2.7. Modules Spanning Multiple Files

Sometimes it makes sense to divide a kernel module between several source files. In this case, you need to:

  1. In all the source files but one, add the line #define __NO_VERSION__. This is important because module.h normally includes the definition of kernel_version, a global variable with the kernel version the module is compiled for. If you need version.h, you need to include it yourself, because module.h won't do it for you with __NO_VERSION__.

  2. Compile all the source files as usual.

  3. Combine all the object files into a single one. Under x86, use ld -m elf_i386 -r -o <module name.o> <1st src file.o> <2nd src file.o>.

Here's an example of such a kernel module.

Example 2-8. start.c

/*  start.c - Illustration of multi filed modules

#include <linux/kernel.h>       /* We're doing kernel work */
#include <linux/module.h>       /* Specifically, a module */

int init_module(void)
  printk("Hello, world - this is the kernel speaking\n");
  return 0;

The next file:

Example 2-9. stop.c

/*  stop.c - Illustration of multi filed modules

#if defined(CONFIG_MODVERSIONS) && ! defined(MODVERSIONS)
   #include <linux/modversions.h> /* Will be explained later */
   #define MODVERSIONS
#include <linux/kernel.h>  /* We're doing kernel work */
#include <linux/module.h>  /* Specifically, a module  */
#define __NO_VERSION__     /* It's not THE file of the kernel module */
#include <linux/version.h> /* Not included by module.h because of
	                                      __NO_VERSION__ */
void cleanup_module()
   printk("<1>Short is the life of a kernel module\n");

And finally, the makefile:

Example 2-10. Makefile for a multi-filed module

hello.o:	hello2_start.o hello2_stop.o
   ld -m elf_i386 -r -o hello2.o hello2_start.o hello2_stop.o
start.o: hello2_start.c
   ${CC} ${MODCFLAGS} -c hello2_start.c
stop.o: hello2_stop.c
   ${CC} ${MODCFLAGS} -c hello2_stop.c