Allegro includes a simple cross-platform threading interface. It is a thin layer on top of two threading APIs: Windows threads and POSIX Threads (pthreads). Enforcing a consistent semantics on all platforms would be difficult at best, hence the behaviour of the following functions will differ subtly on different platforms (more so than usual). Your best bet is to be aware of this and code to the intersection of the semantics and avoid edge cases.

These functions are declared in the main Allegro header file:

 #include <allegro5/allegro.h>



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An opaque structure representing a thread.



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An opaque structure representing a mutex.



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An opaque structure representing a condition variable.


ALLEGRO_THREAD *al_create_thread(
   void *(*proc)(ALLEGRO_THREAD *thread, void *arg), void *arg)

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Spawn a new thread which begins executing proc. The new thread is passed its own thread handle and the value arg.

Returns a pointer to the thread on success. Otherwise, returns NULL if there was an error.

See also: al_start_thread, al_join_thread.


void al_start_thread(ALLEGRO_THREAD *thread)

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When a thread is created, it is initially in a suspended state. Calling al_start_thread will start its actual execution.

Starting a thread which has already been started does nothing.

See also: al_create_thread.


void al_join_thread(ALLEGRO_THREAD *thread, void **ret_value)

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Wait for the thread to finish executing. This implicitly calls al_set_thread_should_stop first.

If ret_value is non-NULL, the value returned by the thread function will be stored at the location pointed to by ret_value.

See also: al_set_thread_should_stop, al_get_thread_should_stop, al_destroy_thread.


void al_set_thread_should_stop(ALLEGRO_THREAD *thread)

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Set the flag to indicate thread should stop. Returns immediately.

See also: al_join_thread, al_get_thread_should_stop.


bool al_get_thread_should_stop(ALLEGRO_THREAD *thread)

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Check if another thread is waiting for thread to stop. Threads which run in a loop should check this periodically and act on it when convenient.

Returns true if another thread has called al_join_thread or al_set_thread_should_stop on this thread.

See also: al_join_thread, al_set_thread_should_stop.

Note: We don't support forceful killing of threads.


void al_destroy_thread(ALLEGRO_THREAD *thread)

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Free the resources used by a thread. Implicitly performs al_join_thread on the thread if it hasn't been done already.

Does nothing if thread is NULL.

See also: al_join_thread.


void al_run_detached_thread(void *(*proc)(void *arg), void *arg)

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Runs the passed function in its own thread, with arg passed to it as only parameter. This is similar to calling al_create_thread, al_start_thread and (after the thread has finished) al_destroy_thread - but you don't have the possibility of ever calling al_join_thread on the thread.


ALLEGRO_MUTEX *al_create_mutex(void)

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Create the mutex object (a mutual exclusion device). The mutex may or may not support "recursive" locking.

Returns the mutex on success or NULL on error.

See also: al_create_mutex_recursive.


ALLEGRO_MUTEX *al_create_mutex_recursive(void)

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Create the mutex object (a mutual exclusion device), with support for "recursive" locking. That is, the mutex will count the number of times it has been locked by the same thread. If the caller tries to acquire a lock on the mutex when it already holds the lock then the count is incremented. The mutex is only unlocked when the thread releases the lock on the mutex an equal number of times, i.e. the count drops down to zero.

See also: al_create_mutex.


void al_lock_mutex(ALLEGRO_MUTEX *mutex)

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Acquire the lock on mutex. If the mutex is already locked by another thread, the call will block until the mutex becomes available and locked.

If the mutex is already locked by the calling thread, then the behaviour depends on whether the mutex was created with al_create_mutex or al_create_mutex_recursive. In the former case, the behaviour is undefined; the most likely behaviour is deadlock. In the latter case, the count in the mutex will be incremented and the call will return immediately.

See also: al_unlock_mutex.

We don't yet have al_mutex_trylock.


void al_unlock_mutex(ALLEGRO_MUTEX *mutex)

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Release the lock on mutex if the calling thread holds the lock on it.

If the calling thread doesn't hold the lock, or if the mutex is not locked, undefined behaviour results.

See also: al_lock_mutex.


void al_destroy_mutex(ALLEGRO_MUTEX *mutex)

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Free the resources used by the mutex. The mutex should be unlocked. Destroying a locked mutex results in undefined behaviour.

Does nothing if mutex is NULL.


ALLEGRO_COND *al_create_cond(void)

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Create a condition variable.

Returns the condition value on success or NULL on error.


void al_destroy_cond(ALLEGRO_COND *cond)

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Destroy a condition variable.

Destroying a condition variable which has threads block on it results in undefined behaviour.

Does nothing if cond is NULL.


void al_wait_cond(ALLEGRO_COND *cond, ALLEGRO_MUTEX *mutex)

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On entering this function, mutex must be locked by the calling thread. The function will atomically release mutex and block on cond. The function will return when cond is "signalled", acquiring the lock on the mutex in the process.

Example of proper use:

while (something_not_true) {
   al_wait_cond(cond, mutex);

The mutex should be locked before checking the condition, and should be rechecked al_wait_cond returns. al_wait_cond can return for other reasons than the condition becoming true (e.g. the process was signalled). If multiple threads are blocked on the condition variable, the condition may no longer be true by the time the second and later threads are unblocked. Remember not to unlock the mutex prematurely.

See also: al_wait_cond_until, al_broadcast_cond, al_signal_cond.


int al_wait_cond_until(ALLEGRO_COND *cond, ALLEGRO_MUTEX *mutex,
   const ALLEGRO_TIMEOUT *timeout)

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Like al_wait_cond but the call can return if the absolute time passes timeout before the condition is signalled.

Returns zero on success, non-zero if the call timed out.

See also: al_wait_cond


void al_broadcast_cond(ALLEGRO_COND *cond)

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Unblock all threads currently waiting on a condition variable. That is, broadcast that some condition which those threads were waiting for has become true.

See also: al_signal_cond.

Note: The pthreads spec says to lock the mutex associated with cond before signalling for predictable scheduling behaviour.


void al_signal_cond(ALLEGRO_COND *cond)

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Unblock at least one thread waiting on a condition variable.

Generally you should use al_broadcast_cond but al_signal_cond may be more efficient when it's applicable.

See also: al_broadcast_cond.

Allegro version 5.2.4 - Last updated: 2018-02-25 20:26:07 UTC