Blocking Routines
A blocking send returns only after the send
buffer is safe for reuse.
This can be synchronous.
It can be asynchronous if buffering is used.
A blocking receive returns only after the data
has arrived fully and is ready to use.
MPI_SEND (buf, count, type, dest, tag, comm,
ierr)
MPI_RECV (buf, count, type, src, tag, comm,
status, ierr)