[EMAIL PROTECTED] wrote:
>
> Knowing the history of something like this is very helpful, but I'm not
> sure what you mean by this first paragraph. I think I'm most unclear
> about the meaning of "The 'threading' approach to asynchronous I/O"?
> Its opposite ("separating asynchronous I/O from threading") doesn't
> illuminate it much more. Could you elaborate?
I'll try. Sorry about being unclear - it is one of my failings.
Here is an example draft of some interfaces:
Threading
---------
An I/O operation passes a buffer, length, file and action and receives a
token back. This token can be queried for completion, waited on and so
on, and is cancelled by waiting on it and getting a status back. I.e.
it is a thread-like object. This is the POSIX-style operation, and is
what I say cannot be made to work effectively.
Streaming
---------
An I/O operation either writes some data to a stream or reads some data
from it; such actions are sequenced within a thread, but not between
threads (even if the threads coordinate their I/O). Data written goes
into limbo until it is read, and there is no way for a reader to find
the block boundaries it was written with or whether data HAS been
written. A non-blocking read merely tests if data are ready for
reading, which is not the same.
There are no positioning operations, and only open, close and POSSIBLY a
heavyweight synchronise or rewind (both equivalent to close+open) force
written data to be transferred. Think of Fortran sequential I/O without
BACKSPACE or C I/O without ungetc/ungetchar/fseek/fsetpos.
Transactions
------------
An I/O operation either writes some data to a file or reads some data
from it. There is no synchronisation of any form until a commit. If
two transfers between a pair of commits overlap (including file length
changes), the behaviour is undefined. All I/O includes its own
positioning, and no positioning is relative.
Regards,
Nick Maclaren,
University of Cambridge Computing Service,
New Museums Site, Pembroke Street, Cambridge CB2 3QH, England.
Email: [EMAIL PROTECTED]
Tel.: +44 1223 334761 Fax: +44 1223 334679
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