Comments on: Sending and Receiving Packets

By: Acetate

Acetate — Thu, 06 May 2010 14:32:26 +0000

@Daniel: If you use unsigned chars, shouldn’t you also make sure that they aren’t zeroed out by the shifts occurring just before this assignment (I don’t know if there is any implicit cast): unsigned int destination_address = ( a << 24 ) | ( b << 16 ) | ( c << 8 ) | d; ?

@Glenn: I have been reading your articles (both physics and networking series) on my spare time for a few days now and I learnt A LOT. Those are clear, succinct and overall well written explanations about often overlooked areas of game development. I think I've just made the jump from bad to mediocre game programmer thanks to you.

By: Daniel

Daniel — Tue, 30 Mar 2010 22:14:52 +0000

Hi,
Great series of tutorials, would it be OK to instead of having a non-blocking socket and waste cpu cycles, could I keep the socket blocking and make it load up data into a buffer which another thread processes?

Also,
unsigned int a = 207;
unsigned int b = 45;
unsigned int c = 186;
unsigned int d = 98;
– Shouldn’t those be unsigned chars?

Thanks

By: Glenn Fiedler

Glenn Fiedler — Wed, 03 Feb 2010 16:55:15 +0000

Select is designed for use with TCP so you get notified when one of your (multiple) TCP sockets has data ready to read — there is no reason to use select here, because you only need one UDP socket to service any number of virtual connections. Use non-blocking sockets instead.

By: Geo

Geo — Wed, 03 Feb 2010 16:44:21 +0000

Hello,

when implementing UDP on the primary thread you need non-blocking sockets. That’s obviuos. What about using select with (0,0) timeout and then recvfrom if select returns ok ? Is it the same with your solution or worse?

thank you!

By: Martin Bell

Martin Bell — Wed, 16 Dec 2009 21:52:30 +0000

Hello.

Many thanks for your article. I’m just starting to learn the basics of network programming so this is very useful, I think I may have spotted a bug however. Should the use of fcntl for setting the non-blocking flag be like this?

int flags = fcntl(handle, F_GETFL, 0);
fcntl(handle, F_SETFL, flags | O_NONBLOCK);

Ie it has only 3 parameters and sets all the flags in one go. The 4th seems to be ignored in your example and I’m guessing all other flags are being reset to zero as a side effect. I’m testing this on a Mac.

Thanks

By: Glenn Fiedler

Glenn Fiedler — Fri, 06 Nov 2009 16:36:48 +0000

What you would typically do is put a sequence number at the top of your packet and increment it each time you send one

Then loop and receive all packets, which will typically be very small — then you can use the sequence number to discard all but the most recent packet from each source, per-frame if you wish

cheers

By: David Moran

David Moran — Fri, 06 Nov 2009 08:36:28 +0000

Hello Glenn
Thank you very much for your article, it is the best I have found in internet.
I have one question… what if you want to get only the LAST packet?. I think it has a “time cost” to make a while() and it will be best to get only the last packet (the one you want normally for UDP (real/time) communications; it does not matter was happens before¡)
Is it any way to read directly the last packet?
Thank you very much

By: Glenn Fiedler

Glenn Fiedler — Mon, 07 Sep 2009 17:03:59 +0000

you’re totally correct – in my production code i have an error reporter for all socket operations that logs the error code when they happen, and i have special case handling for handling for EAGAIN

cheers mate

By: thec

thec — Mon, 07 Sep 2009 13:06:35 +0000

right… I see how you work around the problem, but it’s not very wise to skip all error messages like disconnect etc…

By: Glenn Fiedler

Glenn Fiedler — Mon, 07 Sep 2009 00:55:17 +0000

it’s an error message, of sorts – but i don’t treat it like that

the idea is that in your loop you just keep calling recvfrom until it returns <= 0

this way you keep polling until no packets are available… next frame you try again, until <= 0

cheers and see you on #coders!

ps. check http://gafferongames.com/2009/09/04/the-return-of-the-timestep-crusader/ for a good laugh