tevent_context man page

tevent_context — Chapter 1: Tevent context

Tevent context

Tevent context is an essential logical unit of tevent library. For working with events at least one such context has to be created - allocated, initialized. Then, events which are meant to be caught and handled have to be registered within this specific context. Reason for subordinating events to a tevent context structure rises from the fact that several context can be created and each of them is processed at different time. So, there can be 1 context containing just file descriptor events, another one taking care of signal and time events and the third one which keeps information about the rest.

Tevent loops are the part of the library which represents the mechanism where noticing events and triggering handlers actually happens. They accept just one argument - tevent context structure. Therefore if theoretically an infinity loop (tevent_loop_wait) was called, only those arguments which belong to the passed tevent context structure can be caught and invoked within this call. Although some more signal events were registered (but within some other context) they will not be noticed.

Example

First lines which handle mem_ctx belong to talloc library knowledge but because of the fact that tevent uses the talloc library for its mechanisms it is necessary to understand a bit talloc as well. For more information about working with talloc, please visit talloc website where tutorial and documentation are located.

Tevent context structure *event_ctx represents the unit which will further contain information about registered events. It is created via calling tevent_context_init().

TALLOC_CTX *mem_ctx = talloc_new(NULL);
if (mem_ctx == NULL) {
    // error handling
}

struct tevent_context *ev_ctx = tevent_context_init(mem_ctx);
if(ev_ctx == NULL) {
    // error handling
}

Tevent context has a structure containing lots of information. It include lists of all events which are divided according their type and are in order showing the sequence as they came.

In addition to the lists shown in the diagram, the tevent context also contains many other data (e.g. information about the available system mechanism for triggering callbacks).

Tevent loops

Tevent loops are the dispatcher for events. They catch them and trigger the handlers. In the case of longer processes, the program spends most of its time at this point waiting for events, invoking handlers and waiting for another event again. There are 2 types of loop available for use in tevent library:

·
int tevent_loop_wait()
·
int tevent_loop_once()

Both of functions accept just one parameter (tevent context) and the only difference lies in the fact that the first loop can theoretically last for ever but the second one will wait just for a single one event to catch and then the loop breaks and the program continue.

Info

Mon Oct 10 2016 Version 0.9.8 tevent