Understanding Linux Network Internals

Module Initialization Code

Because the examples in the following sections often refer to modules , a couple of initial concepts have to be made clear.

Kernel code can be either statically linked to the main image or loaded dynamically as a module when needed. Not all kernel components are suitable to be compiled as modules. Device drivers and extensions to basic functionalities are good examples of kernel components often compiled as modules. You can refer to Linux Device Drivers for a detailed discussion of the advantages and disadvantages of modules, as well as the mechanisms that the kernel can use to dynamically load them when they are needed and unload them when they are no longer needed.

Every module must provide two special functions, called init_module and cleanup_module. The first one is called at module load time to initialize the module. The second one is invoked by the kernel when removing the module, to release any resources (memory included) that have been allocated for use by the module.

The kernel provides two macros, module_init and module_exit, that allow developers to use arbitrary names for the two routines. The following snapshot is an example from the drivers/net/3c59x.c Ethernet driver:

module_init(vortex_init);
module_exit(vortex_cleanup);

In the section "Memory Optimizations," we will see how those two macros are defined and how their definition can change based on the kernel configuration. Most of the kernel uses these two macros, but a few modules still use ...