[TUHS] Maximum Array Sizes in 16 bit C

[Dan Cross]

On Fri, Sep 20, 2024 at 11:17 AM Dave Horsfall <dave at horsfall.org> wrote:
> On Fri, 20 Sep 2024, Paul Winalski wrote:
> > On Thu, Sep 19, 2024 at 7:52 PM Rich Salz <rich.salz at gmail.com> wrote:
> >       In my first C programming job I saw the source to V7 grep which
> >       had a "foo[-2]" construct.
> >
> > That sort of thing is very dangerous with modern compilers.  Does K&R C
> > require that variables be allocated in the order that they are declared?  If
> > not, you're playing with fire.  To get decent performance out of modern
> > processors, the compiler must perform data placement to maximize cache
> > efficiency, and that practically guarantees that you can't rely on
> > out-of-bounds array references.
>
> [...]
>
> Unless I'm mistaken (quite possible at my age), the OP was referring to
> that in C, pointers and arrays are pretty much the same thing i.e.
> "foo[-2]" means "take the pointer 'foo' and go back two things" (whatever
> a "thing" is).

Where I've usually seen this idiom is in things like:

    char foo[10];
    char *p = foo + 5;
    p[-2] = 'a'; /* set foo[3] to 'a' */

But as Paul pointed out, a) this relies on aliasing the bytes in
`foo`, and b) is UB if the (negative) index falls below the beginning
of the underlying object (e.g., the array `foo`).

> C is just a high level assembly language; there is no such object as a
> "string" for example: it's just an "array of char" with the last element
> being "\0" (viz: "strlen" vs. "sizeof".

Sadly, this hasn't been true for a few decades; arguably since
optimizing compilers for C started to become common in the 70s. Trying
to treat C as a high-level macro assembler is dangerous, as Paul
pointed out, even though a lot of us feel like we can "see" the
assembly that a line of C code will likely emit. While in many cases
we are probably right (or close to it), C _compiler writers_ don't
think in those terms, but rather think in terms of operations
targeting the abstract virtual machine loosely described by the
language standard. Caveat emptor, there be dragons.

> What's the length of "abc" vs. how many bytes are needed to store it?
>
> > Things were much simpler when V7 was written.
>
> Giggle...  In a device driver I wrote for V6, I used the expression
>
>     "0123"[n]
>
> and the two programmers whom I thought were better than me had to ask me
> what it did...

Fortunately, this is still legal. :-)

        - Dan C.