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makefile

What is ?= in Makefile

KDIR ?= $(shell uname -r)

What is the meaning of ?=?

I have understood the difference between :=, += and = from another thread available in Stack Overflow, but unable to find the explanation for ?=.

Solution

  • ?= 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