On Wed, Mar 30, 2016 at 09:16:22AM +0200, Edward Bartolo wrote:
> Hi and many thanks for the replies,
>
> I can understand that a pointer being an address depends heavily on
> machine architecture which means, on 32 bit machines it is 4 bytes
> long and on 64 bit machines it is 8 bytes long. I also understand that
> a pointer variable is essentially made of two parts as illustrated
> below:
>
> address [always allocated] ------------------> data [not allocated
> automatically]
>
> The address does depend on architecture but the data? And what else
> can enter into a pointer's definition other than what I illustrated?

Some machines (I'm thinking of, say, the PDP-10, which had a 36-bit
wors soze, and, if I remember correctly, eighteen-bit addresses) found
that it was good to stuff many characters into a single addressible
machine word. I think the PDP-10 stuffed five seven-bit characters
into a word, leaving the sign bit unmolested. But sometime it was
necessary to address intividual characters within a word. To this
eend, they made special character pointers, which were a few bits
longer than a regular address. So character pointers were not the same
size as regular pointers.

There are have also been systems where a function pointer has two
compononts -- the address of the code to be executed and the address of
the environment the function is to operate in. There are historical
reasos for that (mostly having to do with programming language that are
not C), but on machines designed for that, function pointers will be
bigger than data pointers.

But C is part of the legacy of 8-bit bytes and byte-addressable
machines.

-- hendrik