Basic Kernel Module Programming

See The Linux Kernel Module Programming Guide http://www.faqs.org/docs/kernel/. (or http://www.tldp.org/LDP/lkmpg/2.4/html/index.html)

commands and files for module programming: lsmod, insmod, rmmod, /var/log/messages, console

functions/macros and header files for kernel module:

    - header files:
        + linux/module.h: for kernel module
        + linux/kernel.h: for printk/KERN_ALERT
        + linux/init.h: for macros
    - functions and macros:
        + init_module(): for initializing/loading module
        + cleanup_module(): for removing module
        + module_init(), module_exit()
        + __initdata, __init, __exit
        + MODULE_LICENSE(), MODULE_DESCRIPTION(), MODULE_AUTHOR(), MODULE_SUPPORTED_DEVICE(): for licensing
        + MODULE_PARM(): for passing arguments
        + printk(), KERN_ALERT: print messages to console or log file

compiling methods:

Using Makefile: your Makefile will be

TARGET  := hello
WARN    := -W -Wall -Wstrict-prototypes -Wmissing-prototypes
INCLUDE := -isystem /lib/modules/`uname -r`/build/include
CFLAGS  := -O2 -DMODULE -D__KERNEL__ ${WARN} ${INCLUDE}
CC      := gcc
         
${TARGET}.o: ${TARGET}.c
        
.PHONY: clean
         
clean:
        rm -rf {TARGET}.o

Compile with Makefile:

[user@hostname src]$ make
gcc -O2 -DMODULE -D__KERNEL__ -W -Wall -Wstrict-prototypes -Wmissing-prototypes -isystem /lib/modules/`uname -r`/build/include   -c -o hello.o hello.c
hello.c:5: warning: no previous prototype for `init_module'
hello.c:12: warning: no previous prototype for `cleanup_module'
/lib/modules/2.4.26/build/include/asm/processor.h: In function `copy_segments':
/lib/modules/2.4.26/build/include/asm/processor.h:433: warning: unused parameter `p'
/lib/modules/2.4.26/build/include/asm/processor.h:433: warning: unused parameter `mm'
/lib/modules/2.4.26/build/include/asm/processor.h: In function `release_segments':
/lib/modules/2.4.26/build/include/asm/processor.h:434: warning: unused parameter `mm'
/lib/modules/2.4.26/build/include/linux/prefetch.h: In function `prefetch':
/lib/modules/2.4.26/build/include/linux/prefetch.h:43: warning: unused parameter `x'
/lib/modules/2.4.26/build/include/linux/prefetch.h: In function `prefetchw':
/lib/modules/2.4.26/build/include/linux/prefetch.h:48: warning: unused parameter `x'
[user@hostname src]$

Using just "gcc options": compile with gcc (on 2.4 kernel):

[user@hostname src]$ gcc -D__KERNEL__ -DMODULE -Wall -O2 -isystem /lib/modules/`uname -r`/build/include -c hello.c

loading/checking/removing kernel module:

    - Switch to super user (root)
    - [user@hostname src]$ insmod hello.o
      Warning: loading hello.o will taint the kernel: no license
        See http://www.tux.org/lkml/#export-tainted for information about tainted modules
      Module hello loaded, with warnings
      [user@hostname src]$ lsmod
      Module                  Size  Used by    Tainted: P
      hello                    796   0  (unused)
      soundcore               6404   0  (autoclean)
      ...
      [user@hostname src]$ rmmod hello
    - Also you can use commands like these (including /sbin/):
      [user@hostname src]$ /sbin/insmod
      [user@hostname src]$ /sbin/lsmod
      [user@hostname src]$ /sbin/rmmod

Checking log messages:
log messages are located in the file "/var/log/messages":

      [user@hostname src]$ tail /var/log/messages
      ...
      Jan 25 02:49:57 {hostname} kernel: module inserted
      Jan 25 02:52:14 {hostname} kernel: module deleted
      [user@hostname src]$

Those "module inserted"/"module deleted" messages above are hard-coded in the source code with "printk" function