Re: memory management

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Thomas Maier-Komor <maierkom@lpr.no-spam.e-technik.tu-muenchen.de> writes:

> Robert A Duff wrote:
> > The same is true of pointers in C and C++ -- an implementation is free
> > to implement pointers as something other than a machine address.  In
> > fact, if a C compiler wishes to check array bounds, it pretty much *has*
> > to use fat pointers.  I know of one C compiler that did just that.  My
> > point is that "pointer" is not synonymous with "single machine address",
> > even in C.
> >
> 
> That's definetly an interesting implementation variant for the C
> language. Could you tell us, which compiler did this? Has this been
> documented anywhere?

No, sorry, but I can't remember the name of the company or the product.
I think they went out of business about 10 years ago.  Their compiler
was intended for testing -- detecting array indices out of bounds.

Here's how to do it: allocate one word of dope before each array,
to store the length of the array.  Represent each C pointer as
a fat pointer: pointer to the dope, and pointer to the array
element.  Treat non-array objects like one-element arrays.
Then on each pointer dereference, check that the element pointer is
between the start and end of the array.

Ada compilers can do all this *much* more efficiently.
Ada does not need fat pointers.  GNAT uses them, but
only in rare cases.

- Bob