[Info-vax] Volatile, was: Re: yet another sys$qiow question

[Jan-Erik Soderholm]

Den 2015-08-19 kl. 19:47, skrev John Reagan:
> On Wednesday, August 19, 2015 at 12:58:05 PM UTC-4, VAXman- wrote:
>> In article <>, Simon Clubley <> writes:
>>> On 2015-08-19, VAXman-  @SendSpamHere.ORG <> wrote:
>>>> Question:  If "volatile" is necessary, shouldn't this code keep
>>>> spinning its tail???
>>>>
>>>
>>> No, because you only have a function call in the loop (sys$qiow())
>>> (and the function call additionally directly references iosb BTW).
>>>
>>> This means that regardless of whether the compiler terminates the
>>> caching scope for iosb at any function call or whether it only
>>> terminates the caching scope when a function call references iosb,
>>> your code still causes the compiler to generate code to re-read the
>>> current contents of the iosb variable after the function call
>>> returns.
>>>
>>> The current discussion is based around the belief C compilers
>>> terminate the caching scope at any function call. I don't know if
>>> this is true for all the other languages VMS supports.
>>
>> So, I should put the $QIO in a subroutine.
>>
>
> Well, the compiler saw that somebody took the address of the IOSB and
> passed it along to $QIO.  The compiler doesn't know if that address was
> saved in some global variable.  So the call to your jacket routine is
> still probably sufficient to tell the compiler (this routine call might
> write into every possible global variable and write into every variable
> whose address was ever taken).
>

And what difference would that make? Would the compiler simply concider
*everything* ("every variable whose address was ever taken) beeing
"volatile"? Not particualy efficient...

Isn't "volatile" just that, to hint the compiler about which variables
that might change without a hint from the actual code compiled?