Question

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

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.

Answer 1

Lazy Set

VARIABLE = value 

Normal setting of a variable, but any other variables mentioned with the value field are recursively expanded with their value at the point at which the variable is used, not the one it had when it was declared

Immediate Set

VARIABLE := value 

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

Lazy Set If Absent

VARIABLE ?= value 

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 VARIABLE), undefined)   VARIABLE = value endif 

See the documentation for more details.

Append

VARIABLE += value 

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