source: CIVL/examples/concurrency/barrierBad.cvl

/* Basic shared-variable flag barrier.   Run as follows:
 *      civl verify -inputB=4 barrierBad.cvl
 */
#include <civlc.cvh>

$input int B = 4; // upper bound on number of threads
$input int N;     // number of threads
$assume(1<=N && N<=B);
$proc threads[N]; // the threads
int in_barrier[N]; // am I inside the barrier?
int num_in_barrier = 0; // how many are inside barrier?
int counter = 0; // shared variable used to test barrier

void init() {
  for (int i=0; i<N; i++) in_barrier[i] = 0;
}

void barrier(int tid) {
  in_barrier[tid] = 1; // I am in the barrier
  num_in_barrier++; // increment number in barrier
  if (num_in_barrier == N) { // I am last to enter
    for (int i=0; i<N; i++) in_barrier[i] = 0; // release all
    num_in_barrier = 0; // now none are in barrier
  }
  $when (in_barrier[tid] == 0); // wait till I am released
}

void run(int tid) {
  while ($true) {
    $assert(counter == 0);
    barrier(tid);
    counter++;
    barrier(tid);
    $assert(counter == N);
    barrier(tid);
    counter--;
    barrier(tid);
  }
}

void main() {
  init();
  for (int i=0; i<N; i++) threads[i] = $spawn run(i);
  for (int i=0; i<N; i++) $wait(threads[i]);
}