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diff --git a/doc/tevent_queue.dox b/doc/tevent_queue.dox new file mode 100644 index 0000000..c1d629c --- /dev/null +++ b/doc/tevent_queue.dox @@ -0,0 +1,275 @@ +/** +@page tevent_queue Chapter 5: Tevent queue +@section queue Tevent queue + +There is a possibility that the dispatcher and its handlers may not be able to +handle all the incoming events as quickly as they arrive. One way to deal with +this situation is to buffer the received events by introducing an event queue +into the events stream, between the events generator and the dispatcher. Events +are added to the queue as they arrive, and the dispatcher pops them off the +beginning of the queue as fast as possible. In tevent library it is +similar, but the queue is not automatically set for any event. The queue has to +be created on purpose, and events which should follow the order of the FIFO +queue have to be explicitly pinpointed. Creating such a queue is crucial in +situations when sequential processing is absolutely essential for the +successful +completion of a task, e.g. for a large quantity of data that are about to be +written from a buffer into a socket. The tevent library has its own queue +structure that is ready to use after it has been initialized and started up +once. + +@subsection cr_queue Creation of Queues + +The first and most important step is the creation of the tevent queue +(represented by struct tevent_queue), which will then be in running mode. + +@code +struct tevent_queue* tevent_queue_create (TALLOC_CTX *mem_ctx, const char *name) +@endcode + +When the program returns from this function, the allocated memory, set +destructor and labeled queue as running has been done and the structure is +ready to be filled with entries. Stopping and starting queues on the run. If +you need to stop a queue from processing its entries, and then turn it on +again, a couple of functions which serve this purpose are: + +- bool tevent_queue_stop() +- bool tevent_queue_start() + +These functions actually only provide for the simple setting of a variable, +which indicates that the queue has been stopped/started. Returned value +indicates result. + +@subsection add_queue Adding Requests to a Queue + +Tevent in fact offers 3 possible ways of inserting a request into a queue. +There are no vast differences between them, but still there might be situations +where one of them is more suitable and desired than another. + +@code +bool tevent_queue_add(struct tevent_queue *queue, + struct tevent_context *ev, + struct tevent_req *req, + tevent_queue_trigger_fn_t trigger, + void *private_data) +@endcode + +This call is the simplest of all three. It offers only boolean verification of +whether the operation of adding the request into a queue was successful or not. +No additional deletion of an item from the queue is possible, i.e. it is only +possible to deallocate the whole tevent request, which would cause triggering +of destructor handling and also dropping the request from the queue. + +<strong>Extended Options</strong> + +Both of the following functions have a feature in common - they return tevent +queue entry structure representing the item in a queue. There is no further +possible handling with this structure except the use of the structure’s pointer +for its deallocation (which leads also its removal from the queue). The +difference lies in the possibility that with the following functions it is +possible to remove the tevent request from a queue without its deallocation. +The previous function can only deallocate the tevent request as it was from +memory, and thereby logically cause its removal from the queue as well. There +is no other utilization of this structure via API at this stage of tevent +library. The possibility of easier debugging while developing with tevent could +be considered to be an advantage of this returned pointer. + +@code +struct tevent_queue_entry *tevent_queue_add_entry(struct tevent_queue *queue, + struct tevent_context *ev, + struct tevent_req *req, + tevent_queue_trigger_fn_t trigger, + void *private_data) +@endcode + +The feature that allows for the optimized addition of entries to a queue is +that a check for an empty queue with no items is first of all carried out. If +it is found that the queue is empty, then the request for inserting the entry +into a queue will be omitted and directly triggered. + +@code +struct tevent_queue_entry *tevent_queue_add_optimize_empty(struct tevent_queue *queue, + struct tevent_context *ev, + struct tevent_req *req, + tevent_queue_trigger_fn_t trigger, + void *private_data) +@endcode + +When calling any of the functions serving for inserting an item into a queue, +it is possible to leave out the fourth argument (trigger) and instead of a +function pass a NULL pointer. This usage sets so-called blocking entries. +These entries, since they do not have any trigger operation to be activated, +just sit in their position until they are labeled as a done by another +function. Their purpose is to block other items in the queue from being +triggered. + +@subsection example_q Example of tevent queue + +@code +#include <stdio.h> +#include <unistd.h> +#include <tevent.h> + +struct foo_state { + int local_var; + int x; +}; + +struct juststruct { + TALLOC_CTX * ctx; + struct tevent_context *ev; + int y; +}; + +int created = 0; + +static void timer_handler(struct tevent_context *ev, struct tevent_timer *te, + struct timeval current_time, void *private_data) +{ + // time event which after all sets request as done. Following item from + // the queue may be invoked. + struct tevent_req *req = private_data; + struct foo_state *stateX = tevent_req_data(req, struct foo_state); + + // processing some stuff + + printf("time_handler\n"); + + tevent_req_done(req); + talloc_free(req); + + printf("Request #%d set as done.\n", stateX->x); +} + +static void trigger(struct tevent_req *req, void *private_data) +{ + struct juststruct *priv = tevent_req_callback_data (req, struct juststruct); + struct foo_state *in = tevent_req_data(req, struct foo_state); + struct timeval schedule; + struct tevent_timer *tim; + schedule = tevent_timeval_current_ofs(1, 0); + printf("Processing request #%d\n", in->x); + + if (in->x % 3 == 0) { // just example; third request does not contain + // any further operation and will be finished right + // away. + tim = NULL; + } else { + tim = tevent_add_timer(priv->ev, req, schedule, timer_handler, req); + } + + if (tim == NULL) { + tevent_req_done(req); + talloc_free(req); + printf("Request #%d set as done.\n", in->x); + } +} + +struct tevent_req *foo_send(TALLOC_CTX *mem_ctx, struct tevent_context *ev, + const char *name, int num) +{ + struct tevent_req *req; + struct foo_state *state; + struct foo_state *in; + struct tevent_timer *tim; + + printf("foo_send\n"); + req = tevent_req_create(mem_ctx, &state, struct foo_state); + if (req == NULL) { // check for appropriate allocation + tevent_req_error(req, 1); + return NULL; + } + + // exemplary filling of variables + state->local_var = 1; + state->x = num; + + return req; +} + +static void foo_done(struct tevent_req *req) { + + enum tevent_req_state state; + uint64_t err; + + if (tevent_req_is_error(req, &state, &err)) { + printf("ERROR WAS SET %d\n", state); + return; + } else { + // processing some stuff + printf("Callback is done...\n"); + } +} + +int main (int argc, char **argv) +{ + TALLOC_CTX *mem_ctx; + struct tevent_req* req[6]; + struct tevent_req* tmp; + struct tevent_context *ev; + struct tevent_queue *fronta = NULL; + struct juststruct *data; + int ret; + int i = 0; + + const char * const names[] = { + "first", "second", "third", "fourth", "fifth" + }; + + printf("INIT\n"); + + mem_ctx = talloc_new(NULL); //parent + talloc_parent(mem_ctx); + ev = tevent_context_init(mem_ctx); + if (ev == NULL) { + fprintf(stderr, "MEMORY ERROR\n"); + return EXIT_FAILURE; + } + + // setting up queue + fronta = tevent_queue_create(mem_ctx, "test_queue"); + tevent_queue_stop(fronta); + tevent_queue_start(fronta); + if (tevent_queue_running(fronta)) { + printf ("Queue is running (length: %d)\n", tevent_queue_length(fronta)); + } else { + printf ("Queue is not running\n"); + } + + data = talloc(ev, struct juststruct); + data->ctx = mem_ctx; + data->ev = ev; + + + // create 4 requests + for (i = 1; i < 5; i++) { + req[i] = foo_send(mem_ctx, ev, names[i], i); + tmp = req[i]; + if (req[i] == NULL) { + fprintf(stderr, "Request error! %d \n", ret); + break; + } + tevent_req_set_callback(req[i], foo_done, data); + created++; + } + + // add item to a queue + tevent_queue_add(fronta, ev, req[1], trigger, data); + tevent_queue_add(fronta, ev, req[2], trigger, data); + tevent_queue_add(fronta, ev, req[3], trigger, data); + tevent_queue_add(fronta, ev, req[4], trigger, data); + + printf("Queue length: %d\n", tevent_queue_length(fronta)); + while(tevent_queue_length(fronta) > 0) { + tevent_loop_once(ev); + printf("Queue: %d items left\n", tevent_queue_length(fronta)); + } + + talloc_free(mem_ctx); + printf("FINISH\n"); + + return EXIT_SUCCESS; +} +@endcode + +*/ |