Basics of MPI Programming: Discussion: MPI

CTC Discussion on MPI Basics; Six Basic Calls

1.2.5 Receiving a message

Message passing in MPI-1 requires two-way communication. One-sided communication is one of the features added in MPI-2. Each time one process sends a message, another process must explicitly receive the message. So for every place in your application that you call an MPI send routine, there will be a corresponding place where you call an MPI receive routine. Care must be taken to ensure that the send and receive parameters match. This will be discussed in more detail later in the module.

Like MPI_SEND, MPI_RECV is blocking. This means the call does not return control to your program until all the received data have been stored in the variable(s) you specify in the parameter list. Because of this, you can use the data after the call and be certain it is all there. (There are non-blocking receives where this is not the case.)

C
- int MPI_Recv(void *buf, int count, MPI_Datatype datatype, int source, int tag, MPI_Comm comm, MPI_Status *status)
Fortran
- MPI_RECV(buf, count, datatype, source, tag, comm, status, ierror)
  <type> buf(*)
  integer count, datatype, source, tag, comm, status(MPI_STATUS_SIZE), ierror

The parameters:

buf is the beginning of the buffer where the incoming data are to be stored. For Fortran, this is often the name of an array in your program. For C, it is an address.
count is the number of elements (not bytes) in your receive buffer
datatype is the type of data
source is the rank of the process from which data will be accepted
(This can be a wildcard, by specifying the parameter MPI_ANY_SOURCE.)
tag is an arbitrary number which can be used to distinguish among messages
(This can be a wildcard, by specifying the parameter MPI_ANY_TAG.)
comm is the communicator
status is an array or structure of information that is returned. For example, if you specify a wildcard for source or tag, status will tell you the actual rank or tag for the message received
ierror is a return error code