source: CIVL/examples/mpi/mpiFeature/Test_nonblocking/bug.cvl

#include "mpi.h"
#include "assert.h"

int many_to_one () {
  int comm_size;
  int comm_rank;
  MPI_Comm comm = MPI_COMM_WORLD;
  MPI_Datatype type = MPI_INT;
  
  MPI_Comm_size (comm, &comm_size);
  MPI_Comm_rank (comm, &comm_rank);

  int rbuf[comm_size];
  MPI_Request reqs[comm_size];
  MPI_Status statuses[comm_size];

  MPI_Send(&comm_rank, 1, type, 0, comm_rank, comm);
  if (comm_rank == 0) {
    for (int i = 0; i < comm_size; i++) {
      $choose {
        $when ($true) {
          MPI_Irecv(rbuf + i, 1, type, MPI_ANY_SOURCE, MPI_ANY_TAG, comm, reqs + i);
          assert (reqs[i] != MPI_REQUEST_NULL);
        }
        $when ($true) {
          MPI_Recv(rbuf + i, 1, type, MPI_ANY_SOURCE, MPI_ANY_TAG, comm, statuses + i);
          reqs[i] = MPI_REQUEST_NULL;
        }
      }
    }
    MPI_Waitall(comm_size, reqs, statuses);
    for (int i = 0; i < comm_size; i++) {
      int src = statuses[i].MPI_SOURCE;
      
      assert (statuses[i].MPI_TAG == src);
      assert (rbuf[i] == src);
    }
  }
  return 0;
}

int main() {
  MPI_Init(NULL, NULL);
  many_to_one();
  MPI_Finalize();
  return 0;
}