linux-kernel - Re: [patch 2/5] signalfd v2

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Message-ID: <Pine.LNX.4.64.0703081123210.10832@woody.linux-foundation.org>
Date:	Thu, 8 Mar 2007 11:27:22 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Davide Libenzi <davidel@...ilserver.org>
cc:	"David M. Lloyd" <dmlloyd@...rg.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [patch 2/5] signalfd v2 - signalfd core ...

On Thu, 8 Mar 2007, Davide Libenzi wrote:
>
> So, to cut it short, I can do the pseudo-siginfo read(2), but I don't 
> like it too much (little, actually). The siginfo, as bad as it is, is a 
> standard used in many POSIX APIs (hence even in kernel), and IMO if we 
> want to send that back, a struct siginfo should be.
> No?

I think it's perfectly fine if you make it "struct siginfo" (even though I 
think it's a singularly ugly struct). It's just that then you'd have to 
make your read() know whether it's a compat-read or not, which you really 
can't.

Which is why you introduced a new system call, but that leads to all the 
problems with the file descriptor no longer being *usable*.

Think scripts. It's easy to do reads in perl scripts, and parse the 
output. In contrast, making perl use a new system call is quite 
challenging.

And *that* is why "everything is a stream of bytes" is so important. You 
don't know where the file descriptor has been, or who uses it. Special 
system calls for special file descriptors are just *wrong*.

After all, that's why we'd have a signalfd() in the first place: exactly 
so that you do *not* have to use special system calls, but can just pass 
it on to any event waiting mechanism like select, poll, epoll. The same is 
just *even*more*true* when it comes to reading the data!

		Linus
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