Re: O_NONBLOCK, read(), select(), NFS, Ext2, etc.

Alexander V. Lukyanov (lav@yars.free.net)
Fri, 12 Jan 2001 19:34:38 +0300

On Fri, Jan 12, 2001 at 09:40:55AM +0000, Alan Cox wrote:
> > be true; but perhaps I am doing something wrong. If I open() a file (on
> > 2.2.18) from a floppy or NFS mount (to test in a slow environment) with
> > O_NONBLOCK|O_RDONLY, read() will still block. If I try to select() on
> > the file descriptor, select() always returns 0.
>
> The definition of immediate is not 'instant'. Otherwise no I/O system would
> ever return anything but -EWOULDBLOCK. Its that it won't wait when there is
> no data pending. On a floppy there is always data pending

I have a long standing idea on modifying poll/read/write behavior on
regular files opened in O_NONBLOCK mode. How about the following scheme:

* poll returns POLLIN/POLLOUT on regular files only if the data are
in buffer cache/there is room for write().
* read returns EAGAIN if the data are not in buffer cache.
* write returns EAGAIN if there is no room (yet) to store the data.

All the operation should signal to IO subsystem that the data are
needed in cache/the room has to be freed.

--
   Alexander.
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