Back when we were running v5 at the University of Toronto, we had a graphics device that we accessed, on our split I&D space 11/45, using 0, something like this:

struct x {

   int reg0, reg1, ...;

};

0->reg1 = 0234;

Several old details of old C made this possible as well: Struct tags were global, -> worked on any pointer, and ints and pointers were interchangeable.

-rob

On Tue, Sep 22, 2020 at 6:51 AM John Cowan <cowan@ccil.org> wrote:

On Mon, Sep 21, 2020 at 1:55 AM Steve Nickolas <usotsuki@buric.co> wrote:

 

I've never written anything that uses varargs, so I've never run into
that.  But I've actually done quite a bit of work with an environment
where this isn't true: MS-DOS using the large or huge model.  In this
environment, sizeof(int)=2, and sizeof(void*) is 4. Of course, it's not conformant to pass an int variable as an argument where a pointer variable is expected.

If the compiler was ISO-conformant (which it almost certainly was not), that would not matter.  0 in int context would be a 2-byte int with all bits zero, and 0 in pointer context would be a 4-byte null pointer, probably with all bits zero.

C doesn't require that the address represented by the null pointer (whether or not it is all-bits-zero) is inaccessible, merely that there is no C object or function there.  A simple shim of the appropriate size (1, 2, 4, 8 bytes depending on the CPU's alignment rules) will suffice.