MPI_Probe(3OpenMPI)													       MPI_Probe(3OpenMPI)

NAME

       MPI_Probe - Blocking test for a message.

SYNTAX

C Syntax
       #include <mpi.h>
       int MPI_Probe(int source, int tag, MPI_Comm comm, MPI_Status *status)

Fortran Syntax
       INCLUDE 'mpif.h'
       MPI_PROBE(SOURCE, TAG, COMM, STATUS, IERROR)
	    INTEGER   SOURCE, TAG, COMM, STATUS(MPI_STATUS_SIZE), IERROR

C++ Syntax
       #include <mpi.h>
       void Comm::Probe(int source, int tag, Status& status) const

       void Comm::Probe(int source, int tag) const

INPUT PARAMETERS

       source	 Source rank or MPI_ANY_SOURCE (integer).

       tag	 Tag value or MPI_ANY_TAG (integer).

       comm	 Communicator (handle).

OUTPUT PARAMETERS

       status	 Status object (status).

       IERROR	 Fortran only: Error status (integer).

DESCRIPTION

       The  MPI_Probe  and MPI_Iprobe operations allow checking of incoming messages, without actual receipt of them. The user can then decide how
       to receive them, based on the information returned by the probe in the status variable. For example, the user may allocate memory  for  the
       receive buffer, according to the length of the probed message.

       MPI_Probe behaves like MPI_Iprobe except that it is a blocking call that returns only after a matching message has been found.

       If your application does not need to examine the status field, you can save resources by using the predefined constant MPI_STATUS_IGNORE as
       a special value for the status argument.

       The semantics of MPI_Probe and MPI_Iprobe guarantee progress: If a call to MPI_Probe has been issued by a process, and a send that  matches
       the  probe has been initiated by some process, then the call to MPI_Probe will return, unless the message is received by another concurrent
       receive operation (that is executed by another thread at the probing process). Similarly, if a process busy waits  with	MPI_Iprobe  and  a
       matching  message has been issued, then the call to MPI_Iprobe will eventually return flag = true unless the message is received by another
       concurrent receive operation.

       Example 1: Use blocking probe to wait for an incoming message.

       CALL MPI_COMM_RANK(comm, rank, ierr)
	      IF (rank.EQ.0) THEN
		   CALL MPI_SEND(i, 1, MPI_INTEGER, 2, 0, comm, ierr)
	      ELSE IF(rank.EQ.1) THEN
		   CALL MPI_SEND(x, 1, MPI_REAL, 2, 0, comm, ierr)
	      ELSE   ! rank.EQ.2
		  DO i=1, 2
		     CALL MPI_PROBE(MPI_ANY_SOURCE, 0,
				     comm, status, ierr)
		     IF (status(MPI_SOURCE) = 0) THEN
       100		  CALL MPI_RECV(i, 1, MPI_INTEGER, 0, 0, status, ierr)
		     ELSE
       200		  CALL MPI_RECV(x, 1, MPI_REAL, 1, 0, status, ierr)
		     END IF
		  END DO
	      END IF

       Each message is received with the right type.

       Example 2: A program similar to the previous example, but with a problem.

       CALL MPI_COMM_RANK(comm, rank, ierr)
	      IF (rank.EQ.0) THEN
		   CALL MPI_SEND(i, 1, MPI_INTEGER, 2, 0, comm, ierr)
	      ELSE IF(rank.EQ.1) THEN
		   CALL MPI_SEND(x, 1, MPI_REAL, 2, 0, comm, ierr)
	      ELSE
		  DO i=1, 2
		     CALL MPI_PROBE(MPI_ANY_SOURCE, 0,
				     comm, status, ierr)
		     IF (status(MPI_SOURCE) = 0) THEN
       100		  CALL MPI_RECV(i, 1, MPI_INTEGER, MPI_ANY_SOURCE,
					0, status, ierr)
		     ELSE
       200		  CALL MPI_RECV(x, 1, MPI_REAL, MPI_ANY_SOURCE,
					0, status, ierr)
		     END IF
		  END DO
	      END IF

       We slightly modified Example 2, using MPI_ANY_SOURCE as the source argument in the two receive calls in statements labeled 100 and 200. The
       program	is  now  incorrect:  The receive operation may receive a message that is distinct from the message probed by the preceding call to
       MPI_Probe.

ERRORS

       Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument. C++ func-
       tions  do not return errors. If the default error handler is set to MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism
       will be used to throw an MPI:Exception object.

       Before the error value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job, except  for
       I/O  function  errors. The error handler may be changed with MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN may be
       used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error.

SEE ALSO

       MPI_Iprobe
       MPI_Cancel

Open MPI 1.2							  September 2006					       MPI_Probe(3OpenMPI)