什麼？=在Makefile中做什麼？

Question

通過我的Makefile看，我看到SOMEVAR?=somestuff，我不知道這意味着什麼。

任何幫助，將不勝感激。

謝謝！

Answer 1

根據GNU Make Manual，只有在當前未定義其他值的情況下，纔會設置SOMEVAR。

Answer 2

如果它尚未定義，它將值分配給SOMEVAR。

Answer 3

見make的MAN-網頁：

變量賦值的變量 在化妝很像在shell變量，並通過tradi- 重刑，包括所有的大寫字母。

變量賦值改性劑 可以用於將值分配給變量的五個運算符如下 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.