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Bug 121

Summary: SDLNet_TCP_Send and SIGPIPE
Product: SDL_net Reporter: Sam Lantinga <slouken>
Component: miscAssignee: Sam Lantinga <slouken>
Status: RESOLVED FIXED QA Contact:
Severity: normal    
Priority: P2    
Version: unspecified   
Hardware: All   
OS: Linux   

Description Sam Lantinga 2006-01-31 09:25:25 UTC 
Date: Thu, 26 May 2005 15:48:34 +0300
From: Petri Latvala <adrinael@nuclearzone.org>
Subject: Re: [SDL] SDLNet_TCP_Send and SIGPIPE (Linux)

On Wed, Jan 19, 2005 at 05:16:04PM +0200, Antti Mannisto wrote:
>   My network game has worked for ages in Windows, but now that     
> I made a Linux version of it, there seem to be random crashes     
> every now and then. Everything has been made on top of SDL so the
> code is identical. It is very hard to debug the problem since  
> a server may crash once in a month or so, but debug information     
> suggests that crashes might be related to SDLNet_TCP_Send.
> With Valgrind, I am able to make send/SLNet_TCP_Send signal       
> SIGPIPE (and thus crash) when I disconnect from a server at a
> certain time. It also seems that players with worse connections
> are getting this problem more often.

The normal behaviour for send() is to SIGPIPE when the other end has
reset the connection, unless you pass MSG_NOSIGNAL as flags. Does
SDL_Net not do this?

>   Could this be a bug in SDL_net/SDL? I am using the latest library
> versions (1.2.8 for SDL) and I can read in the change log that SDL
> 1.2.7 should have made SIGPIPE non-fatal, but what could then    
> explain this? Is it just Valgrind that is not working? And what
> should I do to get rid of the problem? Define my own signal handler?

If SDL_Net uses send() without MSG_NOSIGNAL, it's a bug. In SDL_Net.

>   Would something get broken or could the problem be fixed     
> if I add "signal(SIGPIPE, SIG_IGN)" to somewhere in my code?
> Where should it be put? After SDL and SDL_net have been initialized?

That will make you not crash, but will instead make you unable to
detect broken connections. And will make you try to send() repeatedly
to a bad fd. Don't do this.

... or you could, if you try to receive data quite directly after     
trying to send, which will detect the broken connection in a nicer
way.

Disclaimer: I know nothing about the implementation of SDL_Net, I'm
just a POSIX hacker.


--
Petri Latvala
Comment 1 Sam Lantinga 2006-05-01 03:37:34 UTC 
SDL_net now sets a SIGPIPE handler, for maximum portability.  The semantic has always been that if SDLNet_TCP_Send() returns less data then you requested sending, then the socket has been closed, or another error has occurred.

(discussion from http://forum.sun.com/jive/thread.jspa?threadID=84670)
if you can handle EPIPE as a result from your send()/recv(), the best thing to do is a process-wide SIG_IGN for SIGPIPE.

for portable code, some of the BSD-derivatives have a socket option SO_NOSIGPIPE which lets you disable this on a per-socket basis, which is often nicer. Linux has an option to send(2) MSG_NOSIGNAL which disables SIGPIPE on a per-send() basis (not as nice as the socket option, IMO). but the only thing to work portably is to SIG_IGN it.