Event Queue¶
Provides an Event loop.
This module offers an event queue framework like libevent or libuev.
An event queue is basically a FIFO queue of events, with some functions to efficiently and safely handle adding and getting events to / from such a queue. An event queue is bound to a thread, but any thread or ISR can put events into a queue.
An event is a structure containing a pointer to an event handler. It can be extended to provide context or arguments to the handler. It can also be embedded into existing structures (see examples).
Compared to msg or mbox, this some fundamental differences:
- events are “sender allocated”. Unlike
msg.h::msg_send()
,event.h::event_post()
never blocks or fails. - events contain everything necessary to handle them, thus a thread processing the events of an event queue doesn’t need to be changed in order to support new event types.
- events can be safely used (and actually perform best) when used within one thread, e.g., in order to create a state-machine like process flow. This is not (easily) possible using msg queues, as they might fill up.
- an event can only be queued in one event queue at the same time. Notifying many queues using only one event object is not possible with this imlementation.
At the core, event.h::event_wait()
uses thread flags to implement waiting for events to be queued. Thus event queues can be used safely and efficiently in combination with thread flags and msg queues.
Examples:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 | // simple event handler
static void handler(event_t *event)
{
printf("triggered 0x%08x\n", (unsigned)event);
}
static event_t event = { .handler = handler };
static event_queue_t queue;
int main(void)
{
event_queue_init(&queue);
event_loop(&queue);
}
[...] event_post(&queue, &event);
// example for event extended event struct
typedef struct {
event_t super;
const char *text;
} custom_event_t;
static void custom_handler(event_t *event)
{
custom_event_t *custom_event = (custom_event_t *)event;
printf("triggered custom event with text: \"%s\"\n", custom_event->text);
}
static custom_event_t custom_event = { .super.handler = custom_handler, .text = "CUSTOM EVENT" };
[...] event_post(&queue, &custom_event)
|
-
void(*
event_handler_t
()¶ event handler type definition
-
void
event_queue_init
(event_queue_t * queue)¶ Initialize an event queue.
This will set the calling thread as owner of
queue
.Parameters
queue: event queue object to initialize
-
void
event_post
(event_queue_t * queue,event.h::event_t
* event)¶ Queue an event.
The given event will be posted on the given
queue
. If the event is already queued when calling this function, the event will not be touched and remain in the previous position on the queue. So reposting an event while it is already on the queue will have no effect.Parameters
queue: event queue to queue event in event: event to queue in event queue
-
void
event_cancel
(event_queue_t * queue,event.h::event_t
* event)¶ Cancel a queued event.
This will remove a queued event from an event queue.
Note
Due to the underlying list implementation, this will run in O(n).
Parameters
queue: event queue to remove event from event: event to remove from queue
-
event.h::event_t
*event_get
(event_queue_t * queue)¶ Get next event from event queue, non-blocking.
In order to handle an event retrieved using this function, call event->handler(event).
Parameters
queue: event queue to get event from Return values
- pointer to next event
- NULL if no event available
-
event.h::event_t
*event_wait
(event_queue_t * queue)¶ Get next event from event queue, blocking.
This function will block until an event becomes available.
In order to handle an event retrieved using this function, call event->handler(event).
Parameters
queue: event queue to get event from Return values
- pointer to next event
-
void
event_loop
(event_queue_t * queue)¶ Simple event loop.
This function will forever sit in a loop, waiting for events to be queued and executing their handlers.
It is pretty much defined as:
1 2 3
while ((event = event_wait(queue))) { event->handler(event); }
Parameters
queue: event queue to process
-
THREAD_FLAG_EVENT
¶ Thread flag use to notify available events in an event queue.
1
(0x1)
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EVENT_QUEUE_INIT
¶ event_queue_t static initializer
1
{ .waiter = (thread_t *)sched_active_thread }
-
struct
event
¶ event structure
-
clist.h::clist_node_t
list_node
¶ event queue list entry
-
event.h::event_handler_t
handler
¶ pointer to event handler function
-
-
struct
event_queue_t
¶ event queue structure
-
clist.h::clist_node_t
event_list
¶ list of queued events
-
sched.h::thread_t
*waiter
¶ thread ownning event queue
-