The kernel build system, kbuild, is a set of make scripts that take the configuration information from the .config file, work out the dependencies and compile everything that is necessary to produce a kernel image containing all the statically linked components, possibly a device tree binary and possibly one or more kernel modules. The dependencies are expressed in the makefiles that are in each directory with buildable components. For instance, these two lines are taken from drivers/char/Makefile:

obj-y                    += mem.o random.o
obj-$(CONFIG_TTY_PRINTK) += ttyprintk.o

The obj-y rule unconditionally compiles a file to produce the target, so mem.c and random.c are always part of the kernel. In the second line, ttyprintk.c is dependent on a configuration ...

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