Blocking Send and Receive operations

void Comm.send(Object buf, int offset, int count,
               Datatype datatype, int dest, int tag)

buf	send buffer array
offset	initial offset in send buffer
count	number of items to send
datatype	datatype of each item in send buffer
dest	rank of destination
tag	message tag

Blocking send operation. Java binding of the MPI operation MPI_SEND. The data part of the message consists of a sequence of count values, each of the type indicated by datatype. The actual argument associated with buf must be an array. The value offset is a subscript in this array, defining the position of the first item of the message.

The elements of buf may have primitive type or class type. If the elements are objects, they must be serializable objects. If the datatype argument represents an MPI basic type, its value must agree with the element type of buf: the basic MPI datatypes supported, and their correspondence to Java types, are as follows

MPI datatype	Java datatype
MPI.BYTE	byte
MPI.CHAR	char
MPI.SHORT	short
MPI.BOOLEAN	boolean
MPI.INT	int
MPI.LONG	long
MPI.FLOAT	float
MPI.DOUBLE	double
MPI.OBJECT	Object

If the datatype argument represents an MPI derived type, its base type must agree with the element type of buf (see section A.3.12).

• Rationale. The datatype argument is not redundant in Java, because this proposal includes support for MPI derived types. If it was decided to remove derived types from the API, datatype arguments could be removed from various functions, and Java runtime inquiries could be used internally to extract the element type of the buffer, or methods like send could be overloaded to accept buffers with elements of the 9 basic types. (The disadvantage of the latter approach is that it leads to a large proliferation in the number of methods. Historically MPI has eschewed this kind of overloading. Java at least provides runtime mechanisms for checking type correctness, so the extra security provided by overloading is probably not an essential requirement.)(End of rationale.)

If a data type has MPI.OBJECT as its base type, the objects in the buffer will be transparently serialized and unserialized inside the communication operations.

• Advice to implementors. While straightforward in principle, this extension involves several complications for JNI-based wrapper implementations. The size of the physical receive buffer cannot be computed in advance, so it may be necessary to split the physical message into a header message containing buffer size, followed by the data. This notably complicates wrappers for communication methods based on Request classes. Also, the wrapper has to be prepared to interpret derived types with MPI.OBJECT as the base type, because of course the underlying C MPI cannot do this.(End of advice to implementors.)

Status Comm.recv(Object buf, int offset, int count,
                 Datatype datatype, int source, int tag)

buf	receive buffer array
offset	initial offset in receive buffer
count	number of items in receive buffer
datatype	datatype of each item in receive buffer
source	rank of source
tag	message tag
returns:	status object

Blocking receive operation. Java binding of the MPI operation MPI_RECV. The actual argument associated with buf must be an array. The value offset is a subscript in this array, defining the position into which the first item of the incoming message will be copied.

The elements of buf may have primitive type or class type. If the datatype argument represents an MPI basic type, its value must agree with the element type of buf; if datatype represents an MPI derived type, its base type must agree with the element type of buf (see section A.3.12).

The MPI constants MPI_ANY_SOURCE and MPI_ANY_TAG are available as MPI.ANY_SOURCE and MPI.ANY_TAG.

The following methods can be used to interrogate the return status of a receive operation.

int Status.getSource()

returns:

source of the received message.

int Status.getTag()

returns:

tag of the received message.

• Rationale. Following common object-oriented practices, source and tag values are returned by inquiry functions getSource and getTag, rather than through publicly accessible fields in the Status class.(End of rationale.)

int Status.getCount(Datatype datatype)

datatype	datatype of each item in receive buffer
returns:	number of received entries

Java binding of the MPI operation MPI_GET_COUNT.