Ada95 - Determining Machine Byte Order

Ada95 - Determining Machine Byte Order

[Timothy Halloran]

Is there a way in Ada95 (i.e. in package System, Ada, or Standard) to
determine the byte ordering of a machine (big endian, little endian)?

Byte swapping Ada programmer,

Tim Halloran

Re: Ada95 - Determining Machine Byte Order

[Robert Dewar]

Not only is Mark's solution:

        int n = 1, little_endian = (*((char *)&n) == n);

not very clear (though if you study it for a second and can find your
way around * and & in C, you can see how it works), but more significantly
there is no guarantee that this will work, it is equivalent to the use of
unchecked conversion in Ada, and equally problematic. Mark if you don't
believe me, try seeing what this gives on a Cray, or if you want to take
the theoretical approach, tell me where in the ANSI standard it is written
that this will do something well defined.

All in all I think that the Ada way of doing things:

   Little_Endian : constant Boolean :=
        System.Default_Bit_Order = System.Low_Order_First;

maybe just a *little* bit clearer :-)

Re: Ada95 - Determining Machine Byte Order

[Mark de Rooi]

dewar@cs.nyu.edu (Robert Dewar) writes:

>Not only is Mark's solution:

>        int n = 1, little_endian = (*((char *)&n) == n);

>not very clear (though if you study it for a second and can find your
>way around * and & in C, you can see how it works), but more significantly
>there is no guarantee that this will work, it is equivalent to the use of
>unchecked conversion in Ada, and equally problematic. Mark if you don't
>believe me, try seeing what this gives on a Cray, or if you want to take
>the theoretical approach, tell me where in the ANSI standard it is written
>that this will do something well defined.

Unfortunately, I don't have a Cray handy, but I will take your word for
it that my suggestion won't work on one. As I mentioned in my followup,
it is required that sizeof(char) < sizeof(int) for this to work.
Is sizeof(char) == sizeof(int) for the compiler you use on your Cray?
I am curious as to the reason my (admittedly not very clear) solution
doesn't work on a Cray.

As for the ANSI C standard, well, _of course_ casting a pointer-to-int
to a pointer-to-char and then dereferencing it is machine- and compiler-
dependent. If it weren't, there would be no point to my obscure piece of
code!

Note that I specifically mentioned that it wouldn't work on *every*
machine. It works on every PC, Mac and Unix workstation I ever used,
but not on some DSPs, and apparently, on a Cray. I wasn't aware of the
Ada Way (tm) to handle this, so I just gave a quick fix to the guy who
asked, so he could get on with his work.


>All in all I think that the Ada way of doing things:

>   Little_Endian : constant Boolean :=
>        System.Default_Bit_Order = System.Low_Order_First;

>maybe just a *little* bit clearer :-)

No argument there ;-)

Regards,

Mark