linux-kernel - Re: Finding out socket/pipe connectivity status

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Message-ID: <20070812194210.210db3a3@the-village.bc.nu>
Date:	Sun, 12 Aug 2007 19:42:10 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Jan Engelhardt <jengelh@...putergmbh.de>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org
Subject: Re: Finding out socket/pipe connectivity status

> 	write(stdout, request);
> 	/* reference point [A] */
> 	read(stdin, response);
> 
> So my idea had been to launch another thread that monitors stdin for
> 'breakage' and unmount the fs before a user can start an operation on
> myfs. So I've been trying to complete the idea to code. You say a
> broken socket raises the read flag, so:

Your connection can break at any point even mid-write so you must handle
it. You have no real choice.

You can try and spot it earlier by using select/poll and checking for
error. In which case I'd use poll() as it is a bit more clear about what
event you get

Then poll stdin and stdout. The kernel will give you back 

POLLERR - an error condition
POLLHUP - a hangup

if at the point you poll an error has already occurred. You can do this
with select and the socket error and related stuff buts its uglier

Alan

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