关于kbuild中KERNELRELEASE的疑问

雁子依然 2013-07-02 02:48:53

有个小问题，关于KERNELRELEASE，我的理解是不是也可以不用去判断这个变量的定义呢？
LDD3里面hello的例子makefile如下





# To build modules outside of the kernel tree, we run "make"

# in the kernel source tree; the Makefile these then includes this

# Makefile once again.

# This conditional selects whether we are being included from the

# kernel Makefile or not.

ifeq ($(KERNELRELEASE),)



    # 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

    obj-m := hello.o hellop.o seq.o jit.o jiq.o sleepy.o complete.o \

             silly.o faulty.o kdatasize.o kdataalign.o

endif

我试着把ifeq else endif这三行注释，也可以编译成功，我理解是不是这样：
1. 在模块所在目录执行make，根据makefile规则，会先去找第一个目标modules，执行后面的命令$(MAKE) -C $(KERNELDIR) M=$(PWD) modules

2. 进入kernel目录，kbuild开始工作
3. 根据M=参数，生成模块之前回到当前目录再次读取makefile里面的obj-m

所以是不是说KERNELRELEASE定义与否，效果都是一样的呢

当然有一个例外就是，如果在当前目录下make的时候直接就make KERNELRELEASE=y，那就会编译出错，但是应该不会有人这么做吧，或者说不会出现这样的情况吧？

可以帮住解答一下么？

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