[Info-vax] QIO Writes to a mailbox

[VAXman- at SendSpamHere.ORG]

In article <d0c8a111-6a84-47ba-b816-80123a60f5f6 at 8g2000vbw.googlegroups.com>, Bob Gezelter <gezelter at rlgsc.com> writes:
>On Nov 4, 5:04=A0am, Steve Bainbridge <stephen_bainbri... at yahoo.co.uk>
>wrote:
>> Hi,
>>
>> I have a very simple question, but when looking at the Alpha internals
>> manual and talking to colleagues the answer does not seem so obvious.
>>
>> We have some old code that issues a QIO WRITEVBLK to a mailbox - the
>> return code from the QIO is normal. Another process reads the mailbox
>> and never gets the message - this only happens very rarely.
>> Unfortunately the writer does not check or report what value the IOSB
>> returns and I need to try and understand why these messages are
>> getting lost. I can't reproduce this on our test systems and I can't
>> easily install updated diagnostic code onto these production systems.
>>
>> The question I have is when is the data pointed to by P1 (in the QIO
>> call) actually copied ? It is always done within the QIO routine or is
>> it just scheduled to be done in the QIO call ? Can I re-use the buffer
>> pointed to by P1 immediately after the call to the QIO ?
>>
>> Regards,
>> Steve
>
>Steve,
>
>As has been said several times: QIO (or QIOW) without checking the
>IOSB status at completion is simply incorrect. The return from the
>system service merely indicates that the operation was queued, not its
>final disposition.

I believe I said before that the $QIOW is just $QIO followed by $SYNCH.
In the latter, the caller will wait until the I/O completes.  $QIO is
ASYNCHRONOUS.  It's the $SYNCH which makes it a SYNCHRONOUS call.  But,
not specifying the EFN or, more importantly, the IOSB will render the
$SYNCH rather useless.

Specify the IOSB, check it and assume nothing!  Then, the code should
work.

-- 
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