Makefile拾遗（1）对-C和M=参数的理解

1.背景

在集成KVM的虚拟化平台上，为实现host主机和guest VM之间数据通信需要创建共享内存，其驱动的添加方式可参考Add device driver (uio_ivshmem.ko) on guest。由于才接触makefile，不懂的地方在此记录，如果理解有误，希望大佬指正。

2.编译生成uio_ivshmem.ko的makefile文件

# obj-m is a list of what kernel modules to build.  The .o and other
# objects will be automatically built from the corresponding .c file -
# no need to list the source files explicitly.

obj-m := uio_ivshmem.o 

# KDIR is the location of the kernel source.  The current standard is
# to link to the associated source tree from the directory containing
# the compiled modules.
KDIR  := /lib/modules/$(shell uname -r)/build

# PWD is the current working directory and the location of our module
# source files.
PWD   := $(shell pwd)

# default is the default make target.  The rule here says to run make
# with a working directory of the directory containing the kernel
# source and compile only the modules in the PWD (local) directory.
default:
        $(MAKE) -C $(KDIR) M=$(PWD) modules

install:
        cp uio_ivshmem.ko /lib/modules/$(shell uname -r)/kernel/drivers/uio/

clean:
        rm -f *.ko *.o uio_ivshmem.mod.c Module.symvers

3.代码解释

（1）命令行敲入make，读取当前目录下的makefile，执行的target为：default

（2）$(MAKE) -C $(KDIR) M=$(PWD) modules 这行代码里，$(MAKE)相当于make，若去掉参数，整行代码可先看做  make modules，说明这个命令用于编译内核模块。

（3）-C参数是选择路径，加上$(KDIR)，就会去执行该目录下的makefile，M 参数是编译外部模块，生成的模块不会直接加载在内核中，M="dir"则是编译的路径。

参考博文：

https://blog.youkuaiyun.com/importantg/article/details/51763551

https://blog.youkuaiyun.com/xiaowulang20082008/article/details/50586985

https://blog.youkuaiyun.com/lgjjeff/article/details/90489331