Buffered Send
We have already discussed the concept of a buffered
send .
The corresponding routine is
   MPI_BSEND (buf, count, type, dest, tag, comm, ierr)
This is a blocking send which permits the programmer to
allocate required amount of buffer space into which the
data can be copied until it is delivered.
Routine returns after data has been copied from
    application buffer to the allocated buffer.
This must be used in conjunction with
MPI_BUFFER_ATTACH and MPI_BUFFER_DETACH
routines.