[Kernel Programming: #2] module_init and module_exit macro use

Learning Kernel Programming Day2:
Have already explained how to insert/remove a module to/from kernel at runtime in the kernel programming #1 post.
Please go through it before reading this post #2.

We can put our own names for initialization function and cleanup functions using two macros. They are,
1. module_init
2. module_exit
These two macros allows module owners to put their own init() and cleanup() names in the module. These macros are defined in linux/init.h.

Example: module_2.c

  1 #include<linux/module.h> /* Needed by all modules *MUST* */
  2 #include<linux/init.h> /* Needed for module_init and module_exit macros */
  3 int __init module_2_init(void) {
  4         printk(KERN_ALERT "2nd module program: Module Inserted\n");
  5         return 0;
  6 }
  7
  8 void module_2_cleanup() {
  9         printk(KERN_ALERT "2nd module program: Module Removed\n");
 10 }
 11
 12 module_init(module_2_init);
 13 module_exit(module_2_cleanup);

Note: Please define the init() and cleanup() functions before using these macros only, otherwise you end up with compilation errors.

Next steps:
Build module:
Insert module:
Check kernel log:
Remove module: The details for these steps are already provided in Kernel Programming #1 post. Please refer it.

Observation:
The return value of init function is either 0 or -E (error number). Change the value of the return with 1 and -1 values and check the output.

If the return value 1 is used then we see the below message after inserting module using insmod.
[1306301.170326] 2nd module program: Module Inserted
[1306301.170331] sys_init_module: module_2'->init suspiciously returned 1, it should follow 0/-E convention
[1306301.170333] sys_init_module: loading module anyway...
[1306301.170337] Pid: 23621, comm: insmod Tainted: P           2.6.32-28-generic #55-Ubuntu
[1306301.170339] Call Trace:
[1306301.170349]  [<ffffffff810a0e61>] sys_init_module+0x1e1/0x260
[1306301.170356]  [<ffffffff810121b2>] system_call_fastpath+0x16/0x1b

If the return value -1 is (any error code) used then the result will be
insmod: error inserting 'rtc.ko': -1 Operation not permitted

i.e. our module is not inserted/loaded into kernel because of return value -1.