Question

Are := and += same in Linux Kernel Makefiles and could be used interchangeably? If not, then what is the difference between the two ?

Answer 1

As others say, := is assignment.

But there’s a fine difference between := and =. In most cases it doesn’t matter, but it may make a big difference.

X = $(Y) defines X as a recursive variable, which is something like a C preprocessor macro.
Whenever X is referenced, the value of Y will be used.
The expansion happens when X is expanded. So you can define Y after you’ve defined X, and it’s OK.

X := $(Y) defines X as a simple variable. This is more like a C assignment.
Now, Y is expanded at the time of definition, so changing it later will do nothing.

X += $(Y) appends to X, but keeps its type.
If X was previously defined with =, Y will not be expanded immediately. Same if X was never defined.
If X was previously defined with :=, Y will be expanded immediately.

You can try this exmaple makefile:

X = 1

Y  = $(X)
Z := $(X)

X = 2

Y += $(X)
Z += $(X)

X = 3

test:
        @echo "Y = $(Y)"
        @echo "Z = $(Z)"

It prints

Y = 3 3
Z = 1 2