Re: How to create "re-entrant" makefiles?

[Mike Shal]

On 1/2/07, David Wuertele <address@hidden> wrote:
> I want to include a makefile multiple times, and I each time I want it to define
> rules based on the current value of some variables.  But I'm stymied by the fact
> that Make only interpolates variable values for command scripts when they are
> executed, not when they are defined.
>
> For example:
>
> Makefile
> --------
>
> MY_VAR := one
> include variabletest.mk
>
> MY_VAR := two
> include variabletest.mk
>
> variabletest.mk
> ---------------
>
> $(MY_VAR):
>         echo $(MY_VAR) > $(MY_VAR)
>
> all: $(MY_VAR)
>
>
> When I type "make", I expect it to do this:
>
>   echo one > one
>   echo two > two
>
> But the rules for "one" and "two" both contain $(MY_VAR), which isn't evaluated
> until execution time, and at that point MY_VAR contains "two".  So all I see is
> this:
>
>   echo two > two
>
> How can I make make do what I want?

You can use target-specific variables. Try adding this line to the top
of variabletest.mk:

$(MY_VAR): MY_VAR := $(MY_VAR)

That sets the variable MY_VAR for the target $(MY_VAR) (eg: "one" or
"two") to $(MY_VAR). Since it's using := here, it won't be affected by
future changes to MY_VAR.

Also, note that make will build the first target it sees by default,
which would be "one" in your example. You probably want to put an
empty "all" target at the top of the Makefile so make will always
build all by default:

all:
MY_VAR := one
include ...
etc

-Mike