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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  match-ing matching
       ing 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++ functions 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)