Incremental Linking

[Dave Hylands]

Hi Somanath,

On Tue, May 22, 2012 at 2:56 AM, somanath sahoo <bapi_mvit2004 at yahoo.com> wrote:
> Hi,
>
> As newbie in linux kernel, I would like to understand the concept of
> "incremental linking " w.r.t to linux kernel module.

Incremental linking is basically just taking some objects files,
linking them together to produce a larger object file. The object file
still has undefined references. It will also coalesce sections of the
same name. The kernel likes to use sections for storing pointers to
initcall functions and other things like that.

Some people might also call this partial linking. The kernel uses this
technique for the main portion of the kernel as well. If you look
through your build directory, you will find a whole bunch of
built-in.o files. Each one of these is a partially linked object file
containing all of the object files from the current directory and
built-in.o files from directories below.

With kernel modules, there are some special automatically generated C
files which also get linked in (IIRC that have a name like foo.mod.c)

A kernel module is conceptually identical to a shared library (which
is also partially linked and may contain unresolved references).

You can do incremental linking  by doing:

echo "int foo1(void) { return 1; }" > foo1.c
echo "int foo2(void) { return 2; }" > foo2.c
gcc -c foo1.c
gcc -c foo2.c
ld -r -o foo.o foo1.o foo2.o

You'll now have foo.o which has both foo1.o and foo2.o partially
linked together, which you can see by using:

nm foo.o

-- 
Dave Hylands
Shuswap, BC, Canada
http://www.davehylands.com