The selection of a message by a receive operation is governed by selectionmessage selection the value of its message envelope. A message can be received if its envelope matches the source, tag and comm values specified by the receive operation. The receiver may specify a wildcard wildcard value for source (MPI_ANY_SOURCE), MPI_ANY_SOURCE and/or a wildcard value for tag (MPI_ANY_TAG), MPI_ANY_TAG indicating that any source and/or tag are acceptable. One cannot specify a wildcard value for comm.
The argument source, if different from MPI_ANY_SOURCE, sourcemessage source is specified as a rank within the process group associated with the communicator (remote process group, for intercommunicators). The range of valid values for the source argument is {0,...,n-1} U {MPI_ANY_SOURCE}, where n is the number of processes in this group. This range includes the receiver's rank: if comm is an intracommunicator, then a process may receive a message from itself. The range of valid values for the tag argument is {0,...,UB} U {MPI_ANY_TAG}. tagmessage tag