MPI_Neighbor_alltoall(3) on Linux
The text 32e8c2e344b4, as Debian 8 (Jessie) shipped it.
In this function, each process i receives data items from each process j if an edge (j,i) exists in the topology graph or Cartesian topology. Similarly, each process i sends data items to all processes j where an edge (i,j) exists. This call is more general than MPI_NEIGHBOR_ALLGATHER in that different data items can be sent to each neighbor. The k-th block in send buffer is sent to the k-th neighboring process and the l-th block in the receive buffer is received from the l-th neighbor.