Re: Safety of the Booch Ada 95 Components

[Simon Wright <simon@pogner.demon.co.uk>]

xanthian@well.com (Kent Paul Dolan) writes:

> Robert Dewar  <robert_dewar@my-deja.com> wrote:
> >  herwin@gmu.edu (Harry Erwin) wrote:
> >> A deep copy of a pointer data structure may require more
> >> memory than is available.
> 
> >A naive copy may have that characteristic, but it is always
> >possible on modern machines to do such a copy with no additional
> >storage. It just takes a bit of cleverness. Remember that every
> >pointer has at least two spare bits.
> 
> This one sets alarm bells ringing like crazy.  I was a user of
> AmigaBASIC, whose developers assumed that the 24 bits in use of 32 bit
> pointers they saw when the machine in an early OS rev would be the law
> forever, ignored the developer manuals warnings to the contrary, used
> the other 8 bits for string lengths or some such, in a petty search for
> "efficiency" at the expense of clean coding, and had their software die
> a horrible death when the OS was upgraded within a couple of years to
> use all 32 bits of a pointer.

I suspect Robert was thinking of the low-order 2 bits (on the
assumption that malloc() always returns something longword-aligned. I
rather thought it was often 16-byte-aligned??).