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<span id="install-configure-compilers-and-flags-label"></span><h1><span class="section-number">4.7. </span>Specifying compilers and flags<a class="headerlink" href="#specifying-compilers-and-flags" title="Permalink to this heading"></a></h1>
<p>Changing the compilers that Open MPI uses to build itself uses the
standard GNU Autoconf mechanism of setting special environment variables
either before invoking <code class="docutils literal notranslate"><span class="pre">configure</span></code> or on the <code class="docutils literal notranslate"><span class="pre">configure</span></code> command
line itself.</p>
<p>The following environment variables are recognized by <code class="docutils literal notranslate"><span class="pre">configure</span></code>:</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">CC</span></code>: C compiler to use</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">CFLAGS</span></code>: Compile flags to pass to the C compiler</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">CPPFLAGS</span></code>: Preprocessor flags to pass to the C compiler</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">CXX</span></code>: C++ compiler to use</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">CXXCFLAGS</span></code>: Compile flags to pass to the C++ compiler</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">CXXCPPFLAGS</span></code>: Preprocessor flags to pass to the C++ compiler</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">FC</span></code>: Fortran compiler to use</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">FCFLAGS</span></code>: Compile flags to pass to the Fortran compiler</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">LDFLAGS</span></code>: Linker flags to pass to all compilers</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">LIBS</span></code>: Libraries to pass to all compilers (it is rarely
necessary for users to need to specify additional <code class="docutils literal notranslate"><span class="pre">LIBS</span></code>)</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">PKG_CONFIG</span></code>: Path to the <code class="docutils literal notranslate"><span class="pre">pkg-config</span></code> utility</p></li>
</ul>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>Open MPI v5.0.8 does not contain any C++ code. The only
tests that <code class="docutils literal notranslate"><span class="pre">configure</span></code> runs with the C++ compiler is for
the purposes of determining an appropriate value for <code class="docutils literal notranslate"><span class="pre">CXX</span></code>
to use in the <code class="docutils literal notranslate"><span class="pre">mpic++</span></code> <a class="reference internal" href="../building-apps/quickstart.html#label-quickstart-building-apps"><span class="std std-ref">wrapper compiler</span></a>. The <code class="docutils literal notranslate"><span class="pre">CXXCFLAGS</span></code> and
<code class="docutils literal notranslate"><span class="pre">CXXCPPFLAGS</span></code> values are <em>only</em> used in these
<code class="docutils literal notranslate"><span class="pre">configure</span></code> checks to ensure that the C++ compiler works.</p>
</div>
<p>For example, to build with a specific instance of <code class="docutils literal notranslate"><span class="pre">gcc</span></code>, <code class="docutils literal notranslate"><span class="pre">g++</span></code>,
and <code class="docutils literal notranslate"><span class="pre">gfortran</span></code>:</p>
<div class="highlight-sh notranslate"><div class="highlight"><pre><span></span>shell$<span class="w"> </span>./configure<span class="w"> </span><span class="se">\</span>
<span class="w"> </span><span class="nv">CC</span><span class="o">=</span>/opt/gcc-a.b.c/bin/gcc<span class="w"> </span><span class="se">\</span>
<span class="w"> </span><span class="nv">CXX</span><span class="o">=</span>/opt/gcc-a.b.c/bin/g++<span class="w"> </span><span class="se">\</span>
<span class="w"> </span><span class="nv">FC</span><span class="o">=</span>/opt/gcc-a.b.c/bin/gfortran<span class="w"> </span>...
</pre></div>
</div>
<p>Here’s another example, this time showing building with the Intel
compiler suite:</p>
<div class="highlight-sh notranslate"><div class="highlight"><pre><span></span>shell$<span class="w"> </span>./configure<span class="w"> </span><span class="se">\</span>
<span class="w"> </span><span class="nv">CC</span><span class="o">=</span>icc<span class="w"> </span><span class="se">\</span>
<span class="w"> </span><span class="nv">CXX</span><span class="o">=</span>icpc<span class="w"> </span><span class="se">\</span>
<span class="w"> </span><span class="nv">FC</span><span class="o">=</span>ifort<span class="w"> </span>...
</pre></div>
</div>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>The Open MPI community generally suggests using the above
command line form for setting different compilers (vs. setting
environment variables and then invoking <code class="docutils literal notranslate"><span class="pre">./configure</span></code>). The
above form will save all variables and values in the <code class="docutils literal notranslate"><span class="pre">config.log</span></code>
file, which makes post-mortem analysis easier if problems occur.</p>
</div>
<p>Note that the flags you specify must be compatible across all the
compilers. In particular, flags specified to one language compiler
must generate code that can be compiled and linked against code that
is generated by the other language compilers. For example, on a 64
bit system where the compiler default is to build 32 bit executables:</p>
<div class="highlight-sh notranslate"><div class="highlight"><pre><span></span><span class="c1"># Assuming the GNU compiler suite</span>
shell$<span class="w"> </span>./configure<span class="w"> </span><span class="nv">CFLAGS</span><span class="o">=</span>-m64<span class="w"> </span>...
</pre></div>
</div>
<p>will produce 64 bit C objects, but 32 bit objects for Fortran. These
codes will be incompatible with each other, and Open MPI will not build
successfully. Instead, you must specify building 64 bit objects for
<em>all</em> languages:</p>
<div class="highlight-sh notranslate"><div class="highlight"><pre><span></span><span class="c1"># Assuming the GNU compiler suite</span>
shell$<span class="w"> </span>./configure<span class="w"> </span><span class="nv">CFLAGS</span><span class="o">=</span>-m64<span class="w"> </span><span class="nv">FCFLAGS</span><span class="o">=</span>-m64<span class="w"> </span>...
</pre></div>
</div>
<p>The above command line will pass <code class="docutils literal notranslate"><span class="pre">-m64</span></code> to all the compilers, and
therefore will produce 64 bit objects for all languages.</p>
<div class="admonition warning">
<p class="admonition-title">Warning</p>
<p>Note that setting <code class="docutils literal notranslate"><span class="pre">CFLAGS</span></code> (etc.) does <em>not</em> affect the
flags used by the <a class="reference internal" href="../building-apps/quickstart.html#label-quickstart-building-apps"><span class="std std-ref">wrapper compilers</span></a>. In the above, example, you may
also need to add <code class="docutils literal notranslate"><span class="pre">-m64</span></code> to various <code class="docutils literal notranslate"><span class="pre">--with-wrapper-FOO</span></code>
options:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>shell$ ./configure CFLAGS=-m64 FCFLAGS=-m64 \
--with-wrapper-cflags=-m64 \
--with-wrapper-cxxflags=-m64 \
--with-wrapper-fcflags=-m64 ...
</pre></div>
</div>
<p>Failure to do this will result in MPI applications
failing to compile / link properly.</p>
<p>See the <a class="reference internal" href="../building-apps/customizing-wrappers.html#label-customizing-wrapper-compiler"><span class="std std-ref">Customizing wrapper compiler behavior</span></a> section for more
details.</p>
</div>
<p>Note that if you intend to compile Open MPI with a <code class="docutils literal notranslate"><span class="pre">make</span></code> other than
the default one in your <code class="docutils literal notranslate"><span class="pre">PATH</span></code>, then you must either set the <code class="docutils literal notranslate"><span class="pre">$MAKE</span></code>
environment variable before invoking Open MPI’s <code class="docutils literal notranslate"><span class="pre">configure</span></code> script, or
pass <code class="docutils literal notranslate"><span class="pre">MAKE=your_make_prog</span></code> to configure. For example:</p>
<div class="highlight-sh notranslate"><div class="highlight"><pre><span></span>shell$<span class="w"> </span>./configure<span class="w"> </span><span class="nv">MAKE</span><span class="o">=</span>/path/to/my/make<span class="w"> </span>...
</pre></div>
</div>
<p>This could be the case, for instance, if you have a shell alias for
<code class="docutils literal notranslate"><span class="pre">make</span></code>, or you always type <code class="docutils literal notranslate"><span class="pre">gmake</span></code> out of habit. Failure to tell
<code class="docutils literal notranslate"><span class="pre">configure</span></code> which non-default <code class="docutils literal notranslate"><span class="pre">make</span></code> you will use to compile Open MPI
can result in undefined behavior (meaning: don’t do that).</p>
<p>Note that you may also want to ensure that the value of
<code class="docutils literal notranslate"><span class="pre">LD_LIBRARY_PATH</span></code> is set appropriately (or not at all) for your build
(or whatever environment variable is relevant for your operating
system). For example, some users have been tripped up by setting to
use a non-default Fortran compiler via the <code class="docutils literal notranslate"><span class="pre">FC</span></code> environment variable,
but then failing to set <code class="docutils literal notranslate"><span class="pre">LD_LIBRARY_PATH</span></code> to include the directory
containing that non-default Fortran compiler’s support libraries.
This causes Open MPI’s <code class="docutils literal notranslate"><span class="pre">configure</span></code> script to fail when it tries to
compile / link / run simple Fortran programs.</p>
<p>It is required that the compilers specified be compile and link
compatible, meaning that object files created by one compiler must be
able to be linked with object files from the other compilers and
produce correctly functioning executables.</p>
<div class="section" id="statically-linking-to-the-libraries-of-intel-compiler-suite">
<h2><span class="section-number">4.7.1. </span>Statically linking to the libraries of Intel compiler suite<a class="headerlink" href="#statically-linking-to-the-libraries-of-intel-compiler-suite" title="Permalink to this heading"></a></h2>
<p>The Intel compiler suite, by default, dynamically links its runtime libraries
against the Open MPI binaries and libraries. This can cause problems if the
Intel compiler libraries are installed in non-standard locations. For example,
you might get errors like:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>error while loading shared libraries: libimf.so: cannot open shared object file:
No such file or directory
</pre></div>
</div>
<p>To avoid such problems, you can pass flags to Open MPI’s configure
script that instruct the Intel compiler suite to statically link its
runtime libraries with Open MPI:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>shell$ ./configure CC=icc CXX=icpc FC=ifort LDFLAGS=-Wc,-static-intel ...
</pre></div>
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