linux-kernel - Re: [RFC PATCH 0/3] epoll: read(),write(),ioctl() interface

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Message-ID: <CAJ3m7BohjBxGE4UmjEibhB0ndv+9sK6A-C8y3JiHWHUdFtEhwQ@mail.gmail.com>
Date:	Mon, 3 Feb 2014 11:34:36 -0800
From:	Nathaniel Yazdani <n1ght.4nd.d4y@...il.com>
To:	Clemens Ladisch <clemens@...isch.de>
Cc:	viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/3] epoll: read(),write(),ioctl() interface

On 2/3/14, Clemens Ladisch <clemens@...isch.de> wrote:
> Nathaniel Yazdani wrote:
>> Using the normal I/O interface to manipulate eventpolls is much neater
>> than using epoll-specific syscalls
>
> But it introduces a _second_ API, which is epoll-specific too, and does
> not use the standard semantics either.
>
>> while also allowing for greater flexibility (theoretically, pipes could
>> be used to filter access).
>
> I do not understand this.

The idea here was that if epoll is controlled by read()/write(), then
a program could be written so that it expects the epoll to dup()ed
to a second file descriptor, using one exclusively for writing & the
other exclusively for reading. That way, if an application is in
debug mode, for example, it could start up a thread to replace
those two file descriptors with pipes, so that thread would then
be able to tee, preprocess, or do whatever else to the epoll
streams.

>> read() simply waits for enough events to fill the provided buffer.
>
> The usual semantics of read() are to return a partially filled buffer if
> it would block otherwise, i.e., blocking is done only if the returned
> buffer would have been empty.
>
>> As timeout control is essential for polling to be practical, ioctl() is
>> used to configure an optional timeout
>
> This is what the timeout parameter of poll() and friends is for.

I admit that part of this approach isn't the best.

Either way I appreciate your feedback,
Nathaniel Yazdani
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