What is the difference between the GNU Makefile variable assignments =, ?=, := and +=?

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

See the documentation for more details.


VARIABLE += value

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

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