A program usually begins with a main() function, executes a bunch of instructions and terminates upon completion of those instructions. Kernel modules work a bit differently. A module always begin with either the init_module or the function you specify with module_init call. This is the entry function for modules; it tells the kernel what functionality the module provides and sets up the kernel to run the module’s functions when they’re needed. Once it does this, entry function returns and the module does nothing until the kernel wants to do something with the code that the module provides.
All modules end by calling either cleanup_module or the function you specify with the module_exit call. This is the exit function for modules; it undoes whatever entry function did. It unregisters the functionality that the entry function registered.
Every module must have an entry function and an exit function. Since there’s more than one way to specify entry and exit functions.
