Libev, How to pass arguments to relevant callbacks

Question

I'm trapped in a situation of argument passing in libev.

Usually, libev receives packages in a function like *receive_callback*, that's OK, but in practice, we need to dispatch a relative *write_callback* to deal with specific jobs according to the received package. For example:

S_RECV_MSG* pstRecvMsg = (S_RECV_MSG*) recv_buff;
switch(pstRecvMsg->wMsgType) {
    case 1: 
        ev_io_init(w, write_callback1, w->fd, EV_WRITE);
        break;
    case 2:
        ev_io_init(w, write_callback2, w->fd, EV_WRITE);
        break;
    case 3:
        // .......
}

My question is, if the write_callbackX also has to read the specific content in the recv_buff, how can we pass the recv_buff argument to the callbackX? Must we bear the burden and ugliness of global variables here?

That is the correct answer, if you want to you can write it up as a real answer and accept it, makes it easier for other people to find the information. — dnaq, Jul 03 '12 at 19:11
At least you went back and kept researching rather than waiting around for an answer :) Glad you solved it. — Steve Lazaridis, Jan 25 '13 at 16:09
I suggest writing your solution as an answer (and then accept it) instead of editing it in the question — Shahbaz, Feb 05 '13 at 18:10

score 6 · Answer 1 · edited Mar 19 '14 at 09:33

6

The author answered it himself, but in a comment. Since this shows up as unanswered, I am posting his answer in the "answer" section and closing the loop. Not sure if there is a better way... feel free to fix this.

Question author says:

Sorry, I think I got the answer now, and feel deeply ashamed at my carelessness of reading documentation:
struct my_io{
   ev_io io;
   int otherfd;
   void *somedata;
   struct whatever *mostinteresting;
};
// ...
struct my_io w;
ev_io_init (&w.io, my_cb, fd, EV_READ);
And then we use the my_io like this:
static void my_cb (struct ev_loop *loop, ev_io *w_, int revents)
{
   struct my_io *w = (struct my_io *)w_;
   //...
}

edited Mar 19 '14 at 09:33

Axel Advento

2,995
3
24
32

answered Jul 11 '13 at 16:14

lsk

532
4
5

if i'm using heap memory in somedata, what is the correct approach to free it? – Pritesh Acharya Dec 18 '13 at 04:56
I don't get it. How can ev_io* w_ be cast into (struct my_io*) in the my_cb callback? – lppier Jul 27 '15 at 08:13
@lppier The `ev_io` is the first member in `my_io` so its address can be obtained by that cast. – pjhades Nov 22 '16 at 20:34

Winter Ma · Answer 2 · 2015-01-17T12:54:19.273

3

Yes it's explained in libev document, and there is another way. Each watcher has a void *data member that you can read or modify, and libev will completely ignore it, so you can pass the argument like this:

w->data = (void *)recv_buff;
...
static void my_cb (struct ev_loop *loop, ev_io *w_, int revents)
{
    S_RECV_MSG *recv_buff = (S_RECV_MSG*)w_->data;
    ...
}

see libev document.

edited Jan 17 '15 at 12:54

answered Jan 17 '15 at 10:51

Winter Ma

63
6

using ->data is a munch nicer way of doing it than the 'subclassing' technique – matiu Apr 09 '16 at 05:02

Libev, How to pass arguments to relevant callbacks

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