially allowing another thread to acquire the mutex.

function mutexp object

This function returns t if object represents an Emacs mutex, nil otherwise.

function make-mutex \&optional name

Create a new mutex and return it. If name is specified, it is a name given to the mutex. It must be a string. The name is for debugging purposes only; it has no meaning to Emacs.

function mutex-name mutex

Return the name of mutex, as specified to make-mutex.

function mutex-lock mutex

This will block until this thread acquires mutex, or until this thread is signaled using thread-signal. If mutex is already owned by this thread, this simply returns.

function mutex-unlock mutex

Release mutex. If mutex is not owned by this thread, this will signal an error.

macro with-mutex mutex body…

This macro is the simplest and safest way to evaluate forms while holding a mutex. It acquires mutex, invokes body, and then releases mutex. It returns the result of body.