1. Linux Kernel Module Programming
Jason Barto
September 4, 2003
Jason Barto

2. Agenda Introduction Review Standard Functions Write Module v0.1
Compiling Modules
Handling Runtime Parameters
More Kernel Macros
Write Module v0.2
Close and Further Readings
Jason Barto

3. Introduction
Similar to Java Servlets with predefined structure and function names
Common C functions are not available to modules (such as printf, scanf, strcat, or system)
Instead you must use kernel defined functions (/proc/ksyms)
Jason Barto

4. Review Standard Functions
printk ()
8 Priority Levels: KERN_WARNING, KERN_ALERT
Init function sets up variables, tells the kernel what the module provides and registers any necessary functions
Cleanup function undoes changes made by init
module_init and module_exit
Tell kernel which functions are to be used at init and cleanup
Jason Barto

5. Module v0.1 /* module-0.1.c - The simplest kernel module. */
#include <linux/module.h> /* Needed by all modules */
#include <linux/kernel.h> /* Needed for KERN_ALERT */
#include <linux/init.h> /* Needed for macros */
int module_v0_1_init(void) {
printk(KERN_ALERT "Initing module v0.1\n"); /* A non 0 return means init_module failed; module can't be loaded. */
return 0; }
void module_v0_1_stop(void) {
printk(KERN_ALERT “Unloading module v0.1\n");
module_init (module_v0_1_init);
module_exit (module_v0_1_stop);
Jason Barto

6. Compiling Modules
Kernel modules are object files thus the ‘-c’ flag must be used with gcc
‘-O2’ causes the compiler to use optimization and thus translate kernel macros
‘-isystem /lib/modules/`uname -r`/build/include’ will tell gcc to include the proper kernel header files
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7. Compiling Modules cont..
‘-D__KERNEL__’ will define the KERNEL variable indicating this code will run in kernel mode, not as a user process
‘-DMODULE’ indicates to gcc that this code is a kernel module
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8. Handling Runtime Parameters
MODULE_PARM ()
2 arguments variable name and type (“b”, “h”, “i”, “l”, “s”)
A 2nd parameter of “1-5i” indicates an array of integers between 1 and 5 ints long
Declare parameter receiving variables globally
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9. More Kernel Macros MODULE_LICENSE () MODULE_AUTHOR ()
MODULE_DESCRIPTION ()
MODULE_SUPPORTED_DEVICE ()
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10. Module v0.2 /* module-0.2.c - The simplest kernel module. */
#include <linux/module.h> /* Needed by all modules */
#include <linux/kernel.h> /* Needed for KERN_ALERT */
#include <linux/init.h> /* Needed for macros */
MODULE_LICENSE (“GPL”)
MODULE_AUTHOR (“Jason Barto”);
MODULE_DESCRIPTION (“A module to teach module writing”);
MODULE_SUPPORTED_DEVICE (“The mind”);
int intparm = 0;
char *strparm;
MODULE_PARM (strparm, “s”);
MODULE_PARM (intparm, “i”);
Jason Barto

11. Module v0.2 cont.. int module_v0__init (void) {
printk (KERN_ALERT "Initing module v0.2\n");
printk (KERN_ALERT “Received an int of %d and a string of %s\n”, intparm, strparm);
return 0; }
void module_v0_2_stop (void) {
printk (KERN_ALERT “Unloading module v0.2\n");
module_init (module_v0_2_init);
module_exit (module_v0_2_stop);
Jason Barto

12. Further Reading Kernel Module Programming Guide
Kernel Device Drivers Introduction
Kernel Resource Links Page
Jason Barto