Makefile 中四种变量赋值方式的区别

From: http://my.oschina.net/zengsai/blog/9566

Ask:

What is the difference between :

 VARIABLE = value
 VARIABLE ?= value
 VARIABLE := value
 VARIABLE += value

I have read the section in GNU Make's manual, but it still doesn't make sense to me.

 

Explanation:

 

VARIABLE = value

Normal setting of a variable - values within it are recursively expanded when the variable is used, not when it's declared

VARIABLE := value

Setting of a variable with simple expansion of the values inside - values within it are expanded at declaration time.

VARIABLE ?= value

Setting of a variable only if it doesn't have a value

VARIABLE += value

Appending the supplied value to the existing value