What is the difference between the GNU Makefile variable assignments =, ?=, := and +=?
= 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)
:= 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)
?= 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
+= 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
B is expanded with
+= would not cause
B to be expanded.
Can anybody give a clear explanation of how variable assignment really works in Makefiles.
What is the difference between :
VARIABLE = value VARIABLE ?= value VARIABLE := value VARIABLE += value
I have read the section in GNU Make's manual, but it still doesn't make sense to me.
When you use
VARIABLE = value, if
value is actually a reference to another variable, then the value is only determined when
VARIABLE is used. This is best illustrated with an example:
VAL = foo VARIABLE = $(VAL) VAL = bar # VARIABLE and VAL will both evaluate to "bar"
When you use
VARIABLE := value, you get the value of
value as it is now. For example:
VAL = foo VARIABLE := $(VAL) VAL = bar # VAL will evaluate to "bar", but VARIABLE will evaluate to "foo"
VARIABLE ?= val means that you only set the value of
VARIABLE is not set already. If it's not set already, the setting of the value is deferred until
VARIABLE is used (as in example 1).
VARIABLE += value just appends
VARIABLE. The actual value of
value is determined as it was when it was initially set, using either