What I am trying to acheive in this simplified code is:

• 2 types of processes (root, and children, ids/rank = 10 and 0-9 respectively)
• init:
  • root will listen to children "completed"
  • children will listen to root notification when all has completed
• while there is no winner (not all done yet):
  • children will have 20% chance they will be done (and notify root they are done)
  • root will check that all are done
    • if all done: send notification to children of "winner"

I have code like:

int numprocs, id, arr[10], winner = -1;
bool stop = false;
MPI_Request reqs[10], winnerNotification;

MPI_Init(NULL, NULL);
MPI_Comm_size(MPI_COMM_WORLD, &numprocs);
MPI_Comm_rank(MPI_COMM_WORLD, &id);

for (int half = 0; half < 1; half++) {
    for (int round = 0; round < 1; round++) {
        if (id == 10) { // root
            // keeps track of who has "completed"
            fill_n(arr, 10, -1);
            for (int i = 0; i < 10; i++) {
                MPI_Irecv(&arr[i], 1, MPI_INT, i, 0, MPI_COMM_WORLD, &reqs[i]);
            }
        } else if (id < 10) { // children
            // listen to root of winner notification/indication to stop
            MPI_Irecv(&winner, 1, MPI_INT, 10, 1, MPI_COMM_WORLD, &winnerNotification);
        }

        while (winner == -1) {
            //cout << id << " is in loop" << endl;

            if (id < 10 && !stop && ((rand() % 10) + 1) < 3) { 
                // children has 20% chance to stop (finish work)
                MPI_Send(&id, 1, MPI_INT, 10, 0, MPI_COMM_WORLD);
                cout << id << " sending to root" << endl;
                stop = true;
            } else if (id == 10) {
                // root checks number of children completed
                int numDone = 0;
                for (int i = 0; i < 10; i++) {
                    if (arr[i] >= 0) {
                        //cout << "root knows that " << i << " has completed" << endl;
                        numDone++;
                    }
                }
                cout << "numDone = " << numDone << endl;

                // if all done, send notification to players to stop
                if (numDone == 10) {
                    winner = 1;
                    for (int i = 0; i < 10; i++) {
                        MPI_Send(&winner, 1, MPI_INT, i, 1, MPI_COMM_WORLD);
                    }
                    cout << "root sent notification of winner" << endl;
                }
            }
        }
    }
}

MPI_Finalize();

Output from debugging couts look like: problem seems to be root is not receiving all childrens notification that they are completed?

2 sending to root
3 sending to root
0 sending to root
4 sending to root
1 sending to root
8 sending to root
9 sending to root
numDone = 1
numDone = 1
... // many numDone = 1, but why 1 only?
7 sending to root
...

I thought perhaps I can't receive into an array: but I tried

if (id == 1) {
    int x = 60;
    MPI_Send(&x, 1, MPI_INT, 0, 0, MPI_COMM_WORLD);
} else if (id == 0) {
    MPI_Recv(&arr[1], 1, MPI_INT, 1, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
    cout << id << " recieved " << arr[1] << endl;
}

Which works.