Linking kernel module with a static lib

Question

I'm trying to link my kernel module with an external static lib, like this:

obj-m += my_prog.o
my_prog-objs := some/path/lib.a
# all the standard targets...

For some reasone, the above Makefile doesn't compile my_prog.c at all, and the resulting module doesn't contain its code. Certainly, if I remove my_prog-objs line, my_prog.c gets compiled.

What's wrong with such an approach in a Makefile?

Answer 1

When you create the my_prog-objs list, you tell kbuild to use only the object files in that list. kbuild will no longer compile my_prog.c, and including my_prog.o in my_prog-objs results in a circular dependency. Instead, you need to create a unique obj-m and include both my_prog.o and /path/lib.a in its objs list. For example:

obj-m += foo.o
foo-objs += my_prog.o /path/lib.a

Took me about 2 hours to figure out why my module was doing nothing!

Answer 2

You're overriding the default my_prog-objs, which is just my_prog.o. Instead of replacing the contents with the library, add the library to the default:

my_prog-objs := my_prog.o some/path/lib.a

Hopefully you're not trying to link against a general userspace library... that won't work at all in kernelspace.

Answer 3

You must create a synthetic name as well as the source file and it's object name. You can not use my_prog.o directly as there are rules to make it from source. Here is an sample,

 obj-m += full.o
 full-src := my_prog.c
 full-objs := $(full-src:.c=.o) lib.o # yes, make it an object.

Libraries are only supported from some special directories. Your object should be named lib.o_shipped and placed in the same directory. See: item 11 under TODO^note. So, you need to take the external library and provide it locally as a shipped version. You need two object files; one is your compiled 'C' code/driver and the other is it linked together with the library.

Note: I am being facetious; it has a TODO to document this feature.