Makefile
# if not defined KERNELRELEASE, command is running from command line,need invoke kbuild system.
ifeq ($(KERNELRELEASE),)
KERNELDIR ?= /linux-2.6.30.4 # the directory of kernel source file
PWD := $(shell pwd) # the directory of module source file
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 # if defined KERNELRELEASE, kbuild running
else
obj-m:= hello.o # the source file of module endif
#################################################################
该Makefile 在模块构造过程中一般将被调用两次。使用make 时，第一次 KERNELRELEASE 尚未定义。KERNELDIR 将被定义为内核源码路径，PWD 将被赋值为模块源码文件所在路径。然后通过 $(MAKE) -C $(KERNELDIR) M=$(PWD) modules进入内核源码目录调用顶层Makefile 。顶层Makefile 会通过 M=$(PWD) 获得指定的模块源码所在路径，并调用其下的 Makefile。即该Makefile 将被调用第二次，此时将会执行else 后的语句，通过内核顶层Makefile 构建出模块。