.. _mpi_test:


MPI_Test
========

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:ref:`MPI_Test` |mdash| Tests for the completion of a specific send or receive.

.. The following file was automatically generated
.. include:: ./bindings/mpi_test.rst

INPUT PARAMETER
---------------
* ``request``: Communication request (handle).

OUTPUT PARAMETERS
-----------------
* ``flag``: True if operation completed (logical).
* ``status``: Status object (status).
* ``ierror``: Fortran only: Error status (integer).

DESCRIPTION
-----------

A call to :ref:`MPI_Test` returns flag = true if the operation identified by
request is complete. In such a case, the status object is set to contain
information on the completed operation; if the communication object was
created by a nonblocking send or receive, then it is deallocated and the
request handle is set to ``MPI_REQUEST_NULL``. The call returns flag =
false, otherwise. In this case, the value of the status object is
undefined. :ref:`MPI_Test` is a local operation.

The return status object for a receive operation carries information
that can be 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/>`_. The status object for a send
operation carries information that can be accessed by a call to
:ref:`MPI_Test_cancelled` (see the "Probe and Cancel" section of the
`MPI Standard <https://www.mpi-forum.org/docs/>`_).

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.

One is allowed to call :ref:`MPI_Test` with a null or inactive *request*
argument. In such a case the operation returns with *flag* = true and
empty *status*.

The functions :ref:`MPI_Wait` and :ref:`MPI_Test` can be used to complete both sends
and receives.


NOTES
-----

The use of the nonblocking :ref:`MPI_Test` call allows the user to schedule
alternative activities within a single thread of execution. An
event-driven thread scheduler can be emulated with periodic calls to
:ref:`MPI_Test`.


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 requests passed to
:ref:`MPI_TEST` 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_TEST` 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_Comm_set_errhandler`
   * :ref:`MPI_File_set_errhandler`
   * :ref:`MPI_Testall`
   * :ref:`MPI_Testany`
   * :ref:`MPI_Testsome`
   * :ref:`MPI_Wait`
   * :ref:`MPI_Waitall`
   * :ref:`MPI_Waitany`
   * :ref:`MPI_Waitsome`
   * :ref:`MPI_Win_set_errhandler`