Purpose of `#ifdef MODULE` around module_exit()?

Question

I am currently looking through the code of a "third-party" driver in an attempt to figure out/learn how it functions. I've had a look at sites such as this one, so I sort of understand how the basic premise works, but I don't understand the purpose of #ifdef MODULE here. Google isn't really much help, but I think the definition refers to a kernel module? (I am also completely new to this.)

module_init(os_driver_init);
#ifdef MODULE
module_exit(os_driver_cleanup);
#endif

My question is, what happens if I remove the #ifdef statement? Also, why/when would it be necessary to include the #ifdef statement?

Answer 1

In the Linux kernel, most drivers can be either statically linked (built-in) to the kernel image itself, or built as dynamically-loaded modules (.ko files).

The MODULE macro is defined for a C file when it is being compiled as part of a module, and undefined when it is being built directly into the kernel.

The code you're showing is only defining os_driver_cleanup as a module-exit function when it is being compiled as a module. However, this construct is unnecessary in modern kernel code; include/linux/init.h defines module_exit() as a macro, whose implementation depends on #ifdef MODULE.

Basically, you should always provide an exit function, and leave off the #ifdef around module_exit(). You should also mark your exit function with __exit, which will properly control inclusion of the code for your in the modular/non-modular case.

Here's an example of proper init/exit code.

static int  __init foo_init(void)
{
    /* Register driver, etc. */
}

static void __exit foo_cleanup(void)
{
    /* Unregister driver, etc. */
}

module_init(foo_init);
module_exit(foo_cleanup);