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Date: 6 Aug 91 17:23:59 GMT
From: emery@mitre-bedford.arpa  (David Emery)
Subject: Re: Ada and Unix--Blocked Tasks
Message-ID: <EMERY.91Aug6102359@Dr_No.mitre.org>
List-Id: <comp.lang.ada>

In order for non-blocking I/O to work on Unix, several different
'actors' have to cooperate.  First you need a Unix that supports
non-blocking I/O as a concept.  Usually this works by having a system
call that `queues' the operation, and a signal that tells the
application when the I/O is done.  Next you need a device driver that
also plays this game.  Not all device drivers support non-blocking
I/O, even if the operating system provides the facility.  Finally, you
need the Ada runtime to play along, by handling the signal correctly.  

For this reason, it is almost impossible for a compiler vendor to
guarantee non-blocking I/O in all cases.  In particular, the compiler
vendor cannot guarantee that the user`s devices and device drivers
'play along'.  So take any vendor claims that they support
non-blocking I/O on Unix platforms (without specifying additional
requirements on the device drivers, etc.) with a grain of salt.  It's
possible, and it will probably work on "reasonable systems with
reasonable devices", but it's not guaranteed, especially in the face
of third-party I/O devices.

				dave