[Click] Click Kernel Packages and Library Linking . . .

[Eddie Kohler]

Hi John,

Yeah, this sounds like a problem, and one the Click makefiles are not built to 
handle.

With support for "provisions", you can ask Click to build & link with things 
that are not elements.  Although you mention wanting to maintain the code 
separately, I think it is much better to do one of two things:

1. Don't explicitly link with the library, just make the library export its 
symbols (as any linux module would).  The library becomes a separate module 
which must be installed for Click to load.

2. Shift the library's source into Click using ELEMENT_PROVIDES.

I worry about linking with a .a file.  The two methods above ensure that Click 
and the library are compiled for the same kernel version, using the correct 
kernel compile options.  A .a file might be out of sync.

Eddie


On 10/07/2010 07:31 PM, John Russell Lane wrote:
> Hello!
>
> I have a question related to Click package building, which I haven't
> seen on the mailing list or FAQ:
>
> Is there a well-accepted method for getting static, kernel-ready
> libraries linked to a Click kernel module?
>
> Background:
>
> I have a Click package (we'll call it "mypackage") which, for
> userlevel Click, I link with with an external library (call it
> "libmylib.so").  To tell Click that the external library is required,
> I use something like ELEMENT_LIBS("-L ../mylib -lmylib") in one or
> more of my Click package source files; all builds quite nicely and
> links at runtime.
>
> However, I'd like to move part of this into the kernel (i.e., build
> "mypackage.ko").  I modified my the "mylib" source to add precisely
> the same compile options Click adds for building source files (i.e.,
> it basically does a make in the Linux kernel source tree for a
> non-existent source file, has make dump the options used, parses them
> via sed and adds them to the build).  I then built an ar archive
> library (call it "libmylib.a") out of the subset of source files from
> "mylib" which I've modified to run in the Linux kernel.
>
> Now, initially, I thought there might be an easy way to get this
> static library ("libmylib.a") linked with my Click package
> ("mypackage.ko"), but ELEMENT_LIBS definitely seems to be a
> userlevel-only option and I haven't seen anything in the docs related
> to getting external libraries linked into Click-built kernel modules.
>
> For the moment, what I'm doing is having Click build "mypackage.ko",
> rerunning the final linker call to include my "-L ../mylib -lmylib"
> and then installing.  This builds, loads fine (via click-install) and
> is working for the moment, but I'm hoping there's a more automated
> method for doing this.  Ideally, something like an ELEMENT_KLIBS might
> exist to add such libraries to the final linker call.  One option is
> to fully integrate the "mylib" source into the "mypackage" source, but
> since they're maintained separately, this is not really desirable.  A
> similar option is to create symlinks in the "mypackage" source to the
> "mylib" source files.  But I'm hoping there's something that can be
> more easily maintained.
>
> Has anyone else run into this issue?  Any suggestions, workarounds or
> solutions would be greatly appreciated.
>
> Thanks!
>
> jrl.
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