分类：Linux kernel 笔记

inode结构体中有一个i_rdev成员（定义在<linux/fs.h>中）：

struct inode {
    ......
    dev_t           i_rdev;
    ......
}

如果inode代表一个设备，则i_rdev的值为设备号。为了代码更好地可移植性，获取inode的major和minor号应该使用imajor和iminor函数：

static inline unsigned iminor(const struct inode *inode)
{
    return MINOR(inode->i_rdev);
}

static inline unsigned imajor(const struct inode *inode)
{
    return MAJOR(inode->i_rdev);
}

LDD中对file结构体的描述：

struct file, defined in <linux/fs.h>, is the second most important data structure used in device drivers. Note that a file has nothing to do with the FILE pointers of user-space programs. A FILE is defined in the C library and never appears in kernel code. A struct file, on the other hand, is a kernel structure that never appears in user programs.

The file structure represents an open file . (It is not specific to device drivers; every open file in the system has an associated struct file in kernel space.) It is created by the kernel on open and is passed to any function that operates on the file, until the last close. After all instances of the file are closed, the kernel releases the data structure.

In the kernel sources, a pointer to struct file is usually called either file or filp (“file pointer”). We’ll consistently call the pointer filp to prevent ambiguities with the structure itself. Thus, file refers to the structure and filp to a pointer to the structure.

对inode结构体的描述：

The inode structure is used by the kernel internally to represent files. Therefore, it is different from the file structure that represents an open file descriptor. There can be numerous file structures representing multiple open descriptors on a single file, but they all point to a single inode structure.

总结如下：在kernel中，每一个文件都有一个inode结构体来表示，而file结构体是和打开的文件描述符关联的。如果一个文件被打开多次，有多个文件描述符，也就相应地有多个file结构体与这个文件关联。而inode却永远只有一个。

THIS_MODULE是一个macro，定义在<linux/module.h>中：

#ifdef MODULE
#define MODULE_GENERIC_TABLE(gtype,name)            \
extern const struct gtype##_id __mod_##gtype##_table        \
  __attribute__ ((unused, alias(__stringify(name))))

extern struct module __this_module;
#define THIS_MODULE (&__this_module)
#else  /* !MODULE */
#define MODULE_GENERIC_TABLE(gtype,name)
#define THIS_MODULE ((struct module *)0)
#endif

THIS_MODULE即是__this_module这个变量的地址。__this_module会指向这个模块起始的地址空间，恰好是struct module变量定义的位置。

file_operations结构体的第一个成员是struct module类型的指针，定义在<linux/fs.h>中：

struct file_operations {
    struct module *owner;
    ......
}

LDD对其的解释：

struct module *owner

The first file_operations field is not an operation at all; it is a pointer to the module that “owns” the structure. This field is used to prevent the module from being unloaded while its operations are in use. Almost all the time, it is simply initialized to THIS_MODULE , a macro defined in <linux/module.h>.

owner指向绑定file_operations的模块。在大多时候，只需把THIS_MODULE赋给它即可。

参考资料：
Where is the memory allocation of “_thismodule” variable?；
深入淺出 insmod, #1。

在kernel代码中，有时会看到函数声明中有的参数带有__user修饰符：

ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);

LDD给出的解释：

This annotation is a form of documentation, noting that a pointer is a user-space address that cannot be directly dereferenced. For normal compilation, __user has no effect, but it can be used by external checking software to find misuse of user-space addresses.

__user表明参数是一个用户空间的指针，不能在kernel代码中直接访问。也方便其它工具对代码进行检查。

<linux/version.h>是由顶级目录下的Makefile生成的：

......
define filechk_version.h
    (echo \#define LINUX_VERSION_CODE $(shell                         \
    expr $(VERSION) \* 65536 + 0$(PATCHLEVEL) \* 256 + 0$(SUBLEVEL)); \
    echo '#define KERNEL_VERSION(a,b,c) (((a) << 16) + ((b) << 8) + (c))';)
endef

$(version_h): $(srctree)/Makefile FORCE
    $(call filechk,version.h)
    $(Q)rm -f $(old_version_h)
......

它包含了LINUX_VERSION_CODE和KERNEL_VERSION这两个macro定义。以下面这个版本为例：

......
#define LINUX_VERSION_CODE 199680
#define KERNEL_VERSION(a,b,c) (((a) << 16) + ((b) << 8) + (c))
......

199680对应3.12.0版本。

参考资料：
How the “<linux/version.h>” file is generated?。