[Info-vax] yet another sys$qiow question

[Johnny Billquist]

On 2015-08-21 19:00, John Reagan wrote:
> On Friday, August 21, 2015 at 8:57:45 AM UTC-4, Craig A. Berry wrote:
>> On 8/21/15 7:19 AM, Johnny Billquist wrote:
>>> On 2015-08-21 14:07, David Froble wrote:
>>
>>>> Thanks for validating Craig's statement.  You don't test the IOSB for
>>>> completion.  If you're doing async I/O, then you wait for the AST or
>>>> whatever to tell you it's complete.
>>>
>>> Are you saying that it is a poorly written program if it do not use ASTs???
>>
>> It's poorly written if it does not use a documented method to make sure
>> the I/O completes *before* reading the value of the IOSB, a point I have
>> made and you have ignored multiple times. ASTs are one of the documented
>> methods.
>>
>>> My example code will wait on the event flag. And it will fail if you
>>> don't declare the iosb as volatile.
>>
>> There is no event flag in your sample code, which consisted in its
>> entirety of:
>>
>> SYS$QIO(<arguments>);
>> if (iosb == 0) {
>>     <do some stuff>
>>     <wait for I/O completion>
>>     printf("IOSB is now %x\n", iosb);
>> }
>>
>> Unless "wait for I/O completion" means wait for an event flag? If it
>> means SYS$SYNCH, then you'd be fine because that takes the address of
>> the IOSB and the compiler would know someone else might modify it. If it
>> is an event flag firing then yes, the printf might say iosb is zero even
>> though it changed in the meantime, but only because you checked its
>> value before ensuring that the I/O completed. Which is wrong, so don't
>> do that.
>
> Since the IOSB was passed to $QIO, the compiler now worries that $QIO saved the address (it did).  If you call $WAITFR, the compiler would be worried that $WAITFR has access to the saved IOSB address.  I would expect the IOSB to be refetched for that printf.  (The compiler DOES know about RTL routines like printf that don't have side-effects.)

Yes, the potential saving of references to variables is always an issue 
the compiler needs to take into consideration.

	Johnny