Makefile延迟变量扩展

Question

I have some rules in my root makefile that look something like this:

DEPS += a
DEPS += b
...

$(THE_BINARY) : $(DEPS)

Individual app makefiles include this root makefile after providing some required variables. I want to make it so that apps can add to the DEPS variable too. This works:

DEPS += some_other_dep
include root.make

but this doesn't:

include root.make
DEPS += some_other_dep

Is this because ($DEPS) is expanded at the point where the rule is assigned, and not at the end? Is there a way to write the root makefile in such a way that both DEPS += ... lines do the same thing?

Answer 1

Aggieboy's comment is correct as to the issue: if you include the makefile first then the rule using $(DEPS) is already expanded before you add extra things to it, so it doesn't see the extra things. My advice for people writing this sort of makefile environment with common include files is that the included files that define any rules should always be at the end.

It's true the .SECONDEXPANSION will solve this problem, if you want to go that way, and it's not actually that hard to understand. If the description in the manual is too hard to read, it might be useful to look at this discussion of expansion in GNU make , in particular the one on secondary expansion .

I don't typically recommend this because although it will solve this problem, you'll hit the same issue with the next rule... you'll basically have to use secondary expansion for all your rules if you want to allow include files in any order WRT variable assignments. And, secondary expansion only works for variables used in prerequisite lists. You're out of luck if you want to defer expansion of variables used in/as targets.