Question

浏览Makefile，我看到SOMEVAR?=somestuff，我不确定这意味着什么。

任何帮助都将不胜感激。

谢谢！

Answer 1

根据GNU Make Manual，只有在当前未使用其他值定义时才设置SOMEVAR。

Answer 2

如果尚未定义，则将值分配给SOMEVAR。

Answer 3

请参阅make的MAN页面：

可变分配 make中的变量很像shell中的变量，并且通过传统变量由所有大写字母组成。

变量赋值修饰符可用于为变量赋值的五个运算符为如下：

 =       Assign the value to the variable.  Any previous value is overrid-
         den.

 +=      Append the value to the current value of the variable.

 ?=      Assign the value to the variable if it is not already defined.

 :=      Assign with expansion, i.e. expand the value before assigning it
         to the variable.  Normally, expansion is not done until the vari-
         able is referenced.  NOTE: References to undefined variables are
         not expanded.  This can cause problems when variable modifiers
         are used.

 !=      Expand the value and pass it to the shell for execution and
         assign the result to the variable.  Any newlines in the result
         are replaced with spaces.

什么？=在Makefile中做什么？

3 个答案: