One can think of message transfer as consisting of the following three phases.
1. Data is copied out of the send buffer and message is assembled. 2. A message is transfered from sender to receiver. 3. Data is copied from the incoming message and disassembled into a receive buffer.Type matching must be observed at each of these phases. The type of each variable in the sender buffer must match the type specified for that entry by the send operation. The type specified by the send operation must match the type specified by the receive operation. Finally, the type of each variable in the receive buffer must match the type specified for that entry by the receive operation. A program that fails to observe these rules is erroneous.
To define type matching precisely, we need to deal with two issues: matching of types of variables of the host language with types specified in communication operations, and matching of types between sender and receiver.
The types between a send and receive match if
both operations specify identical type names. That is, MPI_INTEGER
matches MPI_INTEGER, MPI_REAL matches MPI_REAL,
and so on. The one exception to this rule is that
the type MPI_PACKED can match
any other type (Section 
).
The type of a variable matches the type specified in the
communication operation
if the datatype name used by that operation corresponds
to the basic type of the host program variable. For example, an entry with type
name MPI_INTEGER matches a Fortran variable of type INTEGER.
Tables showing this correspondence for Fortran and C appear in
Section .
There are two exceptions to this rule: an entry with type name
MPI_BYTE or MPI_PACKED can be used to match
any byte of storage (on a byte-addressable machine),
MPI_PACKEDMPI_BYTE
irrespective of the datatype of the variable that contains this byte.
The type MPI_BYTE allows one to transfer the binary value of a byte in
memory unchanged.
The type MPI_PACKED is used to send data that has been
explicitly packed with calls to MPI_PACK, or receive data that will
be explicitly unpacked with calls to
MPI_UNPACK (Section ).
The following examples illustrate type matching.
Example 2.3 Sender and receiver specify matching types. CALL MPI_COMM_RANK(comm, rank, ierr) IF (rank.EQ.0) THEN CALL MPI_SEND(a(1), 10, MPI_REAL, 1, tag, comm, ierr) ELSE IF (rank.EQ.1) THEN CALL MPI_RECV(b(1), 15, MPI_REAL, 0, tag, comm, status, ierr) END IF