ifeq ($(KERNELRELEASE),)

# 假如沒有定義KERNELRELEASE的話，就會跑到這個分支
# 表示現在是由命令列叫用
# 於是可以再呼叫核心建構系統

# Assume the source tree is where the running kernel was built
# You should set KERNELDIR in the environment if it's elsewhere
KERNELDIR ?= /lib/modules/$(shell uname -r)/build
# The current directory is passed to sub-makes as argument
PWD := $(shell pwd)

modules:
        $(MAKE) -C $(KERNELDIR) M=$(PWD) modules

modules_install:
        $(MAKE) -C $(KERNELDIR) M=$(PWD) modules_install

clean:
        rm -rf *.o *~ core .depend .*.cmd *.ko *.mod.c .tmp_versions

.PHONY: modules modules_install clean

else
# called from kernel build system: just declare what our modules are
# 有定義KERNELRELEASE這個變數的話，會跑來這個分支
# 就可以知道是由核心建構系統所進行的呼叫
# 在這邊就可以定義想要編譯的核心模組

obj-m := hi.o

endif

#Q: modules v.s. modules_install