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<section id="id1">
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<h1>使用条件判断<a class="headerlink" href="#id1" title="永久链接至标题">¶</a></h1>
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<p>使用条件判断，可以让make根据运行时的不同情况选择不同的执行分支。条件表达式可以是比较变量的值，或是比较变量和常量的值。</p>
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<section id="id2">
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<h2>示例<a class="headerlink" href="#id2" title="永久链接至标题">¶</a></h2>
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<p>下面的例子，判断 <code class="docutils literal notranslate"><span class="pre">$(CC)</span></code> 变量是否 <code class="docutils literal notranslate"><span class="pre">gcc</span></code> ，如果是的话，则使用GNU函数编译目标。</p>
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<div class="highlight-makefile notranslate"><div class="highlight"><pre><span></span><span class="nv">libs_for_gcc</span> <span class="o">=</span> -lgnu
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<span class="nv">normal_libs</span> <span class="o">=</span>
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<span class="nf">foo</span><span class="o">:</span> <span class="k">$(</span><span class="nv">objects</span><span class="k">)</span>
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<span class="cp">ifeq ($(CC),gcc)</span>
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<span class="k">$(</span>CC<span class="k">)</span> -o foo <span class="k">$(</span>objects<span class="k">)</span> <span class="k">$(</span>libs_for_gcc<span class="k">)</span>
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<span class="cp">else</span>
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<span class="k">$(</span>CC<span class="k">)</span> -o foo <span class="k">$(</span>objects<span class="k">)</span> <span class="k">$(</span>normal_libs<span class="k">)</span>
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<span class="cp">endif</span>
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</pre></div>
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</div>
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<p>可见，在上面示例的这个规则中，目标 <code class="docutils literal notranslate"><span class="pre">foo</span></code> 可以根据变量 <code class="docutils literal notranslate"><span class="pre">$(CC)</span></code> 值来选取不同的函数库来编译程序。</p>
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<p>我们可以从上面的示例中看到三个关键字： <code class="docutils literal notranslate"><span class="pre">ifeq</span></code> 、 <code class="docutils literal notranslate"><span class="pre">else</span></code> 和 <code class="docutils literal notranslate"><span class="pre">endif</span></code> 。 <code class="docutils literal notranslate"><span class="pre">ifeq</span></code> 的意思表示条件语句的开始，并指定一个条件表达式，表达式包含两个参数，以逗号分隔，表达式以圆括号括起。 <code class="docutils literal notranslate"><span class="pre">else</span></code> 表示条件表达式为假的情况。 <code class="docutils literal notranslate"><span class="pre">endif</span></code> 表示一个条件语句的结束，任何一个条件表达式都应该以 <code class="docutils literal notranslate"><span class="pre">endif</span></code> 结束。</p>
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<p>当我们的变量 <code class="docutils literal notranslate"><span class="pre">$(CC)</span></code> 值是 <code class="docutils literal notranslate"><span class="pre">gcc</span></code> 时，目标 <code class="docutils literal notranslate"><span class="pre">foo</span></code> 的规则是：</p>
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<div class="highlight-makefile notranslate"><div class="highlight"><pre><span></span><span class="nf">foo</span><span class="o">:</span> <span class="k">$(</span><span class="nv">objects</span><span class="k">)</span>
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<span class="k">$(</span>CC<span class="k">)</span> -o foo <span class="k">$(</span>objects<span class="k">)</span> <span class="k">$(</span>libs_for_gcc<span class="k">)</span>
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</pre></div>
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</div>
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<p>而当我们的变量 <code class="docutils literal notranslate"><span class="pre">$(CC)</span></code> 值不是 <code class="docutils literal notranslate"><span class="pre">gcc</span></code> 时（比如 <code class="docutils literal notranslate"><span class="pre">cc</span></code> ），目标 <code class="docutils literal notranslate"><span class="pre">foo</span></code> 的规则是：</p>
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<div class="highlight-makefile notranslate"><div class="highlight"><pre><span></span><span class="nf">foo</span><span class="o">:</span> <span class="k">$(</span><span class="nv">objects</span><span class="k">)</span>
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<span class="k">$(</span>CC<span class="k">)</span> -o foo <span class="k">$(</span>objects<span class="k">)</span> <span class="k">$(</span>normal_libs<span class="k">)</span>
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</pre></div>
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</div>
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<p>当然，我们还可以把上面的那个例子写得更简洁一些：</p>
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<div class="highlight-makefile notranslate"><div class="highlight"><pre><span></span><span class="nv">libs_for_gcc</span> <span class="o">=</span> -lgnu
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<span class="nv">normal_libs</span> <span class="o">=</span>
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<span class="cp">ifeq ($(CC),gcc)</span>
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<span class="nv">libs</span><span class="o">=</span><span class="k">$(</span>libs_for_gcc<span class="k">)</span>
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<span class="cp">else</span>
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<span class="nv">libs</span><span class="o">=</span><span class="k">$(</span>normal_libs<span class="k">)</span>
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<span class="cp">endif</span>
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<span class="nf">foo</span><span class="o">:</span> <span class="k">$(</span><span class="nv">objects</span><span class="k">)</span>
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<span class="k">$(</span>CC<span class="k">)</span> -o foo <span class="k">$(</span>objects<span class="k">)</span> <span class="k">$(</span>libs<span class="k">)</span>
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</pre></div>
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</div>
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</section>
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<section id="id3">
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<h2>语法<a class="headerlink" href="#id3" title="永久链接至标题">¶</a></h2>
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<p>条件表达式的语法为:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o"><</span><span class="n">conditional</span><span class="o">-</span><span class="n">directive</span><span class="o">></span>
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<span class="o"><</span><span class="n">text</span><span class="o">-</span><span class="k">if</span><span class="o">-</span><span class="n">true</span><span class="o">></span>
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<span class="n">endif</span>
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</pre></div>
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</div>
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<p>以及:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o"><</span><span class="n">conditional</span><span class="o">-</span><span class="n">directive</span><span class="o">></span>
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<span class="o"><</span><span class="n">text</span><span class="o">-</span><span class="k">if</span><span class="o">-</span><span class="n">true</span><span class="o">></span>
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<span class="k">else</span>
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<span class="o"><</span><span class="n">text</span><span class="o">-</span><span class="k">if</span><span class="o">-</span><span class="n">false</span><span class="o">></span>
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<span class="n">endif</span>
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</pre></div>
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</div>
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<p>其中 <code class="docutils literal notranslate"><span class="pre"><conditional-directive></span></code> 表示条件关键字，如 <code class="docutils literal notranslate"><span class="pre">ifeq</span></code> 。这个关键字有四个。</p>
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<p>第一个是我们前面所见过的 <code class="docutils literal notranslate"><span class="pre">ifeq</span></code></p>
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<div class="highlight-makefile notranslate"><div class="highlight"><pre><span></span><span class="cp">ifeq (<arg1>, <arg2>)</span>
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<span class="cp">ifeq '<arg1>' '<arg2>'</span>
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<span class="cp">ifeq "<arg1>" "<arg2>"</span>
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<span class="cp">ifeq "<arg1>" '<arg2>'</span>
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<span class="cp">ifeq '<arg1>' "<arg2>"</span>
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</pre></div>
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</div>
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<p>比较参数 <code class="docutils literal notranslate"><span class="pre">arg1</span></code> 和 <code class="docutils literal notranslate"><span class="pre">arg2</span></code> 的值是否相同。当然，参数中我们还可以使用make的函数。如:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>ifeq ($(strip $(foo)),)
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<text-if-empty>
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endif
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</pre></div>
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</div>
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<p>这个示例中使用了 <code class="docutils literal notranslate"><span class="pre">strip</span></code> 函数，如果这个函数的返回值是空（Empty），那么
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<code class="docutils literal notranslate"><span class="pre"><text-if-empty></span></code> 就生效。</p>
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<p>第二个条件关键字是 <code class="docutils literal notranslate"><span class="pre">ifneq</span></code> 。语法是：</p>
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<div class="highlight-makefile notranslate"><div class="highlight"><pre><span></span><span class="cp">ifneq (<arg1>, <arg2>)</span>
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<span class="cp">ifneq '<arg1>' '<arg2>'</span>
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<span class="cp">ifneq "<arg1>" "<arg2>"</span>
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<span class="cp">ifneq "<arg1>" '<arg2>'</span>
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<span class="cp">ifneq '<arg1>' "<arg2>"</span>
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</pre></div>
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</div>
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<p>其比较参数 <code class="docutils literal notranslate"><span class="pre">arg1</span></code> 和 <code class="docutils literal notranslate"><span class="pre">arg2</span></code> 的值是否相同，如果不同，则为真。和 <code class="docutils literal notranslate"><span class="pre">ifeq</span></code> 类似。</p>
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<p>第三个条件关键字是 <code class="docutils literal notranslate"><span class="pre">ifdef</span></code> 。语法是：</p>
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<div class="highlight-makefile notranslate"><div class="highlight"><pre><span></span><span class="cp">ifdef <variable-name></span>
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</pre></div>
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</div>
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<p>如果变量 <code class="docutils literal notranslate"><span class="pre"><variable-name></span></code> 的值非空，那到表达式为真。否则，表达式为假。当然，
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<code class="docutils literal notranslate"><span class="pre"><variable-name></span></code> 同样可以是一个函数的返回值。注意， <code class="docutils literal notranslate"><span class="pre">ifdef</span></code> 只是测试一个变量是否有值，其并不会把变量扩展到当前位置。还是来看两个例子：</p>
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<p>示例一：</p>
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<div class="highlight-makefile notranslate"><div class="highlight"><pre><span></span><span class="nv">bar</span> <span class="o">=</span>
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<span class="nv">foo</span> <span class="o">=</span> <span class="k">$(</span>bar<span class="k">)</span>
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<span class="cp">ifdef foo</span>
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<span class="nv">frobozz</span> <span class="o">=</span> yes
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<span class="cp">else</span>
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<span class="nv">frobozz</span> <span class="o">=</span> no
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<span class="cp">endif</span>
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</pre></div>
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</div>
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<p>示例二：</p>
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<div class="highlight-makefile notranslate"><div class="highlight"><pre><span></span><span class="nv">foo</span> <span class="o">=</span>
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<span class="cp">ifdef foo</span>
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<span class="nv">frobozz</span> <span class="o">=</span> yes
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<span class="cp">else</span>
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<span class="nv">frobozz</span> <span class="o">=</span> no
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<span class="cp">endif</span>
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</pre></div>
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</div>
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<p>第一个例子中， <code class="docutils literal notranslate"><span class="pre">$(frobozz)</span></code> 值是 <code class="docutils literal notranslate"><span class="pre">yes</span></code> ，第二个则是 <code class="docutils literal notranslate"><span class="pre">no</span></code>。</p>
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<p>第四个条件关键字是 <code class="docutils literal notranslate"><span class="pre">ifndef</span></code> 。其语法是：</p>
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<div class="highlight-makefile notranslate"><div class="highlight"><pre><span></span><span class="cp">ifndef <variable-name></span>
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</pre></div>
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</div>
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<p>这个我就不多说了，和 <code class="docutils literal notranslate"><span class="pre">ifdef</span></code> 是相反的意思。</p>
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<p>在 <code class="docutils literal notranslate"><span class="pre"><conditional-directive></span></code> 这一行上，多余的空格是被允许的，但是不能以 <code class="docutils literal notranslate"><span class="pre">Tab</span></code> 键作为开始（不然就被认为是命令）。而注释符 <code class="docutils literal notranslate"><span class="pre">#</span></code> 同样也是安全的。 <code class="docutils literal notranslate"><span class="pre">else</span></code> 和 <code class="docutils literal notranslate"><span class="pre">endif</span></code>
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也一样，只要不是以 <code class="docutils literal notranslate"><span class="pre">Tab</span></code> 键开始就行了。</p>
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<p>特别注意的是，make是在读取Makefile时就计算条件表达式的值，并根据条件表达式的值来选择语句，所以，你最好不要把自动化变量（如 <code class="docutils literal notranslate"><span class="pre">$@</span></code> 等）放入条件表达式中，因为自动化变量是在运行时才有的。</p>
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<p>而且为了避免混乱，make不允许把整个条件语句分成两部分放在不同的文件中。</p>
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<p>© 版权所有 2014-2019, 作者：陈皓；排版：SeisMan.
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<span class="lastupdated">最后更新于 2022年5月22日.
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