Re: Java vs Ada 95 (Was Re: Once again, Ada absent from DoD SBIR solicitation)

[dewar@merv.cs.nyu.edu (Robert Dewar)]

Robert Eachus says

"   It is the case that if you want to have a program which has
multiple tasks which reference the same bounded string, and one of
them does an update, then you have to have a synchronization point
before the next reference.

   Now as long as there are no synchronization points in the string
ops, you don't have a problem.  You can even allow for the case where
an allocation or free can be an external synchronization point.  The
only problem is transitive updates:"


At first I thought this must be wrong, but as I think about it, I think
it is the complete answer to the concern about reference counts, but it
is hardly a satisfactory answer. If you are using reference counts for
unbounded strings, then it is indeed the case that the code manipulating
these refrence counts is highly likely to be erroneous (wrt the reference
counts as shared variables), if there is any sharing of string values
between tasks.

So I think one must conclude that such sharing is not required to work.

Perhaps an example is worthwhile

Suppose we have two variables V1 and V2

We write V1 := V2 (these are unbounded strings by the way)

then we spawn two tasks, T1, T2, then in T1 we assign a completely
new value to V1 and in T2 we assign a completely new value to V2.

Well this program execution may be erroneous, which is at least to 
me a bit of a surprise.