What is the difference between the GNU Makefile variable assignments =, ?=, := and +=?
Lazy Set
VARIABLE = value
Normal setting of a variable, but any other variables mentioned with the value
field are recursively expanded with their value at the point at which the variable is used, not the one it had when it was declared
Immediate Set
VARIABLE := value
Setting of a variable with simple expansion of the values inside - values within it are expanded at declaration time.
Lazy Set If Absent
VARIABLE ?= value
Setting of a variable only if it doesn't have a value. value
is always evaluated when VARIABLE
is accessed. It is equivalent to
ifeq ($(origin FOO), undefined)
FOO = bar
endif
See the documentation for more details.
Append
VARIABLE += value
Appending the supplied value to the existing value (or setting to that value if the variable didn't exist)
Using =
causes the variable to be assigned a value. If the variable already had a value, it is replaced. This value will be expanded when it is used. For example:
HELLO = world
HELLO_WORLD = $(HELLO) world!
# This echoes "world world!"
echo $(HELLO_WORLD)
HELLO = hello
# This echoes "hello world!"
echo $(HELLO_WORLD)
Using :=
is similar to using =
. However, instead of the value being expanded when it is used, it is expanded during the assignment. For example:
HELLO = world
HELLO_WORLD := $(HELLO) world!
# This echoes "world world!"
echo $(HELLO_WORLD)
HELLO = hello
# Still echoes "world world!"
echo $(HELLO_WORLD)
HELLO_WORLD := $(HELLO) world!
# This echoes "hello world!"
echo $(HELLO_WORLD)
Using ?=
assigns the variable a value iff the variable was not previously assigned. If the variable was previously assigned a blank value (VAR=
), it is still considered set I think. Otherwise, functions exactly like =
.
Using +=
is like using =
, but instead of replacing the value, the value is appended to the current one, with a space in between. If the variable was previously set with :=
, it is expanded I think. The resulting value is expanded when it is used I think. For example:
HELLO_WORLD = hello
HELLO_WORLD += world!
# This echoes "hello world!"
echo $(HELLO_WORLD)
If something like HELLO_WORLD = $(HELLO_WORLD) world!
were used, recursion would result, which would most likely end the execution of your Makefile. If A := $(A) $(B)
were used, the result would not be the exact same as using +=
because B
is expanded with :=
whereas +=
would not cause B
to be expanded.
I suggest you do some experiments using "make". Here is a simple demo, showing the difference between =
and :=
.
/* Filename: Makefile*/
x := foo
y := $(x) bar
x := later
a = foo
b = $(a) bar
a = later
test:
@echo x - $(x)
@echo y - $(y)
@echo a - $(a)
@echo b - $(b)
make test
prints:
x - later
y - foo bar
a - later
b - later bar
Check more elaborate explanation here