[Question] Set a variable while building a specific target

[Masahiro Yamada]

Hi.

Is there a good way to set a specific variable 'BUILDING_FOO'
while Make is building the target 'foo' including its prerequisites?


Let's say we have two targets 'vmlinux' and 'modules', but
the actual build rules are factored out into '__common'.

[Not-working code]

------------------>8-------------------------
.PHONY: vmlinux modules __common

vmlinux: export BUILDING_VMLINUX=y
modules: export BUILDING_MODULES=y

vmlinux modules: __common
        @:

__common:
        @echo BUILDING_VMLINUX is $(BUILDING_VMLINUX)
        @echo BUILDING_MODULES is $(BUILDING_MODULES)
        [ common build rules, which internally uses
           BUILDING_{VMLINUX,MODULES} conditionals ]
------------------>8-------------------------

My hope is:
 - 'make vmlinux' builds __common with BUILDING_VMLINUX=y
 - 'make modules' builds __common with BUILDING_MODULES=y
 - 'make vmlinux modules' builds __common with
    BUILDING_VMLINUX=y and BUILDING_MODULES=y

I know the example above does not work like that.


[Working code]
------------------>8-------------------------
.PHONY: vmlinux modules __common

ifneq ($(filter vmlinux,$(MAKECMDGOALS)),)
export BUILDING_VMLINUX=y
endif

ifneq ($(filter modules,$(MAKECMDGOALS)),)
export BUILDING_MODULES=y
endif

vmlinux modules: __common
        @:

__common:
        @echo BUILDING_VMLINUX is $(BUILDING_VMLINUX)
        @echo BUILDING_MODULES is $(BUILDING_MODULES)
        [ common build rules, which internally uses
           BUILDING_{VMLINUX,MODULES} conditionals ]
------------------>8-------------------------


'make foo bar' builds __common, setting
BUILDING_VMLINUX and BUILDING_MODULES,
but we need to add more filtering code to handle
'make', 'make all', and other targets that
depend on 'vmlinux' and 'modules'.



The Linux kernel build system actually does this:

https://github.com/torvalds/linux/blob/v5.12/Makefile#L633
https://github.com/torvalds/linux/blob/v5.12/Makefile#L637

but, the filtering pattern tends to grow uglily if we try to
cover all the possible targets.

Is there any way to do this better?

--
Best Regards
Masahiro Yamada