Re: GNAT "alignment value must be positive"

[marciant@earthlink.net]

On Thursday, April 9, 2015 at 8:21:55 PM UTC-4, Randy Brukardt wrote:
... 

> > Is it not (no longer?) necessary to ever indicate - as per Ada 95 RM 13.3 
> > [14]
> 
> This paragraph is about "Address", not sure as to what you're referring.

Sorry that should have been [24] not [14], as in Ada 95 RM 13.3 [24].

By the way, the project that is still using a compiler that was made
more that ten years ago and we all have copies of RM95;6.0 to read, not
the newer RM.

> But the Ada 95 wording for Alignment contained a lot of nonsense. It was 
> mostly rewritten for Ada 2005, as Binding Interpretation (meaning it applies 
> to Ada 95 compilers as well), so reading the original wording does nothing 
> but confuse yourself and everyone else.

Unfortunately, updating the RM does not cause the project to have access
to an updated compiler. :)

> 
> > - "zero means that the object is not necessarily aligned on a storage 
> > element
> > boundary."?
> 
> That's talking about the value of X'Alignment, when it is *read*, not so 
> much when it is specified. (This is 13.3(23/2)).

But if the "natural"/"default" alignment that an implementation would use when it places an object of some type is - lets say 2, what about the case that a user program  does unchecked conversion on a sequence of bits to a record type that ends up making the alignment of some of the records components not have the default alignment (not 2 in the example - infact not even alignment 1).
I thought that this was a case where the user should specify alignment 0 for the component's type and that without doing that, improper reading of the component by the program might be the result, due to the compiler having assumed that the component/"object" had its default alignment. 

> ...
> old wording was nonsense. I'm not going to try to explain old nonsense. If 
> we could have explained it, we wouldn't have rewritten it!

Okay, that is fair enough.
 
> ...
> 
> Then read the current RM, not older, buggier versions. The bleeding edge can 
> be found at: http://www.ada-auth.org/standards/ada2x.html (this contains 
> everything the ARG has approved to date). Ada 2012 can be found at: 
> http://www.ada-auth.org/arm.html.
> 
> People discover bugs in the RM all the time, and the ARG fixes them 
> periodically. It's often impossible to explain the original wording, because 
> it didn't make sense for one reason or another. There's a reason that newer 
> standard replace the older ones (even if that doesn't always happen 
> practically).

I know about the http://www.ada-auth.org RM archive and also read the ARG meeting minutes when each new one is posted (Thank you for your great work on that!)

Vinny