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.. _ompi-features-extentions-label:
Open MPI extensions
===================
Open MPI contains a framework for extending the MPI API that is
available to applications. Each extension is usually a standalone set
of functionality that is distinct from other extensions (similar to
how Open MPI's plugins are usually unrelated to each other). These
extensions provide new functions and/or constants that are available
to MPI applications.
.. warning:: These extensions are neither standard nor portable to
other MPI implementations!
They are a mechanism for the Open MPI developer community to
provide new functionality to users, typically before it becomes
standardized by the MPI Forum.
Available extensions
--------------------
The following extensions are included in this version of Open MPI:
#. ``shortfloat``: Provides MPI datatypes ``MPIX_C_FLOAT16``,
``MPIX_SHORT_FLOAT``, ``MPIX_SHORT_FLOAT``, and
``MPIX_CXX_SHORT_FLOAT_COMPLEX`` if corresponding language types are
available. See ``ompi/mpiext/shortfloat/README.txt`` for details.
#. ``affinity``: Provides the ``OMPI_Affinity_str()`` API, which returns
a string indicating the resources which a process is bound. For
more details, see its man page.
#. ``cuda``: When the library is compiled with CUDA-aware support, it
provides two things. First, a macro
``MPIX_CUDA_AWARE_SUPPORT``. Secondly, the function
``MPIX_Query_cuda_support()`` that can be used to query for support.
#. ``example``: A non-functional extension; its only purpose is to
provide an example for how to create other extensions.
#. ``ftmpi``: An implementation of the User Level Fault Mitigation
(ULFM) proposal. :ref:`See its documentation section <ulfm-label>`
for more details.
Compiling the extensions
------------------------
Open MPI extensions are all enabled by default; they can be disabled
via the ``--disable-mpi-ext`` command line switch.
Since extensions are meant to be used by advanced users only, this
file does not document which extensions are available or what they do.
Look in the ``ompi/mpiext`` directory in a distribution Open MPI
tarball to see the extensions; each subdirectory of that directory
contains an extension. Each has a ``README`` file that describes what
it does.
Using the extensions
--------------------
To reinforce the fact that these extensions are non-standard, you must
include a separate header file after ``<mpi.h>`` to obtain the function
prototypes, constant declarations, etc. For example:
.. code-block:: c
#include <mpi.h>
#if defined(OPEN_MPI) && OPEN_MPI
#include <mpi-ext.h>
#endif
int main() {
MPI_Init(NULL, NULL);
#if defined(OPEN_MPI) && OPEN_MPI
char ompi_bound[OMPI_AFFINITY_STRING_MAX];
char current_binding[OMPI_AFFINITY_STRING_MAX];
char exists[OMPI_AFFINITY_STRING_MAX];
OMPI_Affinity_str(OMPI_AFFINITY_LAYOUT_FMT, ompi_bound,
current_bindings, exists);
#endif
MPI_Finalize();
return 0;
}
Notice that the Open MPI-specific code is surrounded by the ``#if``
statement to ensure that it is only ever compiled by Open MPI.
The Open MPI wrapper compilers (``mpicc`` and friends) should
automatically insert all relevant compiler and linker flags necessary
to use the extensions. No special flags or steps should be necessary
compared to "normal" MPI applications.
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