Asynchronous Queues

Asynchronous Queues — asynchronous communication between threads

Types and Values

Includes

#include <glib.h>

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.

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.

Parameters

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.

Parameters

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.

Parameters

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.

Parameters

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().

Parameters

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_push_front ()

void
g_async_queue_push_front (GAsyncQueue *queue,
                          gpointer item);

Pushes the item into the queue . item must not be NULL. In contrast to g_async_queue_push(), this function pushes the new item ahead of the items already in the queue, so that it will be the next one to be popped off the queue.

Parameters

queue

a GAsyncQueue

 

item

data to push into the queue

 

Since: 2.46


g_async_queue_remove ()

gboolean
g_async_queue_remove (GAsyncQueue *queue,
                      gpointer item);

Remove an item from the queue.

Parameters

queue

a GAsyncQueue

 

item

the data to remove from the queue

 

Returns

TRUE if the item was removed

Since: 2.46


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.

Parameters

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.

Parameters

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.

Parameters

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.

Parameters

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:

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gint32 id1;
gint32 id2;

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

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

Parameters

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.

Parameters

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.

Parameters

queue

a GAsyncQueue

 

g_async_queue_ref_unlocked ()

void
g_async_queue_ref_unlocked (GAsyncQueue *queue);

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.

Parameters

queue

a GAsyncQueue

 

g_async_queue_unref_and_unlock ()

void
g_async_queue_unref_and_unlock (GAsyncQueue *queue);

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.

Parameters

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.

Parameters

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().

Parameters

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_push_front_unlocked ()

void
g_async_queue_push_front_unlocked (GAsyncQueue *queue,
                                   gpointer item);

Pushes the item into the queue . item must not be NULL. In contrast to g_async_queue_push_unlocked(), this function pushes the new item ahead of the items already in the queue, so that it will be the next one to be popped off the queue.

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

Parameters

queue

a GAsyncQueue

 

item

data to push into the queue

 

Since: 2.46


g_async_queue_remove_unlocked ()

gboolean
g_async_queue_remove_unlocked (GAsyncQueue *queue,
                               gpointer item);

Remove an item from the queue.

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

Parameters

queue

a GAsyncQueue

 

item

the data to remove from the queue

 

Returns

TRUE if the item was removed

Since: 2.46


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.

Parameters

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.

Parameters

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.

Parameters

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.

Parameters

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.

Parameters

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);

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.

Parameters

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);

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.

Parameters

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 .

Types and Values

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.

See Also

GThreadPool