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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 mes-
       sages, 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  prob-
       lem.

       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 mes-
       sage 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(3), MPI_Cancel(3)




Open MPI 1.2                    September 2006             MPI_Probe(3OpenMPI)

Mac OS X 10.6 - Generated Thu Sep 17 20:13:12 CDT 2009
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