10.4 Thread synchronisation

All internal Prolog operations are thread-safe. This implies that two Prolog threads can operate on the same dynamic predicate without corrupting the consistency of the predicate. This section deals with user-level mutexes (called monitors in ADA or critical sections by Microsoft). A mutex is a MUTual EXclusive device, which implies that at most one thread can hold a mutex.

Mutexes are used to realise related updates to the Prolog database. With‘related', we refer to the situation where a‘transaction' implies two or more changes to the Prolog database. For example, we have a predicate address/2 , representing the address of a person and we want to change the address by retracting the old and asserting the new address. Between these two operations the database is invalid: this person has either no address or two addresses, depending on the assert/retract order.

The code below provides a solution to this problem based on with_mutex/2. It also illustrates the problem of mutexes. The predicate with_mutex/2 behaves as once/1 with respect to the guarded goal. This means that our predicate address/2 is no longer a nice logical non-deterministic relation. This could be solved by explicit locking and unlocking a mutex using setup_call_cleanup/3, but at the risk of deadlocking the program if the choice point is left open by accident.

change_address(Id, Address) :-
        with_mutex(addressbook,
                   ( retractall(address(Id, _)),
                     asserta(address_db(Id, Address))
                   )).

address(Id, Address) :-
        with_mutex(addressbook,
                   address_db(Id, Address)).

Message queues (see message_queue_create/2) often provide simpler and more robust ways for threads to communicate. Still, mutexes can be a sensible solution and are therefore provided.

mutex_create(?MutexId)

Create a mutex. If MutexId is an atom, a named mutex is created. If it is a variable, an anonymous mutex reference is returned. Anonymous mutexes are subject to (atom) garbage collection.

mutex_create(-MutexId, +Options)

Create a mutex using options. Defined options are:

alias(Alias)

Set the alias name. Using mutex_create(X, [alias(name)]) is preferred over the equivalent mutex_create(name).

mutex_destroy(+MutexId)

Destroy a mutex. If the mutex is not locked, it is destroyed and further access yields an existence_error exception. As of version 7.1.19, this behaviour is reliable. If the mutex is locked, the mutex is scheduled for delayed destruction: it will be destroyed when it becomes unlocked.

with_mutex(+MutexId, :Goal)

Execute Goal while holding MutexId. If Goal leaves choice points, these are destroyed (as in once/1). The mutex is unlocked regardless of whether Goal succeeds, fails or raises an exception. An exception thrown by Goal is re-thrown after the mutex has been successfully unlocked. See also mutex_create/1 and setup_call_cleanup/3.

Although described in the thread section, this predicate is also available in the single-threaded version, where it behaves simply as once/1.

mutex_lock(+MutexId)

Lock the mutex. Prolog mutexes are recursive mutexes: they can be locked multiple times by the same thread. Only after unlocking it as many times as it is locked does the mutex become available for locking by other threads. If another thread has locked the mutex the calling thread is suspended until the mutex is unlocked.

If MutexId is an atom, and there is no current mutex with that name, the mutex is created automatically using mutex_create/1. This implies named mutexes need not be declared explicitly.

Please note that locking and unlocking mutexes should be paired carefully. Especially make sure to unlock mutexes even if the protected code fails or raises an exception. For most common cases, use with_mutex/2, which provides a safer way for handling Prolog-level mutexes. The predicate setup_call_cleanup/3 is another way to guarantee that the mutex is unlocked while retaining non-determinism.

mutex_trylock(+MutexId)

As mutex_lock/1, but if the mutex is held by another thread, this predicates fails immediately.

mutex_unlock(+MutexId)

Unlock the mutex. This can only be called if the mutex is held by the calling thread. If this is not the case, a permission_error exception is raised.

[deprecated]mutex_unlock_all

Unlock all mutexes held by the current thread. This predicate should not be needed if mutex unlocking is guaranteed with with_mutex/2 or setup_call_cleanup/3.^{201The also deprecated thread_exit/1 bypasses the automatic cleanup.}

mutex_property(?MutexId, ?Property)

True if Property is a property of MutexId. Defined properties are:

alias(Alias)

Mutex has the defined alias name. See mutex_create/2 using the‘alias' option.

status(Status)

Current status of the mutex. One of unlocked if the mutex is currently not locked, or locked(Owner, Count) if mutex is locked Count times by thread Owner. Note that unless Owner is the calling thread, the locked status can change at any time. There is no useful application of this property, except for diagnostic purposes.^{bugAs Owner and Count are fetched separately from the mutex, the values may be inconsistent.}