电脑/linux/makefile/makefile学习笔记.html

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<title>makefile进阶</title>
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<meta  name="author" content="万泽(德山书生)">
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<h1 class="title">makefile进阶</h1>
</header><nav id="table-of-contents">
<h2>Table of Contents</h2>
<div id="text-table-of-contents">
<ul>
<li><a href="#orgheadline1">0.1. makefile</a></li>
</ul>
</li>
<li><a href="#orgheadline2">1. 强制某个目标更新</a></div>
</nav>


<div class="outline-3">
<h3 id="orgheadline1">makefile</h3>
<div class="outline-text-3" id="text-0-1">
<p>
makefile里面内容其实很丰富的，实际上甚至有点过于复杂了。不过这里只是简单使用其基本功能还是很便利的。
</p>

<div class="highlight"><pre><span></span><span class="nv">project</span><span class="o">=</span>helloworld
<span class="nv">cflags</span><span class="o">=</span>-g -Wall  <span class="si">${</span><span class="nv">CCFLAGS</span><span class="si">}</span>

<span class="nf">${project}</span><span class="o">:</span> 
        <span class="si">${</span><span class="nv">CC</span><span class="si">}</span>  helloworld.c -o <span class="nv">$@</span> <span class="si">${</span><span class="nv">cflags</span><span class="si">}</span>

<span class="nf">clean</span><span class="o">:</span>
        rm <span class="si">${</span><span class="nv">project</span><span class="si">}</span>

<span class="nf">install</span><span class="o">:</span>
        cp <span class="si">${</span><span class="nv">project</span><span class="si">}</span> /usr/local/bin

<span class="nf">uninstall</span><span class="o">:</span>
        rm /usr/local/bin/<span class="si">${</span><span class="nv">project</span><span class="si">}</span>

<span class="nf">.PHONY</span><span class="o">:</span>
        clean uninstall install
</pre></div>

<p>
为了便于理解，上面这个makefile我有意采用了一种和bash接近的风格。前面 <code>project=</code> 就是一个定义变量的行为。这个project变量就是本脚本的名字。然后makefile下面的主体部分格式如下:
</p>

<pre>
target: prerequisites
    the command
</pre>
<p>
具体意思就是要生成target这个文件，首先要确保prerequisites这些依赖文件都在而且是最新的，不在或者不是最新的那么查找对应的目标生成规则继续生成。而对于这个target的生成就是执行下面的bash命令。下面是关于上面例子的一些讲解信息:
</p>

<ol class="org-ol">
<li><em>特别要强调，命令前面请用 Tab键 隔开。</em> 还好emacs虽然设置成为Tab自动切换space，其对于makefile还是能够保留Tab键，其他编辑器就不知道了。这个真的算是一个很不好的设计。</li>

<li>关于变量的使用读者看到上面的例子，我有意采用了类似bash脚本的语法。这么写也是支持的。</li>

<li><code>$@</code> 这个特殊的符号并不是什么神秘东西，其意思就是当前目标的文件名，上面例子中当前目标是 <code>${target}</code> ，也就是helloworld，所以这里 <code>$@</code> 就是 helloworld。</li>

<li><code>.PHONY</code> 这后面跟着一些生成目标，具体意思就是这些生成目标是伪目标，或者说其并没有生成文件，只是执行了某个命令。</li>

<li>我们注意到上面的 <code>${CC}</code> 和 <code>${CCFLAGS}</code>  并没有为用户定义，其是make命令的一些默认变量。 <code>$(CC)</code> 就是调用系统默认的c编译器，一般为gcc。</li>

<li><code>make</code> 命令不输入任何子命令时，默认执行输出第一个目标命令，一般是本项目目标。</li>

<li>makefile的每个命令都有一个独立的终端，也就是不同的终端不共享变量，所以最好多个命令连接成为一个命令，换行用\换行。</li>

<li>makefile扫描两边，第一遍变量替换，第二遍依赖关系。变量声明最好跟着对应的规则，还有要保证不能被后面的变量声明改变。</li>
</ol>
</div>
</div>


<div class="outline-2">
<h2 id="orgheadline2">强制某个目标更新</h2>
<div class="outline-text-2" id="text-1">
<p>
参考了 <a href="http://stackoverflow.com/questions/7643838/how-to-force-make-to-always-rebuild-a-file">这个网页</a> 。
</p>

<p>
大致如下面所示，设置一个 <strong>FORCE</strong> 目标，凡是依赖FORCE目标的都将强制没一次都再更新一遍，而原因就是因为这个FORCE目标不不依赖任何目标，这样makefile认为目标不存在，而每次都会再更新生成一遍。
</p>
<pre>
${project}.org: FORCE
	python3 make_${project}org.py

FORCE:
</pre>
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