1tevent_data(3)                      tevent                      tevent_data(3)
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NAME

6       tevent_data - Chapter 3: Accessing data
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Accessing data with tevent

9       A tevent request is (usually) created together with a structure for
10       storing the data necessary for an asynchronous computation. For these
11       private data, tevent library uses void (generic) pointers, therefore
12       any data type can be very simply pointed at. However, this attitude
13       requires clear and guaranteed knowledge of the data type that will be
14       handled, in advance. Private data can be of 2 types: connected with a
15       request itself or given as an individual argument to a callback. It is
16       necessary to differentiate these types, because there is a slightly
17       different method of data access for each. There are two possibilities
18       how to access data that is given as an argument directly to a callback.
19       The difference lies in the pointer that is returned. In one case it is
20       the data type specified in the function’s argument, in another void* is
21       returned.
22
23       void tevent_req_callback_data (struct tevent_req *req, #type)
24       void tevent_req_callback_data_void (struct tevent_req *req)
25
26       To obtain data that are strictly bound to a request, this function is
27       the only direct procedure.
28
29       void *tevent_req_data (struct tevent_req *req, #type)
30
31       Example with both calls which differs between private data within
32       tevent request and data handed over as an argument.
33
34       #include <stdio.h>
35       #include <unistd.h>
36       #include <tevent.h>
37
38       struct foo_state {
39           int x;
40       };
41
42       struct testA {
43           int y;
44       };
45
46
47       static void foo_done(struct tevent_req *req) {
48           // a->x contains 10 since it came from foo_send
49           struct foo_state *a = tevent_req_data(req, struct foo_state);
50
51           // b->y contains 9 since it came from run
52           struct testA *b = tevent_req_callback_data(req, struct testA);
53
54           // c->y contains 9 since it came from run we just used a different way
55           // of getting it.
56           struct testA *c = (struct testA *)tevent_req_callback_data_void(req);
57
58           printf('a->x: %d0, a->x);
59           printf('b->y: %d0, b->y);
60           printf('c->y: %d0, c->y);
61       }
62
63
64       struct tevent_req * foo_send(TALLOC_CTX *mem_ctx, struct tevent_context *event_ctx) {
65
66       printf('_send0);
67       struct tevent_req *req;
68       struct foo_state *state;
69
70       req = tevent_req_create(event_ctx, &state, struct foo_state);
71       state->x = 10;
72
73       return req;
74       }
75
76       static void run(struct tevent_context *ev, struct tevent_timer *te,
77                       struct timeval current_time, void *private_data) {
78           struct tevent_req *req;
79           struct testA *tmp = talloc(ev, struct testA);
80
81           // Note that we did not use the private data passed in
82
83           tmp->y = 9;
84           req = foo_send(ev, ev);
85
86           tevent_req_set_callback(req, foo_done, tmp);
87           tevent_req_done(req);
88
89       }
90
91       int main (int argc, char **argv) {
92
93           struct tevent_context *event_ctx;
94           struct testA *data;
95           TALLOC_CTX *mem_ctx;
96           struct tevent_timer *time_event;
97
98           mem_ctx = talloc_new(NULL); //parent
99           if (mem_ctx == NULL)
100               return EXIT_FAILURE;
101
102           event_ctx = tevent_context_init(mem_ctx);
103           if (event_ctx == NULL)
104               return EXIT_FAILURE;
105
106           data = talloc(mem_ctx, struct testA);
107           data->y = 11;
108
109           time_event = tevent_add_timer(event_ctx,
110                                         mem_ctx,
111                                         tevent_timeval_current(),
112                                         run,
113                                         data);
114           if (time_event == NULL) {
115               fprintf(stderr, ' FAILED0);
116               return EXIT_FAILURE;
117           }
118
119           tevent_loop_once(event_ctx);
120
121           talloc_free(mem_ctx);
122
123           printf('Quit0);
124           return EXIT_SUCCESS;
125       }
126
127       Output of this example is:
128
129       a->x: 10
130       b->y: 9
131       c->y: 9
132
133Version 0.9.8                     12 Apr 2016                   tevent_data(3)