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Mutexes are used for controlling access to a shared resource. One thread is allowed to hold the mutex, others which attempt to take it will be made to wait until it's free. Threads are woken in the order that they go to sleep.
There isn't a timeout on mutex acquisition, but the usual WITH-TIMEOUT macro (which throws a TIMEOUT condition after n seconds) can be used if you want a bounded wait.
(defpackage :demo (:use "CL" "SB-THREAD" "SB-EXT"))
(in-package :demo)
(defvar *a-mutex* (make-mutex :name "my lock"))
(defun thread-fn ()
(format t "Thread ~A running ~%" *current-thread*)
(with-mutex (*a-mutex*)
(format t "Thread ~A got the lock~%" *current-thread*)
(sleep (random 5)))
(format t "Thread ~A dropped lock, dying now~%" *current-thread*))
(make-thread #'thread-fn)
(make-thread #'thread-fn)
Name of a
queue. Can be assingned to usingsetf. Queue names can be arbitrary printable objects, and need not be unique.
Current owner of the mutex,
nilif the mutex is free. May return a stale value, usemutex-ownerinstead.
Acquire
mutexfornew-owner, which must be a thread ornil. Ifnew-ownerisnil, it defaults to the current thread. Ifwaitpis non-NIL and the mutex is in use, sleep until it is available.Note: using
get-mutexto assign amutexto another thread then the current one is not recommended, and liable to be deprecated.
get-mutexis not interrupt safe. The correct way to call it is:(WITHOUT-INTERRUPTS ... (ALLOW-WITH-INTERRUPTS (GET-MUTEX ...)) ...)
without-interruptsis necessary to avoid an interrupt unwinding the call while the mutex is in an inconsistent state whileallow-with-interruptsallows the call to be interrupted from sleep.It is recommended that you use
with-mutexinstead of callingget-mutexdirectly.
Release
mutexby setting it tonil. Wake up threads waiting for this mutex.
release-mutexis not interrupt safe: interrupts should be disabled around calls to it.If the current thread is not the owner of the mutex then it silently returns without doing anything (if
if-not-owneris :PUNT), signals awarning(ifif-not-owneris :WARN), or releases the mutex anyway (ifif-not-owneris :FORCE).
Acquire
mutexfor the dynamic scope ofbody, setting it tovalueor some suitable default value ifnil. Ifwait-pis non-NIL and the mutex is in use, sleep until it is available
Acquires
mutexfor the dynamic scope ofbody. Within that scope further recursive lock attempts for the same mutex succeed. It is allowed to mixwith-mutexandwith-recursive-lockfor the same mutex provided the default value is used for the mutex.