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On Sat, 6 Dec 1997, Henry W Miller wrote:

> Along with non-block reads there is anouther potentiol problem to
> consider: I was told when I first started using files that read(x) [ there
> is anotuehr parameter for a buffer, but I can't recall read schematics off
> the top of my head]  would ALWAYS read x bytes, and return x.  The only
> exception was when dealing with sockets.  I wrote several programs, for
> class, that relied on this behavor.   However an HSM file system may need
> to block read for a long time, thus if I try to read 1500 bytes, and only
> 500 are avaibal it would return 500 bytes.  There may be programs out
> there that do not handle this situation.  Word needs to get out that
> reling on this feature is about to break programs.  (even if it is only
> student programs, I'd hope real world programmers don't do this, but now
> it is critical to not teach this bad habbit)

Read has _never_ been guaranteed to return exactly the requested number
of bytes -- if it were, what would happen if you requested more bytes
than remained in the file?

> I don't agree with wanting non-blocking, since it will probably be sued
> for polling, (though I agree there needs to be a way to ask for a file to
> be moved to local storage, and programs cannot block for that long) 
> however the alternatives I see are requesting a read, and then a signal
> when the read is ready to complete, or a theading model that allows one
> thread to block while anotuer goes about working.  I like the second one
> better, but both of them require major kernel modifications that I don't
> think we are ready for.  If anyone can come up with an ellegant
> alternative to wasting cycles polling, that won't break existing programs
> please do.

There are several interfaces providing this already.  See the man pages
for select() and poll(), and for the O_ASYNC and O_NONBLOCK options to
fcntl.

- --
				Jim Wise
				jim.wise@turner.com

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