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Merge pull request #10624 from jsquyres/pr/docs-rpath-runpath-and-all-that-stuff
docs: update runpath/rpath docs
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‎docs/installing-open-mpi/configure-cli-options/index.rst

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mpi
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oshmem
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misc
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rpath-and-runpath

‎docs/installing-open-mpi/configure-cli-options/installation.rst

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.. This file is included by building-open-mpi.rst
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Installation options
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^^^^^^^^^^^^^^^^^^^^
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Be sure to read the description of ``--without-memory-manager``,
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below; it may have some effect on ``--enable-static``.
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* ``--disable-wrapper-rpath``:
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By default, the wrapper compilers (e.g., ``mpicc``) will enable
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"rpath" support in generated executables on systems that support it.
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That is, they will include a file reference to the location of Open
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MPI's libraries in the application executable itself. This means
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that the user does not have to set ``LD_LIBRARY_PATH`` to find Open
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MPI's libraries (e.g., if they are installed in a location that the
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run-time linker does not search by default).
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On systems that utilize the GNU ``ld`` linker, recent enough versions
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will actually utilize "runpath" functionality, not "rpath". There
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is an important difference between the two:
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#. "rpath": the location of the Open MPI libraries is hard-coded into
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the MPI/OpenSHMEM application and cannot be overridden at
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run-time.
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#. "runpath": the location of the Open MPI libraries is hard-coded into
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the MPI/OpenSHMEM application, but can be overridden at run-time
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by setting the ``LD_LIBRARY_PATH`` environment variable.
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* ``--disable-wrapper-runpath`` / ``--disable-wrapper-rpath``: By
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default, the wrapper compilers (e.g., ``mpicc``) will enable
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"runpath" or "rpath" support in generated executables on systems
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that support it. That is, they will include a filesystem path
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reference to the location of Open MPI's libraries in the application
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executable itself. This means that the user does not have to set
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``LD_LIBRARY_PATH`` to find Open MPI's libraries, which can be
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helpful if they are installed in a location that the run-time linker
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does not search by default.
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.. note:: By default, the wrapper compilers prefer "runpath"
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behavior over "rpath" behavior.
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* Using ``--disable-wrapper-runpath`` alters this
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preference: "runpath" behavior will never be used.
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Instead, the wrapper compilers will use "rpath" behavior
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if it is supported.
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* Using ``--disable-wrapper-rpath`` will disable *both*
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"runpath" and "rpath" behavior in the wrapper compilers.
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.. important:: Note that the ``--disable-wrapper-runpath`` and
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``--disable-wrapper-rpath`` CLI options *only* affect
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the flags that the wrapper compilers use when
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building MPI/OpenSHMEM applications. These options
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do not affect how Open MPI or OpenSHMEM are built (to
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include the wrapper compilers themselves).
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See the :ref:`Linker "rpath" and "runpath"
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functionality
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<building-ompi-cli-options-rpath-and-runpath-label>`
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section for details on how "rpath" and "runpath"
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affect the building and linking of Open MPI itself.
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When either of "runpath" or "rpath" behaviors are enabled, the
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wrapper compilers will hard-code the filesystem path location of the
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Open MPI libraries into the Open MPI/OpenSHMEM application. The
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most notable differences between "runpath" and "rpath" behavior are:
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* runpath
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#. The run-time linker first searches the paths in the
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``LD_LIBRARY_PATH`` environment variable for the relevant Open
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MPI/OpenSHMEM libraries.
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#. If not found there, the run-time linker falls back to checking
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the hard-coded location for the relevant Open MPI/OpenSHMEM
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libraries.
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* rpath
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#. The run-time linker first checks the hard-coded location for
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the relevant Open MPI/OpenSHMEM libraries.
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#. If not found there, the run-time linker falls back to
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searching the paths in the ``LD_LIBRARY_PATH`` environment
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variable for the relevant Open MPI/OpenSHMEM libraries.
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.. warning:: There are other, subtle differences between "runpath"
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and "rpath" which are out of scope for this
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documentation. You may wish to consult other sources
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for more information.
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For example, a decent set of explanations can be found
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in the slides for a Linux course entitled "Building and
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Using Shared Libraries on Linux // `Shared Libraries:
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The Dynamic Linker
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<https://man7.org/training/download/shlib_dynlinker_slides.pdf>`_".
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For example, consider that you install Open MPI vA.B.0 and
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compile/link your MPI/OpenSHMEM application against it. Later, you
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install Open MPI vA.B.1 to a different installation prefix (e.g.,
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``/opt/openmpi/A.B.1`` vs. ``/opt/openmpi/A.B.0``), and you leave the old
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installation intact.
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``/opt/openmpi/A.B.1`` vs. ``/opt/openmpi/A.B.0``), and you leave
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the old installation intact.
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In the runpath case, you can set the ``LD_LIBRARY_PATH`` environment
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variable to point to the A.B.1 installation, and then your MPI
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application will use those libraries, since the runtime will search
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the paths in ``LD_LIBRARY_PATH`` first.
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In the rpath case, your MPI application will always use the
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libraries from your A.B.0 installation. In the runpath case, you
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can set the ``LD_LIBRARY_PATH`` environment variable to point to the
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A.B.1 installation, and then your MPI application will use those
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libraries.
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In the rpath case, since the run-time linker searches the
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``/opt/openmpi/A.B.0`` location that is hard-coded in your MPI
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application first, your application will use the libraries from your
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A.B.0 installation (regardless of the value of the
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``LD_LIBRARY_PATH`` environment variable).
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Note that in both cases, however, if you remove the original A.B.0
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installation and set ``LD_LIBRARY_PATH`` to point to the A.B.1
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installation, your application will use the A.B.1 libraries.
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This rpath/runpath behavior can be disabled via
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As noted above, both runpath/rpath behaviors can be disabled via
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``--disable-wrapper-rpath``.
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If you would like to keep the rpath option, but not enable runpath
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a different ``configure`` option is avalabile
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``--disable-wrapper-runpath``.
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.. note:: You can also :ref:`customize the compiler/linker flags
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that are used by the wrapper compilers
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<label-customizing-wrapper-compiler>` to build Open
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MPI/OpenSHMEM applications.
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* ``--enable-dlopen``: Enable Open MPI to load components as
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standalone Dynamic Shared Objects (DSOs) at run-time. This option

‎docs/installing-open-mpi/configure-cli-options/rpath-and-runpath.rst

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.. _building-ompi-cli-options-rpath-and-runpath-label:
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Linker "rpath" and "runpath" functionality
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Open MPI and OpenSHMEM |ompi_ver| are both composed of multiple
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libraries that depend on each other.
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When built from official distribution tarballs (from `the official
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Open MPI download site <https://www.open-mpi.org/software/ompi/>`_),
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Open MPI and OpenSHMEM are built with the following versions of the
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GNU Autotools:
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* Autoconf |autoconf_min_version|
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* Automake |automake_min_version|
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* Libtool |libtool_min_version|
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This set of GNU Autotools invokes the ``libtool`` executable to build
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Open MPI's libraries and executables with the `-rpath CLI option
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<https://www.gnu.org/software/libtool/manual/html_node/Link-mode.html#Link-mode>`_.
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The behavior of ``libtool -rpath ...`` is, unfortunately, highly
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system-dependent. In conjunction with compiler-, linker-, and other
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system-level settings, the end result may be to effect "rpath"
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behavior, "runpath" behavior, or possibly even neither behavior.
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Although the specific behavior of ``libtool -rpath ..`` is outside the
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scope of this documentation, you can run commands such as ``readelf -d
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...`` to find out with which behavior your Open MPI was built.
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For example:
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.. code::
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shell$ ./configure --prefix=/opt/openmpi/ ...
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...
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shell$ make -j 32 all && make install
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...
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shell$ readelf -d /opt/openmpi/lib/libmpi.so | egrep -i 'rpath|runpath'
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0x000000000000001d (RUNPATH) Library runpath: [/opt/openmpi/lib]
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The above output indicates that ``libmpi.so`` was built with "runpath"
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support, whereas output like this:
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.. code::
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shell$ readelf -d /opt/openmpi/lib/libmpi.so | egrep -i 'rpath|runpath'
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0x000000000000000f (RPATH) Library rpath: [/opt/openmpi/lib]
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indicates that ``libmpi.so`` was built with "rpath" support.
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.. note:: If you want to utilize additional compiler or linker flags
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(such as runpath flags) when building Open MPI/OpenSHMEM,
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you can :ref:`specify these flags on the configure command
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line <install-configure-compilers-and-flags-label>`.
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For example, if invoking ``libtool -rpath ...`` on your
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system actually effects "rpath" behavior, and you wish to
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effect "runpath" behavior, you could set ``LDFLAGS`` when
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invoking ``configure``, like this:
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.. code-block:: sh
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shell$ ./configure LDFLAGS=--enable-new-dtags ...

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