.. _mpi_iprobe:


MPI_Iprobe
==========

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:ref:`MPI_Iprobe` |mdash| Nonblocking test for a message.

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.. include:: ./bindings/mpi_iprobe.rst

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 <https://www.mpi-forum.org/docs/>`_ 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
<https://www.mpi-forum.org/docs/>`_, 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`