## Asynchronous Queues

Asynchronous Queues — asynchronous communication between threads

## Synopsis

    #include <glib.h>

                        GAsyncQueue;
    GAsyncQueue *       g_async_queue_new                   (void);
    GAsyncQueue *       g_async_queue_new_full              (GDestroyNotify item_free_func);
    GAsyncQueue *       g_async_queue_ref                   (GAsyncQueue *queue);
    void                g_async_queue_unref                 (GAsyncQueue *queue);
    void                g_async_queue_push                  (GAsyncQueue *queue,
                                                             gpointer data);
    void                g_async_queue_push_sorted           (GAsyncQueue *queue,
                                                             gpointer data,
                                                             GCompareDataFunc func,
                                                             gpointer user_data);
    gpointer            g_async_queue_pop                   (GAsyncQueue *queue);
    gpointer            g_async_queue_try_pop               (GAsyncQueue *queue);
    gpointer            g_async_queue_timed_pop             (GAsyncQueue *queue,
                                                             GTimeVal *end_time);
    gint                g_async_queue_length                (GAsyncQueue *queue);
    void                g_async_queue_sort                  (GAsyncQueue *queue,
                                                             GCompareDataFunc func,
                                                             gpointer user_data);

    void                g_async_queue_lock                  (GAsyncQueue *queue);
    void                g_async_queue_unlock                (GAsyncQueue *queue);
    void                g_async_queue_ref_unlocked          (GAsyncQueue *queue);
    void                g_async_queue_unref_and_unlock      (GAsyncQueue *queue);
    void                g_async_queue_push_unlocked         (GAsyncQueue *queue,
                                                             gpointer data);
    void                g_async_queue_push_sorted_unlocked  (GAsyncQueue *queue,
                                                             gpointer data,
                                                             GCompareDataFunc func,
                                                             gpointer user_data);
    gpointer            g_async_queue_pop_unlocked          (GAsyncQueue *queue);
    gpointer            g_async_queue_try_pop_unlocked      (GAsyncQueue *queue);
    gpointer            g_async_queue_timed_pop_unlocked    (GAsyncQueue *queue,
                                                             GTimeVal *end_time);
    gint                g_async_queue_length_unlocked       (GAsyncQueue *queue);
    void                g_async_queue_sort_unlocked         (GAsyncQueue *queue,
                                                             GCompareDataFunc func,
                                                             gpointer user_data);

## Description

Often you need to communicate between different threads. In general it's safer not to do this by shared memory, but by explicit message passing. These messages only make sense asynchronously for multi-threaded applications though, as a synchronous operation could as well be done in the same thread.

Asynchronous queues are an exception from most other GLib data structures, as they can be used simultaneously from multiple threads without explicit locking and they bring their own builtin reference counting. This is because the nature of an asynchronous queue is that it will always be used by at least 2 concurrent threads.

For using an asynchronous queue you first have to create one with `g_async_queue_new()`. A newly-created queue will get the reference count 1. Whenever another thread is creating a new reference of (that is, pointer to) the queue, it has to increase the reference count (using `g_async_queue_ref()`). Also, before removing this reference, the reference count has to be decreased (using `g_async_queue_unref()`). After that the queue might no longer exist so you must not access it after that point.

A thread, which wants to send a message to that queue simply calls `g_async_queue_push()` to push the message to the queue.

A thread, which is expecting messages from an asynchronous queue simply calls `g_async_queue_pop()` for that queue. If no message is available in the queue at that point, the thread is now put to sleep until a message arrives. The message will be removed from the queue and returned. The functions `g_async_queue_try_pop()` and `g_async_queue_timed_pop()` can be used to only check for the presence of messages or to only wait a certain time for messages respectively.

For almost every function there exist two variants, one that locks the queue and one that doesn't. That way you can hold the queue lock (acquire it with `g_async_queue_lock()` and release it with `g_async_queue_unlock()`) over multiple queue accessing instructions. This can be necessary to ensure the integrity of the queue, but should only be used when really necessary, as it can make your life harder if used unwisely. Normally you should only use the locking function variants (those without the suffix _unlocked)

## Details

### GAsyncQueue

    typedef struct _GAsyncQueue GAsyncQueue;

The GAsyncQueue struct is an opaque data structure, which represents an asynchronous queue. It should only be accessed through the `g_async_queue_*` functions.

### g_async_queue_new ()

    GAsyncQueue *       g_async_queue_new                   (void);

Creates a new asynchronous queue with the initial reference count of 1.

Returns : | the new GAsyncQueue.

### g_async_queue_new_full ()

    GAsyncQueue *       g_async_queue_new_full              (GDestroyNotify item_free_func);

Creates a new asynchronous queue with an initial reference count of 1 and sets up a destroy notify function that is used to free any remaining queue items when the queue is destroyed after the final unref.

`item_free_func` : | function to free queue elements
Returns : | the new GAsyncQueue.

Since 2.16

### g_async_queue_ref ()

    GAsyncQueue *       g_async_queue_ref                   (GAsyncQueue *queue);

Increases the reference count of the asynchronous `queue` by 1. You do not need to hold the lock to call this function.

`queue` : | a GAsyncQueue.
Returns : | the `queue` that was passed in (since 2.6)

### g_async_queue_unref ()

    void                g_async_queue_unref                 (GAsyncQueue *queue);

Decreases the reference count of the asynchronous `queue` by 1. If the reference count went to 0, the `queue` will be destroyed and the memory allocated will be freed. So you are not allowed to use the `queue` afterwards, as it might have disappeared. You do not need to hold the lock to call this function.

`queue` : | a GAsyncQueue.

### g_async_queue_push ()

    void                g_async_queue_push                  (GAsyncQueue *queue,
                                                             gpointer data);

Pushes the `data` into the `queue`. `data` must not be `NULL`.

`queue` : | a GAsyncQueue.
`data` : | `data` to push into the `queue`.

### g_async_queue_push_sorted ()

    void                g_async_queue_push_sorted           (GAsyncQueue *queue,
                                                             gpointer data,
                                                             GCompareDataFunc func,
                                                             gpointer user_data);

Inserts `data` into `queue` using `func` to determine the new position.

This function requires that the `queue` is sorted before pushing on new elements.

This function will lock `queue` before it sorts the queue and unlock it when it is finished.

For an example of `func` see `g_async_queue_sort()`.

`queue` : | a GAsyncQueue
`data` : | the `data` to push into the `queue`
`func` : | the GCompareDataFunc is used to sort `queue`. This function is passed two elements of the `queue`. The function should return 0 if they are equal, a negative value if the first element should be higher in the `queue` or a positive value if the first element should be lower in the `queue` than the second element.
`user_data` : | user data passed to `func`.

Since 2.10

### g_async_queue_pop ()

    gpointer            g_async_queue_pop                   (GAsyncQueue *queue);

Pops data from the `queue`. This function blocks until data become available.

`queue` : | a GAsyncQueue.
Returns : | data from the queue.

### g_async_queue_try_pop ()

    gpointer            g_async_queue_try_pop               (GAsyncQueue *queue);

Tries to pop data from the `queue`. If no data is available, `NULL` is returned.

`queue` : | a GAsyncQueue.
Returns : | data from the queue or `NULL`, when no data is available immediately.

### g_async_queue_timed_pop ()

    gpointer            g_async_queue_timed_pop             (GAsyncQueue *queue,
                                                             GTimeVal *end_time);

Pops data from the `queue`. If no data is received before `end_time`, `NULL` is returned.

To easily calculate `end_time` a combination of `g_get_current_time()` and `g_time_val_add()` can be used.

`queue` : | a GAsyncQueue.
`end_time` : | a GTimeVal, determining the final time.
Returns : | data from the queue or `NULL`, when no data is received before `end_time`.

### g_async_queue_length ()

    gint                g_async_queue_length                (GAsyncQueue *queue);

Returns the length of the queue, negative values mean waiting threads, positive values mean available entries in the `queue`. Actually this function returns the number of data items in the queue minus the number of waiting threads. Thus a return value of 0 could mean 'n' entries in the queue and 'n' thread waiting. That can happen due to locking of the queue or due to scheduling.

`queue` : | a GAsyncQueue.
Returns : | the length of the `queue`.

### g_async_queue_sort ()

    void                g_async_queue_sort                  (GAsyncQueue *queue,
                                                             GCompareDataFunc func,
                                                             gpointer user_data);

Sorts `queue` using `func`.

This function will lock `queue` before it sorts the queue and unlock it when it is finished.

If you were sorting a list of priority numbers to make sure the lowest priority would be at the top of the queue, you could use:

    1
    2
    3
    4
    5
    6
    7

    gint32 id1;
    gint32 id2;

    id1 = GPOINTER_TO_INT (element1);
    id2 = GPOINTER_TO_INT (element2);

    return (id1 > id2 ? +1 : id1 == id2 ? 0 : -1);

`queue` : | a GAsyncQueue
`func` : | the GCompareDataFunc is used to sort `queue`. This function is passed two elements of the `queue`. The function should return 0 if they are equal, a negative value if the first element should be higher in the `queue` or a positive value if the first element should be lower in the `queue` than the second element.
`user_data` : | user data passed to `func`

Since 2.10

### g_async_queue_lock ()

    void                g_async_queue_lock                  (GAsyncQueue *queue);

Acquires the `queue`'s lock. After that you can only call the `g_async_queue_*_unlocked()` function variants on that `queue`. Otherwise it will deadlock.

`queue` : | a GAsyncQueue.

### g_async_queue_unlock ()

    void                g_async_queue_unlock                (GAsyncQueue *queue);

Releases the queue's lock.

`queue` : | a GAsyncQueue.

### g_async_queue_ref_unlocked ()

    void                g_async_queue_ref_unlocked          (GAsyncQueue *queue);

### Warning

`g_async_queue_ref_unlocked` is deprecated and should not be used in newly-written code.

Increases the reference count of the asynchronous `queue` by 1.

`Deprecated`: Since 2.8, reference counting is done atomically so `g_async_queue_ref()` can be used regardless of the `queue`'s lock.

`queue` : | a GAsyncQueue.

### g_async_queue_unref_and_unlock ()

    void                g_async_queue_unref_and_unlock      (GAsyncQueue *queue);

### Warning

`g_async_queue_unref_and_unlock` is deprecated and should not be used in newly-written code.

Decreases the reference count of the asynchronous `queue` by 1 and releases the lock. This function must be called while holding the `queue`'s lock. If the reference count went to 0, the `queue` will be destroyed and the memory allocated will be freed.

`Deprecated`: Since 2.8, reference counting is done atomically so `g_async_queue_unref()` can be used regardless of the `queue`'s lock.

`queue` : | a GAsyncQueue.

### g_async_queue_push_unlocked ()

    void                g_async_queue_push_unlocked         (GAsyncQueue *queue,
                                                             gpointer data);

Pushes the `data` into the `queue`. `data` must not be `NULL`. This function must be called while holding the `queue`'s lock.

`queue` : | a GAsyncQueue.
`data` : | `data` to push into the `queue`.

### g_async_queue_push_sorted_unlocked ()

    void                g_async_queue_push_sorted_unlocked  (GAsyncQueue *queue,
                                                             gpointer data,
                                                             GCompareDataFunc func,
                                                             gpointer user_data);

Inserts `data` into `queue` using `func` to determine the new position.

This function requires that the `queue` is sorted before pushing on new elements.

This function is called while holding the `queue`'s lock.

For an example of `func` see `g_async_queue_sort()`.

`queue` : | a GAsyncQueue
`data` : | the `data` to push into the `queue`
`func` : | the GCompareDataFunc is used to sort `queue`. This function is passed two elements of the `queue`. The function should return 0 if they are equal, a negative value if the first element should be higher in the `queue` or a positive value if the first element should be lower in the `queue` than the second element.
`user_data` : | user data passed to `func`.

Since 2.10

### g_async_queue_pop_unlocked ()

    gpointer            g_async_queue_pop_unlocked          (GAsyncQueue *queue);

Pops data from the `queue`. This function blocks until data become available. This function must be called while holding the `queue`'s lock.

`queue` : | a GAsyncQueue.
Returns : | data from the queue.

### g_async_queue_try_pop_unlocked ()

    gpointer            g_async_queue_try_pop_unlocked      (GAsyncQueue *queue);

Tries to pop data from the `queue`. If no data is available, `NULL` is returned. This function must be called while holding the `queue`'s lock.

`queue` : | a GAsyncQueue.
Returns : | data from the queue or `NULL`, when no data is available immediately.

### g_async_queue_timed_pop_unlocked ()

    gpointer            g_async_queue_timed_pop_unlocked    (GAsyncQueue *queue,
                                                             GTimeVal *end_time);

Pops data from the `queue`. If no data is received before `end_time`, `NULL` is returned. This function must be called while holding the `queue`'s lock.

To easily calculate `end_time` a combination of `g_get_current_time()` and `g_time_val_add()` can be used.

`queue` : | a GAsyncQueue.
`end_time` : | a GTimeVal, determining the final time.
Returns : | data from the queue or `NULL`, when no data is received before `end_time`.

### g_async_queue_length_unlocked ()

    gint                g_async_queue_length_unlocked       (GAsyncQueue *queue);

Returns the length of the queue, negative values mean waiting threads, positive values mean available entries in the `queue`. Actually this function returns the number of data items in the queue minus the number of waiting threads. Thus a return value of 0 could mean 'n' entries in the queue and 'n' thread waiting. That can happen due to locking of the queue or due to scheduling. This function must be called while holding the `queue`'s lock.

`queue` : | a GAsyncQueue.
Returns : | the length of the `queue`.

### g_async_queue_sort_unlocked ()

    void                g_async_queue_sort_unlocked         (GAsyncQueue *queue,
                                                             GCompareDataFunc func,
                                                             gpointer user_data);

Sorts `queue` using `func`.

This function is called while holding the `queue`'s lock.

`queue` : | a GAsyncQueue
`func` : | the GCompareDataFunc is used to sort `queue`. This function is passed two elements of the `queue`. The function should return 0 if they are equal, a negative value if the first element should be higher in the `queue` or a positive value if the first element should be lower in the `queue` than the second element.
`user_data` : | user data passed to `func`

Since 2.10