Broken pipe when socket is closed

Question

I have a server/client app on a Linux box. If the server is not up when the client attempts to send a request, I get a SIGPIPE and the application terminates.

How can I check if the server is available on the socket before I try to write?

Also of note, I do not want to trap the SIGPIPE because the client is really part of a shared object that is used by many applications that may or may not already define their own signal handling methods.

Thanks

You can forbid local unix sockets. SIGPIPE is only generate for this type of socket. But that may hurt performance (a lot). — Šimon Tóth, Mar 21 '11 at 20:06
Would it be easier to do it "out-of-band", so to speak? E.g. have a `/var/run/server.pid` file or something that the client could check to see if the server was alive. — Ilkka, Mar 21 '11 at 20:13
@Ilkka: You'd just introduce a race condition and make the error more rare. — Erik, Mar 21 '11 at 20:15
I suggest changing the question title to something more descriptive. Maybe "How to handle SIGPIPE from a library without interfering with the main program" — Zan Lynx, Mar 21 '11 at 23:36
Another SO post might be useful: http://stackoverflow.com/questions/108183/how-to-prevent-sigpipes-or-handle-them-properly — ukhardy, Mar 21 '11 at 20:06

score 7 · Accepted Answer · answered Mar 21 '11 at 20:05

7

Pass MSG_NOSIGNAL as flags to send()

answered Mar 21 '11 at 20:05

Erik

88,732
13
198
189

MSG_NOSIGNAL is not portable. – ukhardy Mar 21 '11 at 20:09
@ukhardy: OP states linux in tags – Erik Mar 21 '11 at 20:11
It is definitely Linux only. I will play around with that and let you know. Thanks for the input. – steveo225 Mar 21 '11 at 20:34
Thanks, that is exactly what I needed; `send` now returns -1 on fail rather than a SIGPIPE. Interestingly enough, I found no mention of `MSG_NOSIGNAL` in the man page for `send`. – steveo225 Mar 22 '11 at 11:43

score 0 · Answer 2 · answered Mar 21 '11 at 23:33

This post by kroki describes what seems to be a good method.

To summarize it:

Check if SIGPIPE is pending. Record that in a variable. If it is pending before we even start then someone else blocked SIGPIPE. In that case skip all the signal stuff below and just do the write.
sigblock SIGPIPE.
Do the write.
Check if SIGPIPE is pending.
If it is pending then sigtimedwait for it with zero timeout.
Unblock SIGPIPE.

Broken pipe when socket is closed

2 Answers2