makefile中？=

在Makefile中，"？="是一种赋值运算符，它的作用是给变量赋值，但是只有当该变量未定义时才会进行赋值。如果变量已经定义了，那么"？="运算符将不会对该变量进行赋值操作。因此，"？="可以用来设置变量的默认值，以防止重复定义变量导致的错误。在你提到的例子中，KERNELDIR ?= /lib/modules/是给变量KERNELDIR赋值，如果该变量已经定义了，那么就不会进行赋值操作，否则将其设置为/lib/modules/。<span class="em">1</span><span class="em">2</span><span class="em">3</span>
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