[Info-vax] yet another sys$qiow question

[Craig A. Berry]

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.

You've convinced me that you do understand how volatile works. Whether
you understand how to use SYS$QIO on VMS is more doubtful.