Hi ! I'm the UDP guy Nick just mentioned ;-)
I just happen have an old in-progress email that discusses the details of what I was attempting w.r.t. UDP, so I guess it's the good time to discuss this. My work is at [1] below. Be aware that this haven't been updated (yes, the merge isn't easy ;-)). I'm planning to update it sometime but since the release cycle is currently locked and it has a rather low priority, it's blocked atm. The main issue I had with using UDP in libevent is that (IIRC) you're forced to put your socket in connected state, because evbuffer access the fd using read() and write(). My plan was : - to abstract those calls in their own functions (evsocket_read, evsocket_write), along with all the other possible means currently available (see [2] for the list I had in my branch). This also cleans up - allow the library client to choose whatever implementation it wanted (via evbuffer_set_fd_cbs [2]), while defaulting to the best implementation for the descriptor (connected/disconnected, iovecs supported on platform, in that order, see the switch in evbuffer_read()/write() and evsocket_type()). - allow those callbacks to gather/use whatever data they want when the fd gets activity via evsocket_info (right now that limited to the sockaddr to send to when writing, and the sockaddr that sent us data when reading, but I guess it could also handle OOB data)... Issues: - sendfile support is out of the window, mainly because it's a special cased evbuffer. Its implementation has been moved in evsocket_sendfile, and IIRC the call should go in evbuffer_write_atmost. - crashes galore ! [1] - http://github.com/tiennou/Libevent/tree/evbuffer_fd_cbs [2] - http://github.com/tiennou/Libevent/blob/evbuffer_fd_cbs/include/event2/evsocket.h Le 1 déc. 2010 à 16:44, Nick Mathewson a écrit : > On Wed, Dec 1, 2010 at 8:22 AM, Thai Duong <tha...@gmail.com> wrote: >> Hi everybody, >> >> I'm new to this list, so please forgive me if this has been discussed. >> I read the libevent2 book, and it said that bufferevent currently >> supports only TCP socket. I take a look at the code, and see that, >> however, it seems that bufferevent can work with UDP too. > > The evbuffer backend that bufferevents uses to store incoming and > outgoing data currently assumes that it's handling stream data rather > than datagrams, and as such does not preserve packet boundaries when > reading and writing. I'm not sure about this too (in fact, I hadn't though about it...). IIRC my plan was to exchange only packet-sized buffers, with a small header at the start... > There are probably other issues as well, but that one's enough of a > showstopper to make bufferevents+UDP unworkable in the current code > base. > >> So my >> question is why the book said that UDP is not supported? > > Because it's the truth. > >> And if that >> is really the case, what should I do to modify bufferevent so that it >> supports UDP? > > It will probably take a fair amount of programming. You'd want to > start by thinking about the appropriate abstractions at the evbuffer > level, and then considering which abstractions are appropriate for > making evbuffers work on packets rather than streams without making > either implementation inefficient or making either API ugly or hard to > use. At the same time, you'd want to have a look through the > bufferevent code and see what stream-only assumptions (if any) it > makes, and what would need to be done to fix those. > > Etienne Samson was working on a design for this a while back, and I'd > like to revisit it for Libevent 2.1. You can have a look at the state > of his work here: > https://sourceforge.net/tracker/?func=detail&aid=2928202&group_id=50884&atid=461324 > . It makes some choices that I didn't agree with at the time, though > to be honest I haven't looked at it recently and can't currently > remember which they were. > > This is also probably a good time to apologize to Etienne for being so > slow here; the Libevent 2.0 release cycle has been entirely too long. > Once 2.0.x-stable is out, it's probably a good idea to consider ways > to make sure that 2.1 does not spend so long in alpha as 2.0.x did. Apologies taken ;-). In fact, I was just playing around with libevent when I found out that it didn't like UDP sockets, so that not like I was in a hurry... > > yrs, > -- > Nick -- Etienne Samson samson.etie...@gmail.com*********************************************************************** To unsubscribe, send an e-mail to majord...@freehaven.net with unsubscribe libevent-users in the body.
