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