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I'm trying to compile a simple hello world module following this guide and I'm confused about what the Makefile is actually doing.

obj-m += hello-1.o

all:
    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules

clean:
    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

I understand that when i type the make command it will run the all recipe which runs make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules. So now it runs the Makefile found at the path given after the -C flag but what does the M=$(PWD) modules do?

arrowd
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Jordan Camp
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2 Answers2

15
  1. 'obj-m' :- specifies object files which are built as loadable kernel modules.

  2. 'all and clean' :- If you run 'make' by default it will run "all :". But we can use all and clean with make. it will run only those specific command.

    Example :-
        'make all' will run "make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules"
        'make clean will run "make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean"

3.'uname -r' :- get name and information about current kernel.

 Example :- for me, my kernel is "4.6.0-rc1".

  1. Option ‘-C dir’ :- Change to directory dir before reading the makefiles.

    Example :- "make -C /lib/modules/$(shell uname -r)/build" will change to "make -C /lib/modules/4.6.0-rc1/build.
  2. '$pwd':- get the path of your current directory.

Now you want to create your loadable module by using "make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules".

Your source code need environment to run. That's why we have to use -C option to change build directory. Which have all needed definition, header file, Macro and etc. Now after changing to build directory you need to tell where is your module present, that's why we are using M=$PWD.

Arvind Yadav
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Linux developer
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  • Actually, if you run ̀make`, by default it will run the first defined target, unless a variable named .DEFAULT_GOAL is defined to the name of another target. – Aurélien Martin Feb 14 '20 at 16:46
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To compile kernel module, the make command you normally need is in this form:

make -C /lib/modules/3.16.0-70-generic/build M=/home/test/ldd3/hello modules

in which, -C means switching to another path.

/lib/modules/3.16.0-70-generic/

is the path to the kernel in used, and

/home/test/ldd3/hello

is where the source of your module resides.

what does the M=$(PWD) modules do?

So as I said M=$(PWD) is simply a shell variable that stores the current path to your kernel module. make needs to store this as it switches to the kernel build path.

artm
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  • I downloaded my device kernel source code from http://openlinux.amlogic.com:8000/download/ARM/kernel/?C=M;O=D , but there is no build folder! – Dr.jacky Dec 11 '16 at 13:30