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<section id="cross-compiling-for-webassembly-with-emscripten">
<span id="developers-cpp-emscripten"></span><h1>Cross compiling for WebAssembly with Emscripten<a class="headerlink" href="#cross-compiling-for-webassembly-with-emscripten" title="Link to this heading">#</a></h1>
<section id="prerequisites">
<h2>Prerequisites<a class="headerlink" href="#prerequisites" title="Link to this heading">#</a></h2>
<p>You need CMake and compilers etc. installed as per the normal build instructions. Before building with Emscripten, you also need to install Emscripten and
activate it using the commands below (see <a class="reference external" href="https://emscripten.org/docs/getting_started/downloads.html">https://emscripten.org/docs/getting_started/downloads.html</a> for details).</p>
<div class="highlight-shell notranslate"><div class="highlight"><pre><span></span>git<span class="w"> </span>clone<span class="w"> </span>https://github.com/emscripten-core/emsdk.git
<span class="nb">cd</span><span class="w"> </span>emsdk
<span class="c1"># replace &lt;version&gt; with the desired EMSDK version.</span>
<span class="c1"># e.g. for Pyodide 0.26, you need EMSDK version 3.1.58</span>
<span class="c1"># the versions can be found in the Makefile.envs file in the Pyodide repo:</span>
<span class="c1"># https://github.com/pyodide/pyodide/blob/10b484cfe427e076c929a55dc35cfff01ea8d3bc/Makefile.envs</span>
./emsdk<span class="w"> </span>install<span class="w"> </span>&lt;version&gt;
./emsdk<span class="w"> </span>activate<span class="w"> </span>&lt;version&gt;
<span class="nb">source</span><span class="w"> </span>./emsdk_env.sh
</pre></div>
</div>
<p>If you want to build PyArrow for <a class="reference external" href="https://pyodide.org">Pyodide</a>, you
need <code class="docutils literal notranslate"><span class="pre">pyodide-build</span></code> installed via <code class="docutils literal notranslate"><span class="pre">pip</span></code>, and to be running with the
same version of Python that Pyodide is built for, along with the same
versions of emsdk tools.</p>
<div class="highlight-shell notranslate"><div class="highlight"><pre><span></span><span class="c1"># install Pyodide build tools.</span>
<span class="c1"># e.g., for version 0.26 of Pyodide, pyodide-build 0.26 and later work</span>
pip<span class="w"> </span>install<span class="w"> </span><span class="s2">&quot;pyodide-build&gt;=0.26&quot;</span>
</pre></div>
</div>
<p>Then build with the <code class="docutils literal notranslate"><span class="pre">ninja-release-emscripten</span></code> CMake preset,
like below:</p>
<div class="highlight-shell notranslate"><div class="highlight"><pre><span></span>emcmake<span class="w"> </span>cmake<span class="w"> </span>--preset<span class="w"> </span><span class="s2">&quot;ninja-release-emscripten&quot;</span>
ninja<span class="w"> </span>install
</pre></div>
</div>
<p>This will install a built static library version of <code class="docutils literal notranslate"><span class="pre">libarrow</span></code> it into the
Emscripten sysroot cache, meaning you can build things that depend on it
and they will find <code class="docutils literal notranslate"><span class="pre">libarrow</span></code>.</p>
<p>e.g. if you want to build for Pyodide, run the commands above, and then
go to <code class="docutils literal notranslate"><span class="pre">arrow/python</span></code> and run</p>
<div class="highlight-shell notranslate"><div class="highlight"><pre><span></span>pyodide<span class="w"> </span>build
</pre></div>
</div>
<p>It should make a wheel targeting the currently enabled version of
Pyodide in the <code class="docutils literal notranslate"><span class="pre">dist</span></code> subdirectory.</p>
</section>
<section id="manual-build">
<h2>Manual Build<a class="headerlink" href="#manual-build" title="Link to this heading">#</a></h2>
<p>If you want to manually build for Emscripten, take a look at the
<code class="docutils literal notranslate"><span class="pre">CMakePresets.json</span></code> file in the <code class="docutils literal notranslate"><span class="pre">arrow/cpp</span></code> directory for a list of things
you will need to override. In particular you will need:</p>
<ol class="arabic simple">
<li><p>Build dependencies set to <code class="docutils literal notranslate"><span class="pre">BUNDLED</span></code>, so it uses properly cross
compiled build dependencies.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">CMAKE_TOOLCHAIN_FILE</span></code> set by using <code class="docutils literal notranslate"><span class="pre">emcmake</span> <span class="pre">cmake</span></code> instead of just <code class="docutils literal notranslate"><span class="pre">cmake</span></code>.</p></li>
<li><p>You will need to set <code class="docutils literal notranslate"><span class="pre">ARROW_ENABLE_THREADING</span></code> to <code class="docutils literal notranslate"><span class="pre">OFF</span></code> for builds
targeting single-threaded Emscripten environments such as Pyodide.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">ARROW_FLIGHT</span></code> and anything else that uses network probably won’t
work.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">ARROW_JEMALLOC</span></code> and <code class="docutils literal notranslate"><span class="pre">ARROW_MIMALLOC</span></code> again probably need to be
<code class="docutils literal notranslate"><span class="pre">OFF</span></code></p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">ARROW_BUILD_STATIC</span></code> set to <code class="docutils literal notranslate"><span class="pre">ON</span></code> and <code class="docutils literal notranslate"><span class="pre">ARROW_BUILD_SHARED</span></code> set to
<code class="docutils literal notranslate"><span class="pre">OFF</span></code> is most likely to work.</p></li>
</ol>
</section>
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