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

Question

I've started learning about building external Linux kernel modules, and in the documentation examples of kbuild (here) I've come across the line

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

which is then used in

make -C $(KDIR) M=$(PWD)

and sets the directory to the kernel source. Now, what's the meaing of that ?= operator? Google gives me nothing! Is that a conditional assignment? Sort of, "define variable as follows unless it already exists in the environment"?

Thanks!

Thank you so much for the reference, I didn't realise it was a make-specific operator. — jstrange, May 30 '15 at 19:19

score 1 · Accepted Answer · answered May 30 '15 at 19:14

1

In a makefile, this sets the value of KDIR only if it doesn't yet have one.

From the GNU make documentation on "How to Use Variables":

This is called a conditional variable assignment operator

answered May 30 '15 at 19:14

Denys Séguret

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

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