Makefile中四种变量赋值的区别

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