[Info-vax] QIO Writes to a mailbox

[VAXman- at SendSpamHere.ORG]

In article <j9dpir$vs2$1 at dont-email.me>, Albrecht Schlosser <vms-news at go4more.de> writes:
>On 08.11.2011 22:36, VAXman- @SendSpamHere.ORG wrote:
>> In article<j9c6ug$r1s$1 at news.albasani.net>, Jan-Erik Soderholm<jan-erik.soderholm at telia.com>  writes:
>....
>>> One efficient way to do it is to check the previous IO (say, by
>>> checking the IOSB) right *before* you will do the *next* $QIO.
>>> In that way, you can do any other processing you like in your
>>> app while the I/O subsystem completes the IO asyncronicaly.
>>>
>>> Always waiting right *after* the $QIO is inefficent and you could
>>> anyway use $QIOW and let the system take care of the "waiting".
>>
>> Even when using $QIO and an AST, one needs to check the IOSB.  Like I said,
>> specify the IOSB, check it and assume nothing!
>
>Yep, that's true, of course, but may not be sufficient in
>Steve(the OP)'s case. He did not write anything about how
>often mailbox writes are generated (maybe asynchronously)
>in the writer program.
>
>If we assume that he reuses buffers (as well as the IOSB),
>then it might be possible to overwrite one message with the
>next message, so that two consecutive QIO's will send the
>same message to the mailbox. I know, this has already been
>said, but it should be considered as a possible cause of
>errors too. Thus, the reader process might read one message
>twice, and - depending on the processing in the reader
>process - may not realize the duplicate, and also miss the
>overwritten message. This *could* explain what Steve is
>seeing, even if all I/O's were working correctly (QIO and
>IOSB success).
>
>Jan-Erik's idea to check the previous IOSB value just
>before the next I/O might help, if the code also makes
>sure that the I/O buffers are written *after* the check.

Didn't I say: "specify the IOSB, check it and assume nothing!"?  Ignoring
the IOSB may just be the "tip of the iceberg" but it's a starting point.



>Of course, it would be better not to overwrite I/O buffers
>and to use different IOSB's as long as it is possible to
>do asynchronous writes to the mailbox in the first place.
>
>I'd check this possible overwriting as well as the IOSB
>return value, especially if all messages are of the same
>sizes (so that overwriting the buffer contents wouldn't
>be so obvious).

If I had a dime for each time I've fixed $QIO programming errors of this
nature I'd retire to my own island.

-- 
VAXman- A Bored Certified VMS Kernel Mode Hacker    VAXman(at)TMESIS(dot)ORG

All your spirit rack abuses, come to haunt you back by day.
All your Byzantine excuses, given time, given you away.