mpi.get.count finds the length of a received message.
1 for integer, 2 for double, 3 for char.
a status number
mpi.recv is used to receive a message, the receiver
buffer can be set to be bigger than the incoming message. To find the
exact length of the received message,
mpi.get.count is used to
find its exact length.
mpi.get.count must be called
immediately after calling
mpi.recv otherwise the status may be
the length of a received message.