MPI_Neighbor_allgather(3) on Linux
The text 912feae17ed1, as Debian 9 (Stretch) shipped it.
In this function, each process i gathers data items from each process j if an edge (j,i) exists in the topology graph, and each process i sends the same data items to all processes j where an edge (i,j) exists. The send buffer is sent to each neighboring process and the l-th block in the receive buffer is received from the l-th neighbor.