2 modules from 1 source ?

[Greg KH]

On Mon, Dec 19, 2022 at 10:02:42AM -0700, jim.cromie at gmail.com wrote:
> On Mon, Dec 19, 2022 at 9:37 AM <jim.cromie at gmail.com> wrote:
> >
> > Id like to build 2 modules (with different names)
> > from a single source file, with 2nd being dependent
> > on the 1st.
> >
> > Specifically, Ive got:
> > lib/test_dynamic_debug.c
> >
> > I want
> > A:  lib/test_dynamic_debug.ko
> > B:  lib/test_dynamic_debug_submod.ko
> >
> > I expect that the code just needs an #ifdef #else #endif
> > block to clearly put the dependor & dependee elements
> > next to each other.
> >
> > My question is how to do this in the Makefile ?
> > this does most of it
> >
> > --- a/lib/Makefile
> > +++ b/lib/Makefile
> > @@ -78,7 +78,8 @@ obj-$(CONFIG_TEST_SORT) += test_sort.o
> >  obj-$(CONFIG_TEST_USER_COPY) += test_user_copy.o
> >  obj-$(CONFIG_TEST_STATIC_KEYS) += test_static_keys.o
> >  obj-$(CONFIG_TEST_STATIC_KEYS) += test_static_key_base.o
> > -obj-$(CONFIG_TEST_DYNAMIC_DEBUG) += test_dynamic_debug.o
> > +obj-$(CONFIG_TEST_DYNAMIC_DEBUG) += test_dynamic_debug.o
> > test_dynamic_debug_submod.o
> > +CFLAGS_test_dynamic_debug_submod.o += -DTEST_DYNAMIC_DEBUG_SUBMOD
> >  obj-$(CONFIG_TEST_PRINTF) += test_printf.o
> >
> > but how do I tell it the 2nd target ?
> 
> more to the point, this doesnt work (nor do permutations), I miss something:
> 
> +($obj)/test_dynamic_debug_submod.o: ($obj)/test_dynamic_debug.c

Pleaase don't do that :(

If you _REALLY_ need to do this, then just #include your .c file into
two other .c files.  And keep everything in your #included .c file
static to make sure you don't have duplicate global symbols.

Otherwise it is impossible for the same .o to be linked into two
different modules (or into the kernel image itself) as there will be
duplicate global symbols (as that .c file had to have something global
for you to be able to call into it.)

But step back, what is the problem that you really want to solve here
that you feel somehow a single .c file built twice would be correct?
Especially with code in lib/ that should not be needed at all as you are
building "library" functions that only get included if they are actually
used in the large kernel image.

thanks,

greg k-h