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I'm working on my own netfilter module for the first time. According to internet documentation, The simplest module contains the following C code:

//'Hello World' kernel module, logs call to init_module
// and cleanup_module to /var/log/messages

// In Ubuntu 8.04 we use make and appropriate Makefile to compile kernel module

#define __KERNEL__
#define MODULE

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

int init_module(void)
{
 printk(KERN_INFO "init_module() called\n");
 return 0;
}

void cleanup_module(void)
{
 printk(KERN_INFO "cleanup_module() called\n");
}

Then the same page suggests the following contents for a makefile:

obj-m := hello.o
KDIR := /lib/modules/$(shell uname -r)/build
PWD := $(shell pwd)
default:
    $(MAKE) -C $(KDIR) SUBDIRS=$(PWD) modules

When I executed make on the command line, I got a no targets for "default" message.

However, when I changed the makefile to the following:

obj-m := hello.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

Executing "make" alone here worked correctly and the C compiler actually ran, and inserting and removing the module worked as expected.

I'm curious. Is the last makefile I showed compatible with every unix operating system (beyond version 2.24)? Currently I'm using Slackware 12 32-bit and I'll be also testing my code on CentOS 6 64-bit and If theres a common makefile that I can create, I'd rather do that, then create a separate makefile for each system.

Can someone give me advice here?

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AFAIK, this looks good. The default one I use is a little different. It comes from the Linux Device Drivers book

# 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 *.order *.symvers

.PHONY: modules modules_install clean

else
    # called from kernel build system: just declare what our modules are
    obj-m := hello.o
endif

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