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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.
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.
Since: 2.16
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.
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.
void g_async_queue_push (GAsyncQueue *queue
,gpointer data
);
Pushes the data
into the queue
. data
must not be NULL
.
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()
.
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the |
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the GCompareDataFunc is used to sort |
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Since: 2.10
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.
Since: 2.46
gboolean g_async_queue_remove (GAsyncQueue *queue
,gpointer item
);
Remove an item from the queue.
Since: 2.46
gpointer
g_async_queue_pop (GAsyncQueue *queue
);
Pops data from the queue
. If queue
is empty, this function
blocks until data becomes available.
gpointer
g_async_queue_try_pop (GAsyncQueue *queue
);
Tries to pop data from the queue
. If no data is available,
NULL
is returned.
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.
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.
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); |
Since: 2.10
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.
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.
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.
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.
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.
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 |
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data |
the |
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func |
the GCompareDataFunc is used to sort |
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user_data |
user data passed to |
Since: 2.10
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.
Since: 2.46
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.
Since: 2.46
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.
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.
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.
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.
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.
Since: 2.10
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.
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.
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.
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.