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>

# ALLEGRO_THREAD

    typedef struct ALLEGRO_THREAD ALLEGRO_THREAD;

Source Code

An opaque structure representing a thread.

Examples:

- ex_threads.c
- ex_threads2.c
- ex_loading_thread.c

# ALLEGRO_MUTEX

    typedef struct ALLEGRO_MUTEX ALLEGRO_MUTEX;

Source Code

An opaque structure representing a mutex.

Examples:

- ex_threads2.c
- ex_loading_thread.c

# ALLEGRO_COND

    typedef struct ALLEGRO_COND ALLEGRO_COND;

Source Code

An opaque structure representing a condition variable.

Examples:

- ex_threads2.c

# al_create_thread

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

Source Code

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.

Examples:

- ex_threads.c
- ex_threads2.c
- ex_loading_thread.c

# al_create_thread_with_stacksize

    ALLEGRO_THREAD *al_create_thread_with_stacksize(
       void *(*proc)(ALLEGRO_THREAD *thread, void *arg), void *arg, size_t stacksize)

Source Code

Spawn a new thread with the give stacksize in bytes 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.

Since: 5.2.5

> Unstable API: New API, may want a better way to specify thread options.

See also: al_start_thread, al_join_thread.

# al_start_thread

    void al_start_thread(ALLEGRO_THREAD *thread)

Source Code

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.

Examples:

- ex_threads.c
- ex_threads2.c
- ex_loading_thread.c

# al_join_thread

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

Source Code

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.

Examples:

- ex_threads.c
- ex_threads2.c
- ex_loading_thread.c

# al_set_thread_should_stop

    void al_set_thread_should_stop(ALLEGRO_THREAD *thread)

Source Code

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

See also: al_join_thread, al_get_thread_should_stop.

Examples:

- ex_threads2.c

# al_get_thread_should_stop

    bool al_get_thread_should_stop(ALLEGRO_THREAD *thread)

Source Code

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.

Examples:

- ex_threads2.c
- ex_loading_thread.c

# al_destroy_thread

    void al_destroy_thread(ALLEGRO_THREAD *thread)

Source Code

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.

Examples:

- ex_threads.c
- ex_threads2.c
- ex_native_filechooser.c

# al_run_detached_thread

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

Source Code

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.

# al_create_mutex

    ALLEGRO_MUTEX *al_create_mutex(void)

Source Code

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.

Examples:

- ex_threads2.c
- ex_loading_thread.c

# al_create_mutex_recursive

    ALLEGRO_MUTEX *al_create_mutex_recursive(void)

Source Code

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.

# al_lock_mutex

    void al_lock_mutex(ALLEGRO_MUTEX *mutex)

Source Code

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.

Examples:

- ex_threads2.c
- ex_loading_thread.c

# al_unlock_mutex

    void al_unlock_mutex(ALLEGRO_MUTEX *mutex)

Source Code

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.

Examples:

- ex_threads2.c
- ex_loading_thread.c

# al_destroy_mutex

    void al_destroy_mutex(ALLEGRO_MUTEX *mutex)

Source Code

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`.

Examples:

- ex_loading_thread.c

# al_create_cond

    ALLEGRO_COND *al_create_cond(void)

Source Code

Create a condition variable.

Returns the condition value on success or `NULL` on error.

Examples:

- ex_threads2.c

# al_destroy_cond

    void al_destroy_cond(ALLEGRO_COND *cond)

Source Code

Destroy a condition variable.

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

Does nothing if `cond` is `NULL`.

# al_wait_cond

    void al_wait_cond(ALLEGRO_COND *cond, ALLEGRO_MUTEX *mutex)

Source Code

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:

    al_lock_mutex(mutex);
    while (something_not_true) {
       al_wait_cond(cond, mutex);
    }
    do_something();
    al_unlock_mutex(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.

Examples:

- ex_threads2.c

# al_wait_cond_until

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

Source Code

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

# al_broadcast_cond

    void al_broadcast_cond(ALLEGRO_COND *cond)

Source Code

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.

Examples:

- ex_threads2.c

# al_signal_cond

    void al_signal_cond(ALLEGRO_COND *cond)

Source Code

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 5.2.11 - Last updated: 2026-02-09 02:00:42 UTC