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FOO ?= bar is equivalent to ifeq ($(origin FOO), undefined) FOO = bar endif However, when CC is not set its value is 'default' rather than 'undefined'. >From [1]: 'undefined': if variable was never defined. 'default': if variable has a default definition, as is usual with CC and so on. See Variables Used by Implicit Rules. Note that if you have redefined a default variable, the origin function will return the origin of the later definition. [1] https://www.gnu.org/software/make/manual/make.html Signed-off-by: Radostin Stoyanov <rstoyanov1@gmail.com> Acked-by: Cyrill Gorcunov <gorcunov@gmail.com> Signed-off-by: Andrei Vagin <avagin@gmail.com>