send-recv deadlock

[Stephen Siegel]

To do a message-passing data exchange, you need to send first, receive second. Otherwise deadlock will ensue. In diffusion example, the receives happen first (check for this in other examples as well):

void exchange_ghost_cells() {
  if(left >= 0) send(v[1], left, 0);
  if(right < nprocs){
	recv(v[nxl+1], right, 0);
   	send(v[nxl], right, 0);
  }
  if(left >= 0) recv(v[0], left, 0);
}

I guess in this case it won't deadlock because of the boundaries, but it will force sequentialization.