Re: GNAT bug with respect to an Implementation Advise

["Randy Brukardt" <randy@rrsoftware.com>]

Florian Weimer wrote in message <87aea0xfgk.fsf@deneb.enyo.de>...

>GNAT is following the IA, but the IA is surprising.  There's also an
>AI on this issue, which involved a lot of discussion.  (Some vendors
>do not follow the IA and want to change the standard to officially
>sanction the derivation, IMHO.

No, that's an oversimplification. The problem is that the Ada 95
standard never specifies what the base range is. That is left up to
compilers. Thus, there is no way to tell how many bytes are written out
by a particular type. (There is also the issue that you can't read
hetrogenus data structures defined outside of Ada at all without some
control over this.)

For instance, if you have the type:
    type Byte is range 0 .. 255;
    for Byte'Size is 8;

the base range might be:
    Byte'Base'First = -32768, Byte'Base'Last = 32767 (i.e. 16 bits
read/written by streams)
or it might be
    Byte'Base'First = -2**31, Byte'Base'Last = 2**31-1 (i.e. 32 bits
read/written by streams)
or something else again (say on a 24-bit machine).

These are all legal, correct implementations of Ada, and even if all of
the implementations follow the IA, they won't be compatible.

                Randy.