11 Multiprocessing

11.3 Locks

Locks can be used to control access to shared data by several processes.

A lock has the following components: name (a string), lock (t ornil, that is, whether the lock is set or not), owner (a process, ornil) and count (an integer showing the number of times the lock has been set).

The two main symbols used in locking are the functionmake-lock, to create a lock, and the macrowith-lock, to execute a body of code while holding the specified lock.

mp:make-lock

Function

mp:make-lock &key important-p &allow-other-keys

: Creates a lock object. Ifimportant-p ist the lock is added to the list held in the global variablemp:*important-locks*. The functionmp:free-important-locks frees all important locks associated with a given process (or all the important locks if called onnil). Other keywords should be names of the lock components.

mp:process-lock

Function

mp:process-lock lock &optional whostate timeout

: Blocks the current process until the lock is claimed ortimeout elapses if it has been specified. Returnst if lock was claimed,nil otherwise.

mp:process-unlock

Function

mp:process-unlock lock &optional errorp

: Releases the lock. If errorp is non-nil it signals an error if the current process does not own the lock. The default value of errorp ist.

mp:with-lock

Macro

mp:with-lock ((lock &rest lock-args) &body body

: Executes the body with lock held. Arguments to pass on tomp:process-lock are specified using lock-args.
: The following accessors are available for locks:lock-owner,lock-count,lock-name andlock-lock.