Message Passing

int MPI_Bcast( void *buf, int count, MPI_Datatype datatype,
int srcPid, MPI_Comm comm )

int MPI_Send( void *buf, int count, MPI_Datatype datatype, 
int destPid, int tag, MPI_Comm comm )

int MPI_Recv( void *buf, int count, MPI_Datatype datatype, 
int srcPid, int tag, MPI_Comm comm, MPI_Status *status )

srcPid: The id of the process from whom you want to receive a message. This can be set to MPI_ANY_SOURCE to receive a message from any process within the give communications group.
tag: A code indicating the type of message to be received. This can be set to MPI_ANY_TAG to receive a message with any type of tag associated with it.

int MPI_Reduce( void *sendbuf, void *recvbuf, int count, 
MPI_Datatype datatype, MPI_Op op, int destPid, MPI_Comm comm )

int MPI_Probe( int srcPid, int tag, MPI_Comm comm, MPI_Status *status );

Blocks and waits for a message with the given data tag from the indicated process. The function does not actually receive the message. It can be used in conjunction with MPI_Get_count().

srcPid: the id of the process from which you are waiting for a message.
tag: the tag associated with the type of message for which you are waiting.
comm: the communications group being probed.
status: address of a struct into which information related to the message can be stored.

int MPI_Get_count( MPI_Status *status, MPI_Datatype datatype, int *count )

Determines the number of data items contained in a message just received or the next message to be received. This function is used with either MPI_Recv() or MPI_Probe().

status: the address of the MPI_Status struct filled in by either the MPI_Recv() or MPI_Probe() function.
datatype: the type of data items being received.
count: the address of an integer variable into which the number of items received is stored.

The following code segment illustrates the use of this function

MPI_Status msgStatus;
int numItems;

MPI_Probe( ROOT_PROC, DATA_TAG, MPI_COMM_WORLD, &msgStatus );
MPI_Get_count( &msgStatus, MPI_DOUBLE, &numItems );

Preliminaries

MPI Reference

Message Passing Non-blocking