Understanding the Linux Kernel, 3rd Edition

Module Implementation

Modules are stored in the filesystem as ELF object files and are linked to the kernel by executing the insmod program (see the later section, "Linking and Unlinking Modules“). For each module, the kernel allocates a memory area containing the following data:

• A module object

• A null-terminated string that represents the name of the module (all modules must have unique names)

• The code that implements the functions of the module

The module object describes a module; its fields are shown in Table B-1. A doubly linked circular list collects all module objects; the list head is stored in the modules variable, while the pointers to the adjacent elements are stored in the list field of each module object.

Table B-1. The module object

Type	Name	Description
enum module_state	state	The internal state of the module
struct list_head	list	Pointers for the list of modules
char [60]	name	The module name
struct module_kobject	mkobj	Includes a kobject data structure and a pointer to this module object
struct module_param_attrs *	param_attrs	Pointer to an array of module parameter descriptors
const struct kernel_symbol *	syms	Pointer to an array of exported symbols
unsigned int	num_syms	Number of exported symbols
const unsigned long *	crcs	Pointer to an array of CRC values for the exported symbols
const struct kernel_symbol *	gpl_syms	Pointer to an array of GPL-exported symbols
unsigned int	num_gpl_syms	Number of GPL-exported symbols
const unsigned long *