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In Kbuild, Makefiles can add the same object to obj-y multiple times. So, obj-y += foo.o obj-y += foo.o is fine. However, this is not true when the same object is added multiple times via composite objects. For example, obj-y += foo.o bar.o foo-objs := foo-bar-common.o foo-only.o bar-objs := foo-bar-common.o bar-only.o causes build error because two instances of foo-bar-common.o are linked into the vmlinux. Makefiles tend to invent ugly work-around, for example - lib/zstd/Makefile - drivers/net/ethernet/cavium/liquidio/Makefile The technique used in Kbuild to avoid the multiple definition error is to use $(filter $(obj-y), $^). Here, $^ lists the names of all the prerequisites with duplicated names removed. By replacing it with $(filter $(real-obj-y), $^) we can do likewise for composite objects. For built-in objects, we do not need to keep the composite object structure. We can simply expand them, and link $(real-obj-y) to built-in.a. Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.