Re: Pascal Calling Convention

[Adam Beneschan <adam@irvine.com>]

On Mar 28, 1:29 pm, Hyman Rosen <hyro...@mail.com> wrote:
> On 3/26/2011 4:46 PM, Robert A Duff wrote:
>
> > It doesn't seem reasonable to me.  Uninitialized variables do not
> > contain random data.  They contain arbitrary data.  And it's quite
> > likely that such arbitrary data happens to be the same every time,
> > or has some other non-random pattern.  Bad Guys sometimes exploit
> > such patterns to cause security problems.
>
> Time for my favorite data structure! This is a structure which
> represents an unordered set of the integers [0,N) and which has
> constant time initialization, insertion, deletion, and membership
> test. It relies on reading uninitialized data as some arbitrary
> value. Compilers which regard doing this as undefined behavior
> will break the code. I'm going to write it in C++, but it should
> be straightforward to translate to Ada:

I hope your "favorite data structure" comment was sarcastic.  I would
expect this code to behave incorrectly a large portion of the time;
specifically, if I read the code right, once you add any element to
the set, the program is likely to think 0 is in the set whether you've
added it or not.

                        -- Adam



> template <unsigned N>
> class set
> {
>      unsigned n;    // number of members in set
>      unsigned d[N]; // members are in d[0..n-1]
>      unsigned s[N]; // d[s[k]] == k if k is a member
>      // unspecified elements of d and s are arbitrary
>
> public:
>      set() : n(0) { }
>      bool has(unsigned k)
>          { return k < N && s[k] < n && d[s[k]] == k; }
>      void add(unsigned k)
>          { if (k < N && !has(k)) { d[n] = k; s[k] = n++; }
>      void del(unsigned k)
>          { if (has(k)) { unsigned i = s[k]; d[i] = d[--n]; s[d[i]] = i; }
>      void clr() { n = 0; }
>
>
>
> };