A simple program on linux device driver

Question

include

#include<linux/module.h>
#include<linux/init.h>

int my_init(void){
        printk("<1> Angus : Module Insertion is successful!");
        return 0;
}

void my_cleanup(void){
        printk("<1> Angus : Module unloading successful!");
}

module_init(my_init);
module_cleanup(my_cleanup);

Makefile :

obj-m:=simple.o
aoll:
        make -C /usr/src/linux-headers-3.2.0-25-generic-pae/ M=$(PWD) modules
clean:

        make -C /usr/src/linux-headers-3.2.0-25-generic-pae/ M=$(PWD) clean

make -C => will change to the directory before doing a make, In this path /usr/src/linux-headers-3.2.0-25-generic-pae/ I have Makefile , why is the M=$(PWD) needed ? what does it do, where I can check for $PWD ? The Makefile inside the /usr/src/linux-headers-3.2.0-25-generic-pae/ has the target all:modules and target modules and has the target clean. What is obj-m ?

Answer 1

You'd better to read the paragraph at page 24 of the book Linux Device Drivers, 3rd edition (freely available at http://oreilly.com/openbook/linuxdrive3/book/index.html ).

The -C option makes it change the directory to the one provided. There, it finds the kernel's top-level Makefile. Then, the M= option causes that Makefile to move back to your module source directory before trying to build the modules target ( $PWD is a variable containing the path of your current directory).

obj-m is a variable containing the list of kernel modules to be build (see https://www.kernel.org/doc/Documentation/kbuild/makefiles.txt ) .

Answer 2

You could change your Makefile rules:

aoll:
    (cd /usr/src/linux-headers-3.2.0-25-generic-pae/;echo $(PWD);make m=$(PWD) module)

Answer 3

why is the M=$(PWD) needed ?

The M= option causes that makefile to move back into your module source directory before trying to build the modules target . This target, in turn, refers to the list of modules found in the obj-m variable.

What is obj-m ?

The assignment above states that there is one module to be built from the object file hello.o. The resulting module is named hello.ko after being built from the object file.