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man MPI_Send

The function used to send data is MPI_Send.

It takes 6 parameters and the first three identify where the data to
send is stored, how many elements there are to send, and which types
they are.

The destination is identified by the fourth parameter indicating the
rank of the destination and the sixth parameter indicates which
communicator to use.

The fifth parameter is a tag that is used to sort out what type of
message it is. You can think of it as a subject header for an email.

man MPI_Recv

The receiver calls MPI_Recv.

The first three parameters indicate where to put the data once it is
received, how much data can be received at most, and the type of the
data to receive.

The fourth parameter indicates who is sending the message and the
sixth parameter indicated in which communicator the message is using.

The receiver indicates it is waiting on a message of the tag
identified in the fifth parameter.

The last parameter is a pointer to an MPI_Status structure that will
store metadata about the communication.