[bug #56206] ifndef false positive for exported variable

[Paul D. Smith]

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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