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<li class="toctree-l3"><a class="reference internal" href="#using-the-showme-option">9.3.1. Using the <code class="docutils literal notranslate"><span class="pre">--showme</span></code> option</a></li>
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</style><div class="section" id="extracting-flags-from-the-wrapper-compilers">
<h1><span class="section-number">9.3. </span>Extracting flags from the wrapper compilers<a class="headerlink" href="#extracting-flags-from-the-wrapper-compilers" title="Permalink to this heading"></a></h1>
<p>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.</p>
<div class="section" id="using-the-showme-option">
<h2><span class="section-number">9.3.1. </span>Using the <code class="docutils literal notranslate"><span class="pre">--showme</span></code> option<a class="headerlink" href="#using-the-showme-option" title="Permalink to this heading"></a></h2>
<p>The wrapper compilers all support a <code class="docutils literal notranslate"><span class="pre">--showme</span></code> command line option
that will show what commands would have been invoked.</p>
<div class="highlight-sh notranslate"><div class="highlight"><pre><span></span><span class="c1"># Show the flags necessary to compile MPI C applications</span>
shell$<span class="w"> </span>mpicc<span class="w"> </span>--showme:compile
<span class="c1"># Show the flags necessary to link MPI C applications</span>
shell$<span class="w"> </span>mpicc<span class="w"> </span>--showme:link
<span class="c1"># Show all the flags necessary to build MPI C applications</span>
shell$<span class="w"> </span>mpicc<span class="w"> </span>--showme
</pre></div>
</div>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>If you pass <code class="docutils literal notranslate"><span class="pre">--showme</span></code> <em>and additional command line
parameters</em> to the wrapper compiler, be sure to <em>also</em> pass in a
filename. Otherwise, the <code class="docutils literal notranslate"><span class="pre">--showme</span></code> functionality will not
display output as expected.</p>
</div>
<div class="admonition warning">
<p class="admonition-title">Warning</p>
<p>It is almost never a good idea to hard-code these results
in a <code class="docutils literal notranslate"><span class="pre">Makefile</span></code> (or other build system). It is almost always
best to run (for example) <code class="docutils literal notranslate"><span class="pre">mpicc</span> <span class="pre">--showme:compile</span></code> in a dynamic
fashion to find out what you need. For example, GNU Make allows
running commands and assigning their results to variables:</p>
<div class="highlight-make notranslate"><div class="highlight"><pre><span></span><span class="nv">MPI_COMPILE_FLAGS</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">$(</span>shell<span class="w"> </span>mpicc<span class="w"> </span>--showme:compile<span class="k">)</span>
<span class="nv">MPI_LINK_FLAGS</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">$(</span>shell<span class="w"> </span>mpicc<span class="w"> </span>--showme:link<span class="k">)</span>
<span class="nf">my_app</span><span class="o">:</span><span class="w"> </span><span class="n">my_app</span>.<span class="n">c</span>
<span class="w"> </span><span class="k">$(</span>CC<span class="k">)</span><span class="w"> </span><span class="k">$(</span>MPI_COMPILE_FLAGS<span class="k">)</span><span class="w"> </span>my_app.c<span class="w"> </span><span class="k">$(</span>MPI_LINK_FLAGS<span class="k">)</span><span class="w"> </span>-o<span class="w"> </span>my_app
</pre></div>
</div>
</div>
</div>
<div class="section" id="using-pkg-config">
<h2><span class="section-number">9.3.2. </span>Using <code class="docutils literal notranslate"><span class="pre">pkg-config</span></code><a class="headerlink" href="#using-pkg-config" title="Permalink to this heading"></a></h2>
<p>Alternatively, Open MPI also installs <code class="docutils literal notranslate"><span class="pre">pkg-config(1)</span></code> configuration
files under <code class="docutils literal notranslate"><span class="pre">$libdir/pkgconfig</span></code>. If <code class="docutils literal notranslate"><span class="pre">pkg-config</span></code> is configured to
find these files (e.g., if you add <code class="docutils literal notranslate"><span class="pre">$libdir/pkgconfig</span></code> — which
is usually <code class="docutils literal notranslate"><span class="pre">$prefix/lib/pkgconfig</span></code> — to the
<code class="docutils literal notranslate"><span class="pre">PKG_CONFIG_PATH</span></code> environment variable), then compiling / linking
Open MPI programs can be performed like this:</p>
<div class="highlight-sh notranslate"><div class="highlight"><pre><span></span>shell$<span class="w"> </span><span class="nb">export</span><span class="w"> </span><span class="nv">PKG_CONFIG_PATH</span><span class="o">=</span>/opt/openmpi/lib/pkgconfig
shell$<span class="w"> </span>gcc<span class="w"> </span>hello_world_mpi.c<span class="w"> </span>-o<span class="w"> </span>hello_world_mpi<span class="w"> </span>-g<span class="w"> </span><span class="se">\</span>
<span class="w"> </span><span class="sb">`</span>pkg-config<span class="w"> </span>ompi-c<span class="w"> </span>--cflags<span class="w"> </span>--libs<span class="sb">`</span>
shell$
</pre></div>
</div>
<p>Open MPI supplies multiple <code class="docutils literal notranslate"><span class="pre">pkg-config</span></code> configuration files; one for
each different wrapper compiler (language):</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">ompi</span></code>: Synonym for <code class="docutils literal notranslate"><span class="pre">ompi-c</span></code>; Open MPI applications using the C
MPI bindings</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">ompi-c</span></code>: Open MPI applications using the C MPI bindings</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">ompi-cxx</span></code>: Open MPI applications using the C MPI bindings</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">ompi-fort</span></code>: Open MPI applications using the Fortran MPI bindings</p></li>
</ul>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>Open MPI’s <code class="docutils literal notranslate"><span class="pre">pkg-config</span></code> files <em>work properly</em>, but they
probably aren’t <em>technically correct</em>.</p>
<p>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 <code class="docutils literal notranslate"><span class="pre">.pc</span></code> file and should
therefore be listed as a <code class="docutils literal notranslate"><span class="pre">Requires</span></code> and/or
<code class="docutils literal notranslate"><span class="pre">Requires.private</span></code> in Open MPI’s <code class="docutils literal notranslate"><span class="pre">.pc</span></code> files.</p>
<p>For example, Open MPI lists <code class="docutils literal notranslate"><span class="pre">-lpmix</span></code> in both <code class="docutils literal notranslate"><span class="pre">Libs</span></code> and
<code class="docutils literal notranslate"><span class="pre">Libs.private</span></code>. But since PMIx provides its own
<code class="docutils literal notranslate"><span class="pre">pmix.pc</span></code> file, it would be more correct for Open MPI to
<em>not</em> list <code class="docutils literal notranslate"><span class="pre">-lpmix</span></code> in <code class="docutils literal notranslate"><span class="pre">Libs</span></code> / <code class="docutils literal notranslate"><span class="pre">Libs.private</span></code>, and
instead include:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>Requires: pmix
Requires.private: pmix
</pre></div>
</div>
<p>The end result is likely immaterial, but we document this
just in case it ever becomes an issue someday.</p>
</div>
</div>
<div class="section" id="using-ompi-info">
<h2><span class="section-number">9.3.3. </span>Using <code class="docutils literal notranslate"><span class="pre">ompi_info</span></code><a class="headerlink" href="#using-ompi-info" title="Permalink to this heading"></a></h2>
<p>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.</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>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
</pre></div>
</div>
<p>This installation is <em>only</em> adding options in the <code class="docutils literal notranslate"><span class="pre">xLIBS</span></code> areas of the
wrapper compilers; all other values are blank (remember: the <code class="docutils literal notranslate"><span class="pre">-I</span></code>’s
and <code class="docutils literal notranslate"><span class="pre">-L</span></code>’s are implicit).</p>
<p>Note that the <code class="docutils literal notranslate"><span class="pre">--parsable</span></code> option can be used to obtain
machine-parsable versions of this output. For example:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>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
</pre></div>
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