linux - What's the meaning of ?= in a kernel makefile? -

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• what difference between gnu makefile variable assignments =, ?=, := , +=? 5 answers

i've started learning building external linux kernel modules, , in documentation examples of kbuild (here) i've come across line

kdir ?= /lib/modules/$(shell uname -r)/build 

which used in

make -c $(kdir) m=$(pwd) 

and sets directory kernel source. now, what's meaing of ?= operator? google gives me nothing! conditional assignment? sort of, "define variable follows unless exists in environment"?

thanks!

in makefile, sets value of kdir if doesn't yet have one.

from gnu make documentation on "how use variables":

this called conditional variable assignment operator