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unix make 文档

</span></span><span style="font-family: 楷体_GB2312">直接展开式变量</span></a></h3>
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为了避免“递归展开式”变量存在的问题和不方便。</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">GNU 
make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">支持另外一种风格的变量，称为“直接展开”式。这种风格的变量使用“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">:=</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”定义。在使用“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">:=</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”定义变量时，变量值中对其他量或者函数的引用在定义变量时被展开（对变量进行替换）。所以变量被定义后就是一个实际需要的文本串，其中不再包含任何变量的引用。因此</span></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial">x := foo</span></i></b></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial">y := $(x) bar</span></i></b></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial">x := later</span></i></b></p>
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&nbsp;&nbsp;&nbsp; </span></p>
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就等价于：</span></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial">y := foo bar</span></i></b></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial">x := later</span></i></b></p>
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和递归展开式变量不同：此风格变量在定义时就完成了对所引用变量和函数的展开，因此不能实现对其后定义变量的引用。如：</span></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial">CFLAGS := $(include_dirs) 
-O</span></i></b></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial">include_dirs := 
-Ifoo -Ibar</span></i></b></p>
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由于变量“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">include_dirs</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”的定义出现在“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">CFLAGS</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”定义之后。因此在“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">CFLAGS</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”的定义中，“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">include_dirs</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”的值为空。“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">CFLAGS</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”的值为“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">-O</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”而不是“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">-Ifoo 
-Ibar -O</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”。这一点也是直接展开式和递归展开式变量的不同点。注意这里的两个变量都是“直接展开”式的。大家不妨试试将其中某一个变量使用递归展开式定义后看一下又会出现什么样的结果。</span></p>
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下边我们来看一个复杂一点的例子。分析一下直接展开式变量定义（</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">:=</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">）的用法，这里也用到了</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">的</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">shell</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">函数和变量“</span><span lang="SV" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">MAKELEVEL</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”（此变量在</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">的递归调用时代表</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">的调用深度）。</span></p>
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其中包括了对函数、条件表达式和系统变量“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">MAKELEVEL</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”的使用：</span></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial; color: black">&nbsp;</span></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial">ifeq (0,${MAKELEVEL})</span></i></b></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial">cur-dir&nbsp;&nbsp; := 
$(shell pwd)</span></i></b></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial">whoami&nbsp; := 
$(shell whoami)</span></i></b></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial">host-type := 
$(shell arch)</span></i></b></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial">MAKE := ${MAKE} 
host-type=${host-type} whoami=${whoami}</span></i></b></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial">endif</span></i></b></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial">&nbsp;</span></p>
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第一行是一个条件判断，如果是顶层</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">Makefile</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">，就定义下列变量。否则不定义任何变量。第二、三、四、五行分别定义了一个变量，在进行变量定义时对引用到的其它变量和函数展开。最后结束定义。利用直接展开式的特点我们可以书写这样一个规则：</span></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial">${subdirs}:</span></i></b></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial">${MAKE} 
cur-dir=${cur-dir}/$@ -C $@ all</span></i></b></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial">&nbsp;</span></p>
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它实现了在不同子目录下变量“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">cur_dir</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”使用不同的值（为当前工作目录）。</span></p>
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<span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">
在复杂的</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">Makefile</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">中，推荐使用直接展开式变量。因为这种风格变量的使用方式和大多数编程语言中的变量使用方式基本上相同。它可以使一个比较复杂的</span><span lang="EN-US" style="font-size: 12.0pt; l