Re: [Kiobuf-io-devel] RFC: Kernel mechanism: Compound event wait

From: Stephen C. Tweedie (
Date: Thu Feb 08 2001 - 08:22:18 EST


On Thu, Feb 08, 2001 at 12:15:13AM +0100, Pavel Machek wrote:
> > EAGAIN is _not_ a valid return value for block devices or for regular
> > files. And in fact it _cannot_ be, because select() is defined to always
> > return 1 on them - so if a write() were to return EAGAIN, user space would
> > have nothing to wait on. Busy waiting is evil.
> So you consider inability to select() on regular files _feature_?

Select might make some sort of sense for sequential access to files,
and for random access via lseek/read but it makes no sense at all for
pread and pwrite where select() has no idea _which_ part of the file
the user is going to want to access next.

> How do you write high-performance ftp server without threads if select
> on regular file always returns "ready"?

Select can work if the access is sequential, but async IO is a more
general solution.

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