什么？=在Makefile中做什么？

Question

通过我的Makefile看，我看到SOMEVAR?=somestuff，我不知道这意味着什么。

任何帮助，将不胜感激。

谢谢！

Answer 1

根据GNU Make Manual，只有在当前未定义其他值的情况下，才会设置SOMEVAR。

Answer 2

如果它尚未定义，它将值分配给SOMEVAR。

Answer 3

见make的MAN-网页：

变量赋值的变量 在化妆很像在shell变量，并通过tradi- 重刑，包括所有的大写字母。

变量赋值改性剂 可以用于将值分配给变量的五个运算符如下 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.