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

       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

Fortran 2008 Syntax

       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

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