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MPI_Probe.3.rst

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MPI_Probe

:ref:`MPI_Probe` |mdash| 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

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_PROBE(SOURCE, TAG, COMM, STATUS, IERROR)
     INTEGER SOURCE, TAG, COMM, STATUS(MPI_STATUS_SIZE), IERROR

Fortran 2008 Syntax

USE mpi_f08
MPI_Probe(source, tag, comm, status, ierror)
     INTEGER, INTENT(IN) :: source, tag
     TYPE(MPI_Comm), INTENT(IN) :: comm
     TYPE(MPI_Status) :: status
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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 :ref:`MPI_Probe` and :ref:`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.

:ref:`MPI_Probe` behaves like :ref:`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 :ref:`MPI_Probe` and :ref:`MPI_Iprobe` guarantee progress: If a call to :ref:`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 :ref:`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 :ref:`MPI_Iprobe` and a matching message has been issued, then the call to :ref:`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 == 0) THEN
   CALL MPI_SEND(i, 1, MPI_INTEGER, 2, 0, comm, ierr)
ELSE IF(rank == 1) THEN
   CALL MPI_SEND(x, 1, MPI_REAL, 2, 0, comm, ierr)
ELSE   ! rank == 2
   DO i=1, 2
      CALL MPI_PROBE(MPI_ANY_SOURCE, 0,
                     comm, status, ierr)
      IF (status(MPI_SOURCE) = 0) THEN
         CALL MPI_RECV(i, 1, MPI_INTEGER, 0, 0, status, ierr)
      ELSE
         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 == 0) THEN
   CALL MPI_SEND(i, 1, MPI_INTEGER, 2, 0, comm, ierr)
ELSE IF(rank == 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
         CALL MPI_RECV(i, 1, MPI_INTEGER, MPI_ANY_SOURCE, &
                       0, status, ierr)
      ELSE
         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 :ref:`MPI_Probe`.

ERRORS

Note that per the "Return Status" section in the "Point-to-Point Communication" chapter in the MPI Standard, MPI errors on messages queried by :ref:`MPI_Probe` do not set the status.MPI_ERROR field in the returned status. The error code is always passed to the back-end error handler and may be passed back to the caller through the return value of :ref:`MPI_Probe` if the back-end error handler returns it. The pre-defined MPI error handler MPI_ERRORS_RETURN exhibits this behavior, for example.

.. seealso::
   * :ref:`MPI_Iprobe`
   * :ref:`MPI_Cancel`