Hello World (part 4): Licensing and Module Documentation

If you're running kernel 2.4 or later, you might have noticed something like this when you loaded the previous example modules:

# insmod hello-3.o
Warning: loading hello-3.o will taint the kernel: no license
  See http://www.tux.org/lkml/#export-tainted for information about tainted modules
Hello, world 3
Module hello-3 loaded, with warnings

In kernel 2.4 and later, a mechanism was devised to identify code licensed under the GPL (and friends) so people can be warned that the code is non open-source. This is accomplished by the MODULE_LICENSE() macro which is demonstrated in the next piece of code. By setting the license to GPL, you can keep the warning from being printed. This license mechanism is defined and documented in linux/module.h.

Similarly, MODULE_DESCRIPTION() is used to describe what the module does, MODULE_AUTHOR() declares the module's author, and MODULE_SUPPORTED_DEVICE() declares what types of devices the module supports.

These macros are all defined in linux/module.h and aren't used by the kernel itself. They're simply for documentation and can be viewed by a tool like objdump. As an exercise to the reader, try grepping through linux/drivers to see how module authors use these macros to document their modules.

Example 2-6. hello-4.c

/*  hello-4.c - Demonstrates module documentation.
 *  Copyright (C) 2001 by Peter Jay Salzman
 *  08/02/2006 - Updated by Rodrigo Rubira Branco <rodrigo@kernelhacking.com>

/* Kernel Programming */
#define MODULE
#define LINUX
#define __KERNEL__

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#define DRIVER_AUTHOR "Peter Jay Salzman <p@dirac.org>"
#define DRIVER_DESC   "A sample driver"

int init_hello_3(void);
void cleanup_hello_3(void);

static int init_hello_4(void)
   printk(KERN_ALERT "Hello, world 4\n");
   return 0;

static void cleanup_hello_4(void)
   printk(KERN_ALERT "Goodbye, world 4\n");


/*  You can use strings, like this:
MODULE_LICENSE("GPL");           // Get rid of taint message by declaring code as GPL.

/*  Or with defines, like this:
MODULE_AUTHOR(DRIVER_AUTHOR);    // Who wrote this module?
MODULE_DESCRIPTION(DRIVER_DESC); // What does this module do?

/*  This module uses /dev/testdevice.  The MODULE_SUPPORTED_DEVICE macro might be used in
 *  the future to help automatic configuration of modules, but is currently unused other
 *  than for documentation purposes.