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From: zenith@ensmp.fr (Steven Ericsson Zenith)
Newsgroups: comp.parallel
Subject: Ease description addendum---errata
Message-ID: <12873@hubcap.clemson.edu>
Date: 1 Feb 91 11:42:12 GMT
Sender: fpst@hubcap.clemson.edu
Lines: 66
Approved: parallel@hubcap.clemson.edu
To: fpst@hubcap.clemson.edu
In-Reply-To: Steve Stevenson's message of Thu, 31 Jan 91 17:00:43 -0500 <9101312200.AA05686@hubcap.clemson.edu>


Well, this morning on rereading my posting revealed at least two errors.
First, the phrase "non-local (automatic)", should read "non-local
(non-automatic)". Secondly - this'll teach me to crow - the example
given has an incorrect bound in "(i = 3 for LIMIT)" so my claim that the
program works is false. It should read:

void main()
{
  int   i, ok;
  %share [LIMIT] int prime;

  %write (prime[2], 1);

  %cooperate (i = 3 for LIMIT-3) prime(i);

  for(i = 2; i < LIMIT; ++i) {
    %get (primes[i], ok);
    if (ok) printf("%d\n", i);
  }
}

void prime(n)
     int        n;
{
  int           i, limit, ok;
  double        function sqrt();

  limit = sqrt((double) me) + 1;

  for (i = 2; i < limit; ++i) {
    %read (primes[i], ok);
    if (ok && (me%i == 0)) %write(prime[n], 0);
  }
  %write(prime[n], 1);
}

This probably went unnoticed since I didn't explain "replication". The 3
numbers we don't test for primality are 0,1,2 - the last number we test
is (LIMIT-1). The phrase (i = b for n) instances n indexes named "i"
with the values b..(b+n-1).

In fact my bounds checking uncovered a second error propogated from the
original Linda program (the for loop in main has an incorrect condition
bound). This is corrected here also.

*Solution to the exercise:*

The reason the original Linda example deadlocks is corrected by the line

  %write (prime[2], 1);

which provides the "prime" processes with something to start with.
Otherwise the program will deadlock, as does the Linda version, in the
"prime" function on the line

    %read (primes[i], ok);

Steven
--
Steven Ericsson Zenith * Email: zenith@ensmp.fr  *    Fax:(1)64.69.47.09
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