GNU Makefile变量赋值=，?=，:=和+ =有什么区别? [英] What is the difference between the GNU Makefile variable assignments =, ?=, := and +=?

问题描述

任何人都可以清楚地说明变量赋值在Makefiles中是如何工作的.

Can anybody give a clear explanation of how variable assignment really works in Makefiles.

之间的区别是什么:

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

我已经阅读了GNU Make的部分.手册，但对我来说仍然没有意义.

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

推荐答案

懒惰集

VARIABLE = value

变量的常规设置-使用变量而不是声明变量时，递归扩展其中的值

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

VARIABLE := value

使用内部值的简单扩展设置变量-声明时扩展其中的值.

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

VARIABLE ?= value

仅在没有值的情况下设置变量.访问VARIABLE时始终评估value.等同于

Setting of a variable only if it doesn't have a value. value is always evaluated when VARIABLE is accessed. It is equivalent to

ifeq ($(origin FOO), undefined)
  FOO = bar
endif

有关详细信息，请参见文档.

See the documentation for more details.

VARIABLE += value

将提供的值附加到现有值(如果变量不存在，则设置为该值)

Appending the supplied value to the existing value (or setting to that value if the variable didn't exist)

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