__init and __exit

The __init and __exit declarations are special kernel macros designed to tell the kernel to flag these
functions for special handling in cases where they are compiled in statically rather than included as
parts of modules.
 The __init declaration allows the kernel to reclaim the space used by initialization
functions, while the __exit declaration tells the kernel to just plain ignore the function altogether. If
you’re only going to write your device driver as a module, with no possibility that it will be included
statically, it is perfectly safe to leave these special declarations out.

1 __init：

/* These macros are used to mark some functions or
 * initialized data (doesn't apply to uninitialized data)
 * as `initialization' functions. The kernel can take this
 * as hint that the function is used only during the initialization
 * phase and free up used memory resources after
 *
 * Usage:
 * For functions:
 *

 * You should add __init immediately before the function name, like:
 *
 * static void __init initme(int x, int y)
 * {
 *    extern int z; z = x * y;
 * }
 *
 * If the function has a prototype somewhere, you can also add
 * __init between closing brace of the prototype and semicolon:
 *
 * extern int initialize_foobar_device(int, int, int) __init;
 *
 * For initialized data:
 * You should insert __initdata between the variable name and equal
 * sign followed by value, e.g.:
 *
 * static int init_variable __initdata = 0;
 * static char linux_logo[] __initdata = { 0x32, 0x36, ... };
 *
 * Don't forget to initialize data not at file scope, i.e. within a function,
 * as gcc otherwise puts the data into the bss section and not into the init
 * section.
 *
 * Also note, that this data cannot be "const".

其主要作用是初始化.

2)module_init：
/**
 * module_init() - driver initialization entry point
 * @x: function to be run at kernel boot time or module insertion
 *
 * module_init() will add the driver initialization routine in
 * the "__initcall.int" code segment if the driver is checked as
 * "y" or static, or else it will wrap the driver initialization
 * routine with init_module() which is used by insmod and
 * modprobe when the driver is used as a module.
 */
/**
 * module_exit() - driver exit entry point
 * @x: function to be run when driver is removed
 *
 * module_exit() will wrap the driver clean-up code
 * with cleanup_module() when used with rmmod when
 * the driver is a module.  If the driver is statically
 * compiled into the kernel, module_exit() has no effect.
 */

注意一下init和cleanup这两个函数定义的变化。__init宏使内建模块中的init函数在执行完成后释放掉，不过可装载的模块不受影响。如果你关心init函数什么时候调用，这一点是很有用的。

还有个__initdata，和__init的作用基本上一样，不过它是针对变量而不是函数的。

__exit宏会使那些内建到内核的模块省略掉cleanup函数，不过和__init一样，对loadable模块没影响。再说一遍，如果你关心cleanup运行的时机，这是重要的。Built-in的驱动不需要cleanup，反正它们也不能退出，不过loadable式的模块显然是需要一个的。

这些宏都定义在linux/init.h中，它们会释放内核的内存。你启动内核的时候会看到一些诸如Freeing unused kernel memory:236k freed，之类的信息，这多半就是它们干的。

_init修饰，以及.init开头的节，在模块插入完毕运行之后，这一部分
的内存空间会被释放，因此dump的时候这些信息是无法得到的。

内核模块至少要有两个函数：一个叫做init_module()的“start”初始化函数供insmod的时候调用，一个叫clean_module()的“end”清除函数供rmmod的时候调用。实际上，2.3.13之后的内核这两个函数不再必须使用这两个名字了。你给它们起什么名字都可以，2.3节我会将具体怎么做。给这两个函数自己起名字其实是个挺好的主意，不过还是有很多人使用init_module和clean_module函数。L

很经典的，init_module函数会向内核注册一些什么东西，或者用自己的代码替换内核的一些什么功能（通常它们做点什么动作之后还是会调用原来的函数）。而clean_module函数则会把init_module干的事情做个了结，以便安全的卸载模块。

最后，每个内核模块都要包含linux/module.h。仅仅为了KERL_ALERT（2.1.1节会讲到它）还需要在这个例子中包括linux/kernel.h。