Question

I am trying to parse some Makefile files to read some configs them them and I encountered a wide range of expressions like:

AAA := Some, text
BBB_NAME := @AAA@ (c)
CCC = value
DDD = Some other $(CCC) xxx

I would like to know if all of these are valid and what if there is any difference between them (so I can properly parse them).

Answer 1

They are all valid, as you can tell by putting them in a Makefile and running it. If you want to know what values they actually take, you can try

$(info $(AAA))

(Note that the only real problem is with the (c) in BBB_NAME, it can cause problems if you pass it into other functions.)

The one tricky part is the difference between = and := (and other assignment operators). Full details are in the manual, but basically := evaluates the right-hand side at once, while = holds off until the left-hand side is evaluated somewhere. Consider

CCC = value
DDD := Some other $(CCC) xxx
EEE = Some other $(CCC) xxx

The value of DDD is now Some other value xxx, while the value of EEE is Some other $(CCC) xxx. If you use them somewhere:

$(info $(DDD))
$(info $(EEE))

Make expands $(DDD) and $(EEE) to the same thing and you see

Some other value xxx
Some other value xxx

But there are differences:

CCC = value
DDD := Some other $(CCC) xxx
EEE = Some other $(CCC) xxx

DDD := $(DDD) and yyy   # This is perfectly legal.
EEE := $(EEE) and yyy   # Infinite recursion. Make will not allow this.

CCC = dimension

$(info $(DDD))          # Produces "Some other value xxx and yyy"
$(info $(EEE))          # Produces "Some other dimension xxx"