Lock Class

A Lock is used to protect a resource from concurrent access by multiple threads or processes. This is done by following the rule that the shared resource is only accessed after the lock has been obtained. The operating system ensures that only one thread can have taken a lock at any one time. Once the resource is no longer required, the lock is released so that other threads can gain access.

Lock Creation

The create function is used to create a Lock. The following creates an un-named lock:

    (create Lock)

If this Lock object is shared between threads, it can be used by those threads to synchronise access to another shared resource.

Named locks can be used to lock a resource across different processes -- this mirrors the named/un-named Event object usage. Locks of the same name are essentially the 'same' lock. A named lock is created as follows:

    (create Lock "fred")

Locks are most often used with the LifetimeLock object, which holds a lock for as long as the lifetime-lock is in existence. This ensures that the lock is released properly even if an Error is thrown.

Lock Properties

property name	property type	description
`name`	String	the name of the lock

Lock Methods

method name	description
`lock`	take the lock -- so that no other thread can take it
`unlock`	release the lock -- so that another thread can take it
`isLocked`	test if the lock has been taken

`lock`

Take the lock -- so that no other thread can take it. The time (in milliseconds) to wait for the lock is supplied. If the lock is successfully taken in time, the Symbol ok is returned, otherwise the empty List () is returned.

Arguments:

timeout Integer

Return type: Symbol or the empty List ()

`unlock`

Release the lock -- so that another thread can take it. If the lock is successfully released, the Symbol ok is returned, otherwise the empty List () is returned.

Arguments: none

Return type: Symbol or the empty List ()

`isLocked`

Test if the lock has been taken. If so, the Symbol ok is returned, otherwise the empty List () is returned.

Arguments: none

Return type: Symbol or the empty List ()

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