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6.2.4 Conditional Variable Assignment

There is another assignment operator for variables, ‘?=’. This is called a conditional variable assignment operator, because it only has an effect if the variable is not yet defined. This statement:

FOO ?= bar

is exactly equivalent to this (see The origin Function):

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

Note that a variable set to an empty value is still defined, so ‘?=’ will not set that variable.