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Extracting flags from the wrapper compilers
===========================================
If you cannot use the wrapper compilers for some reason, there are
multiple supported ways to extract the compiler/linker flags that you
will need.
Using the ``--showme`` option
-----------------------------
The wrapper compilers all support a ``--showme`` command line option
that will show what commands would have been invoked.
.. code-block:: sh
# Show the flags necessary to compile MPI C applications
shell$ mpicc --showme:compile
# Show the flags necessary to link MPI C applications
shell$ mpicc --showme:link
# Show all the flags necessary to build MPI C applications
shell$ mpicc --showme
.. note:: If you pass ``--showme`` *and additional command line
parameters* to the wrapper compiler, be sure to *also* pass in a
filename. Otherwise, the ``--showme`` functionality will not
display output as expected.
.. warning:: It is almost never a good idea to hard-code these results
in a ``Makefile`` (or other build system). It is almost always
best to run (for example) ``mpicc --showme:compile`` in a dynamic
fashion to find out what you need. For example, GNU Make allows
running commands and assigning their results to variables:
.. code-block:: make
MPI_COMPILE_FLAGS = $(shell mpicc --showme:compile)
MPI_LINK_FLAGS = $(shell mpicc --showme:link)
my_app: my_app.c
$(CC) $(MPI_COMPILE_FLAGS) my_app.c $(MPI_LINK_FLAGS) -o my_app
Using ``pkg-config``
--------------------
Alternatively, Open MPI also installs ``pkg-config(1)`` configuration
files under ``$libdir/pkgconfig``. If ``pkg-config`` is configured to
find these files (e.g., if you add ``$libdir/pkgconfig`` |mdash| which
is usually ``$prefix/lib/pkgconfig`` |mdash| to the
``PKG_CONFIG_PATH`` environment variable), then compiling / linking
Open MPI programs can be performed like this:
.. code-block:: sh
shell$ export PKG_CONFIG_PATH=/opt/openmpi/lib/pkgconfig
shell$ gcc hello_world_mpi.c -o hello_world_mpi -g \
`pkg-config ompi-c --cflags --libs`
shell$
Open MPI supplies multiple ``pkg-config`` configuration files; one for
each different wrapper compiler (language):
* ``ompi``: Synonym for ``ompi-c``; Open MPI applications using the C
MPI bindings
* ``ompi-c``: Open MPI applications using the C MPI bindings
* ``ompi-cxx``: Open MPI applications using the C MPI bindings
* ``ompi-fort``: Open MPI applications using the Fortran MPI bindings
.. note:: Open MPI's ``pkg-config`` files *work properly*, but they
probably aren't *technically correct*.
Specifically: Open MPI will list all of its dependent
libraries that are necessary to link an MPI application,
even if a given dependency has a ``.pc`` file and should
therefore be listed as a ``Requires`` and/or
``Requires.private`` in Open MPI's ``.pc`` files.
For example, Open MPI lists ``-lpmix`` in both ``Libs`` and
``Libs.private``. But since PMIx provides its own
``pmix.pc`` file, it would be more correct for Open MPI to
*not* list ``-lpmix`` in ``Libs`` / ``Libs.private``, and
instead include:
.. code-block::
Requires: pmix
Requires.private: pmix
The end result is likely immaterial, but we document this
just in case it ever becomes an issue someday.
Using ``ompi_info``
-------------------
This method is not directly suitable for getting all the compiler /
linker flags needed to compile MPI applications because it does not
include the relevant flags to find Open MPI's headers and libraries.
But it does show a breakdown of all other flags.
.. code-block::
shell$ ompi_info --all | grep -i wrapper
Wrapper extra CFLAGS:
Wrapper extra CXXFLAGS:
Wrapper extra FFLAGS:
Wrapper extra FCFLAGS:
Wrapper extra LDFLAGS:
Wrapper extra LIBS: -lutil -lnsl -ldl -Wl,--export-dynamic -lm
This installation is *only* adding options in the ``xLIBS`` areas of the
wrapper compilers; all other values are blank (remember: the ``-I``'s
and ``-L``'s are implicit).
Note that the ``--parsable`` option can be used to obtain
machine-parsable versions of this output. For example:
.. code-block::
shell$ ompi_info --all --parsable | grep wrapper:extra
option:wrapper:extra_cflags:
option:wrapper:extra_cxxflags:
option:wrapper:extra_fflags:
option:wrapper:extra_fcflags:
option:wrapper:extra_ldflags:
option:wrapper:extra_libs:-lutil -lnsl -ldl -Wl,--export-dynamic -lm
|
