.. _mpi_iprobe: MPI_Iprobe ========== .. include_body :ref:`MPI_Iprobe` |mdash| Nonblocking test for a message. SYNTAX ------ C Syntax ^^^^^^^^ .. code-block:: c #include int MPI_Iprobe(int source, int tag, MPI_Comm comm, int *flag, MPI_Status *status) Fortran Syntax ^^^^^^^^^^^^^^ .. code-block:: fortran USE MPI ! or the older form: INCLUDE 'mpif.h' MPI_IPROBE(SOURCE, TAG, COMM, FLAG, STATUS, IERROR) LOGICAL FLAG INTEGER SOURCE, TAG, COMM, STATUS(MPI_STATUS_SIZE), IERROR Fortran 2008 Syntax ^^^^^^^^^^^^^^^^^^^ .. code-block:: fortran USE mpi_f08 MPI_Iprobe(source, tag, comm, flag, status, ierror) INTEGER, INTENT(IN) :: source, tag TYPE(MPI_Comm), INTENT(IN) :: comm LOGICAL, INTENT(OUT) :: flag TYPE(MPI_Status) :: status INTEGER, OPTIONAL, INTENT(OUT) :: ierror INPUT PARAMETERS ---------------- * ``source``: Source rank or ``MPI_ANY_SOURCE`` (integer). * ``tag``: Tag value or ``MPI_ANY_TAG`` (integer). * ``comm``: Communicator (handle). OUTPUT PARAMETERS ----------------- * ``flag``: Message-waiting flag (logical). * ``status``: Status object (status). * ``ierror``: Fortran only: Error status (integer). DESCRIPTION ----------- The :ref:`MPI_Probe` and :ref:`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 (basically, the information returned by status). In particular, the user may allocate memory for the receive buffer, according to the length of the probed message. ``MPI_Iprobe(source, tag, comm, flag, status)`` returns *flag* = true if there is a message that can be received and that matches the pattern specified by the arguments source, tag, and comm. The call matches the same message that would have been received by a call to ``MPI_Recv(..., source, tag, comm, status)`` executed at the same point in the program, and returns in status the same value that would have been returned by :ref:`MPI_Recv`. Otherwise, the call returns *flag* = false, and leaves status undefined. If :ref:`MPI_Iprobe` returns *flag* = true, then the content of the status object can be subsequently accessed as described in the "Return Status" subsection of the "Point-to-Point Communication" chapter in the `MPI Standard `_ to find the source, tag, and length of the probed message. A subsequent receive executed with the same context, and the source and tag returned in status by :ref:`MPI_Iprobe` will receive the message that was matched by the probe if no other intervening receive occurs after the probe. If the receiving process is multithreaded, it is the user's responsibility to ensure that the last condition holds. The source argument of :ref:`MPI_Probe` can be ``MPI_ANY_SOURCE``, and the tag argument can be ``MPI_ANY_TAG``, so that one can probe for messages from an arbitrary source and/or with an arbitrary tag. However, a specific communication context must be provided with the comm argument. 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. It is not necessary to receive a message immediately after it has been probed for, and the same message may be probed for several times before it is received. NOTE ---- Multi-threaded application developers should remember that two threads calling :ref:`MPI_Iprobe` may return true for the same message in both threads. ERRORS ------ .. include:: ./ERRORS.rst Note that per the "Return Status" section in the "Point-to-Point Communication" chapter in the `MPI Standard `_, MPI errors on messages queried by :ref:`MPI_Iprobe` do not set the ``status.MPI_ERROR`` field in the returned *status*. The error code is always passed to the back-end error handler and may be passed back to the caller through the return value of :ref:`MPI_Iprobe` if the back-end error handler returns it. The pre-defined MPI error handler ``MPI_ERRORS_RETURN`` exhibits this behavior, for example. .. seealso:: * :ref:`MPI_Probe` * :ref:`MPI_Cancel`