# MPI_Alltoall

[MPI_Alltoall](https://www.mpich.org/static/docs/v3.4.1/www3/MPI_Ialltoall.html)
MPI_Alltoall(void* sendBuf, int sendCount,MpIDatatype sendType,void* recvBufm, int recvCount,MPI_Datatype recvType,MPI_Commcomm)

### Comm:
##### Communicator over which data is to be exchanged (handle).
##### EX: MPI_COMM_WROLD

MPI_Comm_size(comm, &n);
    for (i = 0, i < n; i++)
        MPI_Send(sendbuf + i * sendcount * extent(sendtype),
            sendcount, sendtype, i, ..., comm);
    for (i = 0, i < n; i++)
        MPI_Recv(recvbuf + i * recvcount * extent(recvtype),
            recvcount, recvtype, i, ..., comm);

% mpicc hello.c

% cat nodefile
node1
node2

% mpirun -np 1 -hostfile nodefile a.out（由1節點來執行）
Hello World from node1

% mpirun -np 2 -hostfile nodefile a.out（由2節點來執行）
Hello World from node1
Hello World from node2