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

10       A tevent request is (usually) created together with a structure for
11       storing the data necessary for an asynchronous computation. For these
12       private data, tevent library uses void (generic) pointers, therefore
13       any data type can be very simply pointed at. However, this attitude
14       requires clear and guaranteed knowledge of the data type that will be
15       handled, in advance. Private data can be of 2 types: connected with a
16       request itself or given as an individual argument to a callback. It is
17       necessary to differentiate these types, because there is a slightly
18       different method of data access for each. There are two possibilities
19       how to access data that is given as an argument directly to a callback.
20       The difference lies in the pointer that is returned. In one case it is
21       the data type specified in the function’s argument, in another void* is
22       returned.
23
24       void tevent_req_callback_data (struct tevent_req *req, #type)
25       void tevent_req_callback_data_void (struct tevent_req *req)
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27       To obtain data that are strictly bound to a request, this function is
28       the only direct procedure.
29
30       void *tevent_req_data (struct tevent_req *req, #type)
31
32       Example with both calls which differs between private data within
33       tevent request and data handed over as an argument.
34
35       #include <stdio.h>
36       #include <unistd.h>
37       #include <tevent.h>
38
39       struct foo_state {
40           int x;
41       };
42
43       struct testA {
44           int y;
45       };
46
47
48       static void foo_done(struct tevent_req *req) {
49           // a->x contains 10 since it came from foo_send
50           struct foo_state *a = tevent_req_data(req, struct foo_state);
51
52           // b->y contains 9 since it came from run
53           struct testA *b = tevent_req_callback_data(req, struct testA);
54
55           // c->y contains 9 since it came from run we just used a different way
56           // of getting it.
57           struct testA *c = (struct testA *)tevent_req_callback_data_void(req);
58
59           printf("a->x: %d0, a->x);
60           printf("b->y: %d0, b->y);
61           printf("c->y: %d0, c->y);
62       }
63
64
65       struct tevent_req * foo_send(TALLOC_CTX *mem_ctx, struct tevent_context *event_ctx) {
66
67       printf("_send0);
68       struct tevent_req *req;
69       struct foo_state *state;
70
71       req = tevent_req_create(event_ctx, &state, struct foo_state);
72       state->x = 10;
73
74       return req;
75       }
76
77       static void run(struct tevent_context *ev, struct tevent_timer *te,
78                       struct timeval current_time, void *private_data) {
79           struct tevent_req *req;
80           struct testA *tmp = talloc(ev, struct testA);
81
82           // Note that we did not use the private data passed in
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84           tmp->y = 9;
85           req = foo_send(ev, ev);
86
87           tevent_req_set_callback(req, foo_done, tmp);
88           tevent_req_done(req);
89
90       }
91
92       int main (int argc, char **argv) {
93
94           struct tevent_context *event_ctx;
95           struct testA *data;
96           TALLOC_CTX *mem_ctx;
97           struct tevent_timer *time_event;
98
99           mem_ctx = talloc_new(NULL); //parent
100           if (mem_ctx == NULL)
101               return EXIT_FAILURE;
102
103           event_ctx = tevent_context_init(mem_ctx);
104           if (event_ctx == NULL)
105               return EXIT_FAILURE;
106
107           data = talloc(mem_ctx, struct testA);
108           data->y = 11;
109
110           time_event = tevent_add_timer(event_ctx,
111                                         mem_ctx,
112                                         tevent_timeval_current(),
113                                         run,
114                                         data);
115           if (time_event == NULL) {
116               fprintf(stderr, " FAILED0);
117               return EXIT_FAILURE;
118           }
119
120           tevent_loop_once(event_ctx);
121
122           talloc_free(mem_ctx);
123
124           printf("Quit0);
125           return EXIT_SUCCESS;
126       }
127
128       Output of this example is:
129
130       a->x: 10
131       b->y: 9
132       c->y: 9
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134Version 0.9.8                   Mon Aug 8 2022                  tevent_data(3)