Send/receive

# examples/04-sendrecv.jl
using MPI
MPI.Init()

comm = MPI.COMM_WORLD
rank = MPI.Comm_rank(comm)
size = MPI.Comm_size(comm)

dst = mod(rank+1, size)
src = mod(rank-1, size)

N = 4

send_mesg = Array{Float64}(undef, N)
recv_mesg = Array{Float64}(undef, N)

fill!(send_mesg, Float64(rank))

rreq = MPI.Irecv!(recv_mesg, comm; source=src, tag=src+32)

print("$rank: Sending   $rank -> $dst = $send_mesg\n")
sreq = MPI.Isend(send_mesg, comm; dest=dst, tag=rank+32)

stats = MPI.Waitall([rreq, sreq])

print("$rank: Received $src -> $rank = $recv_mesg\n")

MPI.Barrier(comm)

> mpiexecjl -n 4 julia examples/04-sendrecv.jl
0: Sending   0 -> 1 = [0.0, 0.0, 0.0, 0.0]
1: Sending   1 -> 2 = [1.0, 1.0, 1.0, 1.0]
2: Sending   2 -> 3 = [2.0, 2.0, 2.0, 2.0]
3: Sending   3 -> 0 = [3.0, 3.0, 3.0, 3.0]
0: Received 3 -> 0 = [3.0, 3.0, 3.0, 3.0]
1: Received 0 -> 1 = [0.0, 0.0, 0.0, 0.0]
3: Received 2 -> 3 = [2.0, 2.0, 2.0, 2.0]
2: Received 1 -> 2 = [1.0, 1.0, 1.0, 1.0]