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From: chris@alsys.UUCP (Christopher Gray)
Newsgroups: comp.sys.transputer
Subject: Polling using a hard channel
Message-ID: <8685.9012070015@pyrltd.pyra.co.uk>
Date: 6 Dec 90 10:29:36 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 20

I am developing an application in which it would be desirable to poll
a hard channel to determine whether or not a remote process is ready
to communicate over the hard channel.

Unfortunately this cannot be done in occam and there appears to be no
machine level instruction which allows this.

The application uses the routine OutputOrFail.t (from the occam
support library xlink.lib). With a "delay" of zero, this routine
always returns "aborted" for a hard channel even if there is a remote
process ready to communicate.  This is because communication over a
hard channel always results in descheduling of the communicating
process.  Unfortunately, setting timeout = (small value) also causes
problems due to the unknown length of time that the message takes to
read/write on a hard link.

Significantly, polling using soft channels produces no such problems
and works fine.  However, when using hard channels there appears to be
no way to achieve polling using software, at whichever level: occam;
occam support library; machine code.