What is ?= in Makefile

linux makefile linux-kernel

Solution 1:

?= indicates to set the KDIR variable only if it's not set/doesn't have a value.

For example:

KDIR ?= "foo"
KDIR ?= "bar"

test:
    echo $(KDIR)

Would print "foo"

GNU manual: http://www.gnu.org/software/make/manual/html_node/Setting.html

Solution 2:

Thanks to Simon and R.T. for their quick and correct response.

Also, I have found the GNU manual that explains everything in detail: http://www.gnu.org/software/make/manual/html_node/Setting.html

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