MPI_Probe(3)                       Open MPI                       MPI_Probe(3)



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++
       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




1.4.1                            Jan 14, 2010                     MPI_Probe(3)
