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_timeout_pop           (GAsyncQueue *queue,
                                                         guint64 timeout);
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_timeout_pop_unlocked  (GAsyncQueue *queue,
                                                         guint64 timeout);
gint                g_async_queue_length_unlocked       (GAsyncQueue *queue);
void                g_async_queue_sort_unlocked         (GAsyncQueue *queue,
                                                         GCompareDataFunc func,
                                                         gpointer user_data);

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

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(). GAsyncQueue structs are reference counted, use g_async_queue_ref() and g_async_queue_unref() to manage your references.

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_timeout_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 _unlocked suffix).

In many cases, it may be more convenient to use GThreadPool when you need to distribute work to a set of worker threads instead of using GAsyncQueue manually. GThreadPool uses a GAsyncQueue internally.

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.

Returns :

 a new GAsyncQueue. Free with g_async_queue_unref()

g_async_queue_new_full ()

GAsyncQueue *       g_async_queue_new_full              (GDestroyNotify item_free_func);

Creates a new asynchronous queue 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 :

 a new GAsyncQueue. Free with g_async_queue_unref()

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, see g_async_queue_sort().

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

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. If queue is empty, this function blocks until data becomes 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_timeout_pop ()

gpointer            g_async_queue_timeout_pop           (GAsyncQueue *queue,
                                                         guint64 timeout);

Pops data from the queue. If the queue is empty, blocks for timeout microseconds, or until data becomes available.

If no data is received before the timeout, NULL is returned.

queue :

 a GAsyncQueue

timeout :

 the number of microseconds to wait

Returns :

 data from the queue or NULL, when no data is received before the timeout.

g_async_queue_length ()

gint                g_async_queue_length                (GAsyncQueue *queue);

Returns the length of the queue.

Actually this function returns the number of data items in the queue minus the number of waiting threads, so a negative value means waiting threads, and a positive value means available entries in the queue. A return value of 0 could mean n entries in the queue and n threads waiting. This 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.

The sort function func is passed two elements of the queue. It 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.

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

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. If another thread is already holding the lock, this call will block until the lock becomes available.

Call g_async_queue_unlock() to drop the lock again.

While holding the lock, you can only call the g_async_queue_*_unlocked() functions on queue. Otherwise, deadlock may occur.

queue :

 a GAsyncQueue

g_async_queue_unlock ()

void                g_async_queue_unlock                (GAsyncQueue *queue);

Releases the queue's lock.

Calling this function when you have not acquired the with g_async_queue_lock() leads to undefined behaviour.

queue :

 a GAsyncQueue

g_async_queue_ref_unlocked ()

void                g_async_queue_ref_unlocked          (GAsyncQueue *queue);

Warning

g_async_queue_ref_unlocked has been deprecated since version 2.8 and should not be used in newly-written code. Reference counting is done atomically. so g_async_queue_ref() can be used regardless of the queue's lock.

Increases the reference count of the asynchronous queue by 1.

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 has been deprecated since version 2.8 and should not be used in newly-written code. Reference counting is done atomically. so g_async_queue_unref() can be used regardless of the queue's lock.

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.

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.

The sort function func is passed two elements of the queue. It 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.

This function requires that the queue is sorted before pushing on new elements, see g_async_queue_sort().

This function must be 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

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. If queue is empty, this function blocks until data becomes 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_timeout_pop_unlocked ()

gpointer            g_async_queue_timeout_pop_unlocked  (GAsyncQueue *queue,
                                                         guint64 timeout);

Pops data from the queue. If the queue is empty, blocks for timeout microseconds, or until data becomes available.

If no data is received before the timeout, NULL is returned.

This function must be called while holding the queue's lock.

queue :

 a GAsyncQueue

timeout :

 the number of microseconds to wait

Returns :

 data from the queue or NULL, when no data is received before the timeout.

g_async_queue_length_unlocked ()

gint                g_async_queue_length_unlocked       (GAsyncQueue *queue);

Returns the length of the queue.

Actually this function returns the number of data items in the queue minus the number of waiting threads, so a negative value means waiting threads, and a positive value means available entries in the queue. A return value of 0 could mean n entries in the queue and n threads waiting. This 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.

The sort function func is passed two elements of the queue. It 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.

This function must be called while holding the queue's lock.

queue :

 a GAsyncQueue

func :

 the GCompareDataFunc is used to sort queue

user_data :

 user data passed to func

Since 2.10

g_async_queue_timed_pop ()

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

Warning

g_async_queue_timed_pop is deprecated and should not be used in newly-written code. use g_async_queue_timeout_pop().

Pops data from the queue. If the queue is empty, blocks until end_time or until data becomes available.

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_timed_pop_unlocked ()

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

Warning

g_async_queue_timed_pop_unlocked is deprecated and should not be used in newly-written code. use g_async_queue_timeout_pop_unlocked().

Pops data from the queue. If the queue is empty, blocks until end_time or until data becomes available.

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.

This function must be called while holding the queue's lock.

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.

See Also

GThreadPool
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