source: CIVL/examples/concurrency/ring.cvl

#include <civlc.cvh>
/* Create NPROCS processes.  Have them exchange data in a cycle.
 * Command line verification example:
 *     civl verify -inputNPROCS_BOUND=10 -inputN_BOUND=5 ring.cvl
 */
#include<comm.cvh>
#define TAG 0

$input int NPROCS_BOUND = 10; // upper bound of the number of MPI processes
$input int NPROCS; // number of MPI processes
$assume(0 < NPROCS && NPROCS <= NPROCS_BOUND);
$assume(NPROCS >= 2);
$input int N_BOUND = 5; // upper bound on number times to loop
$input int N; // number of times to loop
$assume(0 < N && N <= N_BOUND);
$gcomm GCOMM_WORLD  = $gcomm_create($here, NPROCS); // global comm object

void MPI_Process (int rank) {
  $comm comm = $comm_create($here, GCOMM_WORLD, rank);
  int left = (rank+NPROCS-1)%NPROCS;
  int right = (rank+NPROCS+1)%NPROCS;
  int x=rank, y; // send a message from x, recv into y
  $message in, out;
  
  for (int i=0; i<N; i++) {
    if (rank%2==0) { // send first, then recv
      out = $message_pack(rank, right, TAG, &x, sizeof(int));
      $comm_enqueue(comm, out);
      in = $comm_dequeue(comm, left, TAG);
      $message_unpack(in, &y, sizeof(int));
      $assert(y==left);
    } else { // recv first, then send
      in = $comm_dequeue(comm, left, TAG);
      $message_unpack(in, &y, sizeof(int));
      $assert(y==left);
      out = $message_pack(rank, right, TAG, &x, sizeof(int));
      $comm_enqueue(comm, out);
    }
  }
  $comm_destroy(comm);
}

void main() {
  $proc procs[NPROCS];
  
  for (int i=0; i<NPROCS; i++) 
    procs[i] = $spawn MPI_Process(i);
  for (int i=0; i<NPROCS; i++)
    $wait(procs[i]);
  $gcomm_destroy(GCOMM_WORLD, NULL);
}