Kbuild Makefile#

These aren’t plain GNU Make, Kbuild (the kernel’s build system) reads them with special meaning when you run make -C /lib/modules/$(uname -r)/build M=$(pwd) modules.

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CFLAGS_main.o := -DDEBUG

ccflags-y := -std=gnu99 -DENABLE_DEBUG

proc_fs_basic-objs := main.o

obj-m := proc_fs_basic.o

  • CFLAGS_main.o - per-object compiler flags. Only main.o gets -DDEBUG; if you had main.o and helper.o, only the first sees the macro. Useful for debug-gating one file without recompiling the whole module noisily.

  • ccflags-y - flags applied to every .o in this Makefile (the module-wide equivalent of CFLAGS). Here it forces gnu99 and defines ENABLE_DEBUG globally.

  • proc_fs_basic-objs - when a module is built from more than one source file, this lists what gets linked into the final .ko. Pattern is <module_name>-objs := a.o b.o c.o (Kbuild also accepts -y instead of -objs in newer trees).

  • obj-m - the actual build trigger. obj-m = build as a loadable module → proc_fs_basic.ko. Swap it for obj-y and it’d get compiled directly into the kernel image instead.

Order of resolution: obj-m → Kbuild sees proc_fs_basic.o is wanted → checks for proc_fs_basic-objs → pulls in main.o → applies CFLAGS_main.o + ccflags-y while compiling it.

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