:ref:`MPI_Probe` |mdash| Blocking test for a message.
#include <mpi.h>
int MPI_Probe(int source, int tag, MPI_Comm comm, MPI_Status *status)
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
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
source
: Source rank orMPI_ANY_SOURCE
(integer).tag
: Tag value orMPI_ANY_TAG
(integer).comm
: Communicator (handle).
status
: Status object (status).ierror
: Fortran only: Error status (integer).
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`.
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`