Update of bug #56206 (project make):
Status: None => Not A Bug
Open/Closed: Open => Closed
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Follow-up Comment #1:
This behavior is correct. The confusion is because the ?= operator and the
ifndef operator have different behaviors for variables which are defined to
have an empty value.
ifdef/ifndef test whether the expansion of the variable is the empty string or
not:
ifndef variable-name
If the variable variable-name has an empty value, the text-if-true is
effective;
otherwise, the text-if-false, if any, is effective.
However, the ?= operator tests whether the variable is actually defined or
not, not whether it has an empty value:
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
endif
Note that a variable set to an empty value is still defined, so ‘?=’
will not set that variable.
-verbatim=
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