[Info-vax] Re; Spiralog, RMS Journaling (was Re: FREESPADRIFT)

[David Froble]

Stephen Hoffman wrote:
> On 2016-06-18 20:59:14 +0000, John Reagan said:
> 
>> On Saturday, June 18, 2016 at 3:02:29 PM UTC-4, Stephen Hoffman wrote:
>>> On 2016-06-18 18:07:05 +0000, Paul Sture said:
>>>
>>>> both XAB$L_EBK and XAB$W_FFB Field are "meaningful for sequential 
>>>> files only"
>>>
>>> I'd prefer that any (errant) requests violating that would kick back 
>>> a run-time error when used on inappropriate file formats, but that 
>>> concept and that capability simply does not exist within the 
>>> (current) OpenVMS RMS APIs.
>>>
>>> Ugly case would be an explicit NaN/NaT returned in these fields 
>>> secondary to an errant access; a Not a Number or Not a Thing 
>>> response. OpenVMS and various databases certainly have have examples 
>>> akin to that, but OpenVMS has no particularly consistent 
>>> implementation of that.  In more than a few fields in the various 
>>> data structures, RMS doesn't, though.
>>
>> NaNs are floating point.  NaTs don't exist in memory, only in 
>> registers (and are Itanium specific).
> 
> Okay.  Let me try this a different way.  The RMS interface — and many of 
> the other OpenVMS interfaces — has no mechanism for differentiating a 
> data field that is invalid or uninitialized from a field that contains a 
> valid value, and RMS further has no ready means to flag that mere 
> accesses to these fields are invalid at run-time.   This is the software 
> equivalent of NaN or NaT that can be implemented in various contexts, or 
> of a database field that's marked as null.   This differentiation is 
> analogous to what Itanium and other systems are using NaN, NaT or such 
> to represent.  Something which really isn't.     This as differentiated 
> from a data field that's empty, zero or some other such explicitly-set 
> value.

Ok, dumb Dave is having a problem with this.  And yes, I had problems with the 
concept of Null in Visual Basic.

A data field is a place where data is stored.  Even if it is zeros, or whatever, 
it's still data.  An application can determine if it doesn't like the value of 
the data.  I'm not sure anything else can do so.