Point-to-Point Communication
[End-User Documentation]

Collaboration diagram for Point-to-Point Communication:

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Detailed Description

Sending and receiving of messages by processes is the basic MPI communication mechanism.

The basic point-to-point communication operations are send and receive.

MOC_Send uses either buffered or synchronous send depending on the message-size (Short messages are transmitted using buffered send and long messages are transmitted using synchronous send).

MOC_Recv receives all messages irrespective wether buffered or synchronous communication is used.

Note:

Because there are separate message-queues for each remote process and no distinct global ordering of messages, there may be some situation where a message from a process may never be received. This may occur when MOC_Recv, MOC_Probe and MOC_IProbe are used with MOC_ANY_SOURCE, because then there is the possibility that new messages are received before a specific message from a process with a higher rank than the other senders. This may also occur with lower possibility when using MOC_ANY_TAG between messages from the same sender but with other tags.

Modules
	Return Status
	Receiving operations return a status-object holding information about the message.
	Datatypes
	Datatypes supported by MOC.
	Synchronous Communication
	Direct blocking sending of messages without any intermediate buffer.
	Buffered Communication
	Blocking sending of messages with the use of an intermediate buffer.
	Send-Receive
	The send-receive operations combine in one call the sending of a message to one destination and the receiving of another message, from another process.
	Non-Blocking Communication
	Non-blocking sending and receiving messages.
Defines
#define	MOC_SEND_TRESHOLD   4096
	Indicates, at which message-size MOC_Send switches to synchronous send.
Functions
int	MOC_Send (void *buf, int count, MOC_Datatype datatype, int dest, int tag, MOC_Comm comm)
	Performs a blocking standard mode send operation.
int	MOC_Recv (void *buf, int count, MOC_Datatype datatype, int source, int tag, MOC_Comm comm, MOC_Status *status)
	Performs a blocking standard mode receive operation.
int	MOC_Probe (int source, int tag, MOC_Comm comm, MOC_Status *status)
	Waits till a message is received and returns information without actually receiving the message.
int	MOC_Iprobe (int source, int tag, MOC_Comm comm, int *flag, MOC_Status *status)
	Tests if a message is available and returns information without actually receiving the message.

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Function Documentation

int MOC_Iprobe	(	int	source,
		int	tag,
		MOC_Comm	comm,
		int *	flag,
		MOC_Status *	status
	)

Tests if a message is available and returns information without actually receiving the message.

Parameters:

	source	in source of message
	tag	in message-tag
	comm	in communicator
	flag	out boolean value telling if a message is available
	status	out status-object holding information about message

Return values:

	MOC_SUCCESS	No error; MPI routine completed successfully.
	...	error-code

int MOC_Probe	(	int	source,
		int	tag,
		MOC_Comm	comm,
		MOC_Status *	status
	)

Waits till a message is received and returns information without actually receiving the message.

Parameters:

	source	in source of message
	tag	in message-tag
	comm	in communicator
	status	out status-object holding information about message

Return values:

	MOC_SUCCESS	No error; MPI routine completed successfully.
	...	error-code

int MOC_Recv	(	void *	buf,
		int	count,
		MOC_Datatype	datatype,
		int	source,
		int	tag,
		MOC_Comm	comm,
		MOC_Status *	status
	)

Performs a blocking standard mode receive operation.

Note:

This routine blocks until the message is received.

Parameters:

	buf	inout Buffer into which data should be stored
	count	in Number of elements in receive buffer (not equal to actual byte-value)
	datatype	in Datatype of each receive buffer element
	source	in Rank of source
	tag	in Tag of message
	comm	in Communicator of message
	status	inout Object holding status-information (has to be allocated by user)

Return values:

	MOC_SUCCESS	No error; MPI routine completed successfully.
	...	error-code

int MOC_Send	(	void *	buf,
		int	count,
		MOC_Datatype	datatype,
		int	dest,
		int	tag,
		MOC_Comm	comm
	)

Performs a blocking standard mode send operation.

Note:

This routine may block until the message is received.

Parameters:

	buf	in Pointer to buffer where data is stored
	count	in Number of elements to be send (not equal to actual byte-value)
	datatype	in Type of elements to be send
	dest	in Rank of receiving process
	tag	in Tag of message
	comm	in communicator of message

Return values:

	MOC_SUCCESS	No error; MPI routine completed successfully.
	...	error-code