source: CIVL/examples/concurrency/blockAdderBad.cvl

/* Compare sequential and parallel sum of array by block partition
 * Example run:
 *      civl verify -inputB=6 -inputW=3 blockAdderBad.cvl
 * or (if you want to find the minimal counterexample)
 *              civl verify -inputB=6 -inputW=3 blockAdderBad.cvl -min
 */
#include<civlc.cvh>

$input int B = 6; // upper bound on array length
$input int W = 3; // upper bound on number of workers

$input int N; // array length
$assume(N>=1 && N<=B);
$input int P; // number of workers
$assume(P>=1 && P<=W);
$input double a[N];

double adder_seq() {
  double s = 0.0;

  for (int i=0; i<N; i++)
    s += a[i];
  return s;
}

double adder_par() {
  $proc workers[P]; // the workers
  int lock = 0; // lock for accessing result
  double result = 0.0; // shared by all workers

  void run_worker(int id) {
    int start = (id*N)/P;
    int stop = ((id+1)*N)/P;
    double localSum = 0.0;

    for (int i=start; i<stop; i++)
      localSum += a[i];
    $when (lock==0) lock = 1;
    result += localSum;
    // lock = 0; // forgot to release the lock
  }

  for (int i=0; i<P; i++) ; // hack
  for (int i=0; i<P; i++)
    workers[i] = $spawn run_worker(i);
  $waitall(workers, P);
  return result;
}

void main() {
  double sum_seq = adder_seq();
  double sum_par = adder_par();  

  $assert((sum_seq == sum_par));
}