Creating a Macro

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<strong>6.5</strong>  </span> <p/> <p>Imagine that you need to run a tool as part of your build. For example, you may want to generate or preprocess a source file, or compress a binary. In this tutorial, you are going to create a macro that resizes an image.</p> <p>Macros are suitable for simple tasks. If you want to do anything more complicated, for example add support for a new programming language, consider creating a <a href="/versions/6.5.0/rules/rules">rule</a>. Rules give you more control and flexibility.</p> <p>The easiest way to create a macro that resizes an image is to use a <code translate="no" dir="ltr">genrule</code>:</p> <div></div><devsite-code><pre class="devsite-click-to-copy" translate="no" dir="ltr" is-upgraded syntax="Python"><code translate="no" dir="ltr"><span class="devsite-syntax-n">genrule</span><span class="devsite-syntax-p">(</span> <span class="devsite-syntax-n">name</span> <span class="devsite-syntax-o">=</span> <span class="devsite-syntax-s2">"logo_miniature"</span><span class="devsite-syntax-p">,</span> <span class="devsite-syntax-n">srcs</span> <span class="devsite-syntax-o">=</span> <span class="devsite-syntax-p">[</span><span class="devsite-syntax-s2">"logo.png"</span><span class="devsite-syntax-p">],</span> <span class="devsite-syntax-n">outs</span> <span class="devsite-syntax-o">=</span> <span class="devsite-syntax-p">[</span><span class="devsite-syntax-s2">"small_logo.png"</span><span class="devsite-syntax-p">],</span> <span class="devsite-syntax-n">cmd</span> <span class="devsite-syntax-o">=</span> <span class="devsite-syntax-s2">"convert $< -resize 100x100 $@"</span><span class="devsite-syntax-p">,</span> <span class="devsite-syntax-p">)</span> <span class="devsite-syntax-n">cc_binary</span><span class="devsite-syntax-p">(</span> <span class="devsite-syntax-n">name</span> <span class="devsite-syntax-o">=</span> <span class="devsite-syntax-s2">"my_app"</span><span class="devsite-syntax-p">,</span> <span class="devsite-syntax-n">srcs</span> <span class="devsite-syntax-o">=</span> <span class="devsite-syntax-p">[</span><span class="devsite-syntax-s2">"my_app.cc"</span><span class="devsite-syntax-p">],</span> <span class="devsite-syntax-n">data</span> <span class="devsite-syntax-o">=</span> <span class="devsite-syntax-p">[</span><span class="devsite-syntax-s2">":logo_miniature"</span><span class="devsite-syntax-p">],</span> <span class="devsite-syntax-p">)</span> </code></pre></devsite-code> <p>If you need to resize more images, you may want to reuse the code. To do that, define a function in a separate <code translate="no" dir="ltr">.bzl</code> file, and call the file <code translate="no" dir="ltr">miniature.bzl</code>:</p> <div></div><devsite-code><pre class="devsite-click-to-copy" translate="no" dir="ltr" is-upgraded syntax="Python"><code translate="no" dir="ltr"><span class="devsite-syntax-k">def</span> <span class="devsite-syntax-nf">miniature</span><span class="devsite-syntax-p">(</span><span class="devsite-syntax-n">name</span><span class="devsite-syntax-p">,</span> <span class="devsite-syntax-n">src</span><span class="devsite-syntax-p">,</span> <span class="devsite-syntax-n">size</span><span class="devsite-syntax-o">=</span><span class="devsite-syntax-s2">"100x100"</span><span class="devsite-syntax-p">,</span> <span class="devsite-syntax-o">**</span><span class="devsite-syntax-n">kwargs</span><span class="devsite-syntax-p">):</span> <span class="devsite-syntax-w"> </span><span class="devsite-syntax-sd">"""Create a miniature of the src image.</span> <span class="devsite-syntax-sd"> The generated file is prefixed with 'small_'.</span> <span class="devsite-syntax-sd"> """</span> <span class="devsite-syntax-n">native</span><span class="devsite-syntax-o">.</span><span class="devsite-syntax-n">genrule</span><span class="devsite-syntax-p">(</span> <span class="devsite-syntax-n">name</span> <span class="devsite-syntax-o">=</span> <span class="devsite-syntax-n">name</span><span class="devsite-syntax-p">,</span> <span class="devsite-syntax-n">srcs</span> <span class="devsite-syntax-o">=</span> <span class="devsite-syntax-p">[</span><span class="devsite-syntax-n">src</span><span class="devsite-syntax-p">],</span> <span class="devsite-syntax-n">outs</span> <span class="devsite-syntax-o">=</span> <span class="devsite-syntax-p">[</span><span class="devsite-syntax-s2">"small_"</span> <span class="devsite-syntax-o">+</span> <span class="devsite-syntax-n">src</span><span class="devsite-syntax-p">],</span> <span class="devsite-syntax-n">cmd</span> <span class="devsite-syntax-o">=</span> <span class="devsite-syntax-s2">"convert $< -resize "</span> <span class="devsite-syntax-o">+</span> <span class="devsite-syntax-n">size</span> <span class="devsite-syntax-o">+</span> <span class="devsite-syntax-s2">" $@"</span><span class="devsite-syntax-p">,</span> <span class="devsite-syntax-o">**</span><span class="devsite-syntax-n">kwargs</span> <span class="devsite-syntax-p">)</span> </code></pre></devsite-code> <p>A few remarks:</p> <ul> <li><p>By convention, macros have a <code translate="no" dir="ltr">name</code> argument, just like rules.</p></li> <li><p>To document the behavior of a macro, use <a href="https://www.python.org/dev/peps/pep-0257/">docstring</a> like in Python.</p></li> <li><p>To call a <code translate="no" dir="ltr">genrule</code>, or any other native rule, use <code translate="no" dir="ltr">native.</code>.</p></li> <li><p>Use <code translate="no" dir="ltr">**kwargs</code> to forward the extra arguments to the underlying <code translate="no" dir="ltr">genrule</code> (it works just like in <a href="https://docs.python.org/3/tutorial/controlflow.html#keyword-arguments">Python</a>). This is useful, so that a user can use standard attributes like <code translate="no" dir="ltr">visibility</code>, or <code translate="no" dir="ltr">tags</code>.</p></li> </ul> <p>Now, use the macro from the <code translate="no" dir="ltr">BUILD</code> file:</p> <div></div><devsite-code><pre class="devsite-click-to-copy" translate="no" dir="ltr" is-upgraded syntax="Python"><code translate="no" dir="ltr"><span class="devsite-syntax-n">load</span><span class="devsite-syntax-p">(</span><span class="devsite-syntax-s2">"//path/to:miniature.bzl"</span><span class="devsite-syntax-p">,</span> <span class="devsite-syntax-s2">"miniature"</span><span class="devsite-syntax-p">)</span> <span class="devsite-syntax-n">miniature</span><span class="devsite-syntax-p">(</span> <span class="devsite-syntax-n">name</span> <span class="devsite-syntax-o">=</span> <span class="devsite-syntax-s2">"logo_miniature"</span><span class="devsite-syntax-p">,</span> <span class="devsite-syntax-n">src</span> <span class="devsite-syntax-o">=</span> <span class="devsite-syntax-s2">"image.png"</span><span class="devsite-syntax-p">,</span> <span class="devsite-syntax-p">)</span> <span class="devsite-syntax-n">cc_binary</span><span class="devsite-syntax-p">(</span> <span class="devsite-syntax-n">name</span> <span class="devsite-syntax-o">=</span> <span class="devsite-syntax-s2">"my_app"</span><span class="devsite-syntax-p">,</span> <span class="devsite-syntax-n">srcs</span> <span class="devsite-syntax-o">=</span> <span class="devsite-syntax-p">[</span><span class="devsite-syntax-s2">"my_app.cc"</span><span class="devsite-syntax-p">],</span> <span class="devsite-syntax-n">data</span> <span class="devsite-syntax-o">=</span> <span class="devsite-syntax-p">[</span><span class="devsite-syntax-s2">":logo_miniature"</span><span class="devsite-syntax-p">],</span> <span class="devsite-syntax-p">)</span> </code></pre></devsite-code> </div> <devsite-thumb-rating position="footer"> </devsite-thumb-rating> <devsite-feedback position="footer" project-name="Bazel" product-id="5052038" bucket="https-bazel-build" context="" version="t-devsite-webserver-20241114-r00-rc02.464921008191574316" data-label="Send Feedback Button" track-type="feedback" track-name="sendFeedbackLink" track-metadata-position="footer" class="nocontent" project-icon="https://www.gstatic.com/devrel-devsite/prod/v870e399c64f7c43c99a3043db4b3a74327bb93d0914e84a0c3dba90bbfd67625/bazel/images/touchicon-180.png" > <button> Send feedback </button> </devsite-feedback> <div class="devsite-floating-action-buttons"> </div> </article> <devsite-content-footer class="nocontent"> <p>Except as otherwise noted, the content of this page is licensed under the <a href="https://creativecommons.org/licenses/by/4.0/">Creative Commons Attribution 4.0 License</a>, and code samples are licensed under the <a href="https://www.apache.org/licenses/LICENSE-2.0">Apache 2.0 License</a>. 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