Linux系统编程，Linux中的文件读写操作

1.open函数，打开文件

我们来看下常用的open函数

这个函数最终返回一个文件描述符struct file

我们查看一下它的Ubuntu内核源代码

struct file {

　　union {

　　      struct list_head fu_list; //文件对象链表指针linux/include/linux/list.h

　　      struct rcu_head fu_rcuhead; //RCU(Read-Copy Update)是Linux 2.6内核中新的锁机制

　　 } f_u;

　　struct path f_path; //包含dentry和mnt两个成员，用于确定文件路径

　　#define f_dentry f_path.dentry //f_path的成员之一，当前文件的dentry结构

　　#define f_vfsmnt f_path.mnt //表示当前文件所在文件系统的挂载根目录

　　const struct file_operations *f_op; //与该文件相关联的操作函数

　　atomic_t f_count; //文件的引用计数(有多少进程打开该文件)

　　unsigned int f_flags; //对应于open时指定的flag

　　mode_t f_mode; //读写模式：open的mod_t mode参数

       loff_t     f_pos;//当前文件指针位置

　　off_t f_pos; //该文件在当前进程中的文件偏移量

　　struct fown_struct f_owner; //该结构的作用是通过信号进行I/O时间通知的数据。

　　unsigned int f_uid, f_gid;// 文件所有者id，所有者组id

　　struct file_ra_state f_ra; //在linux/include/linux/fs.h中定义，文件预读相关

　　unsigned long f_version;//记录文件的版本号，每次使用之后递增

　　#ifdef CONFIG_SECURITY

　　     void *f_security;

　　#endif

　　/* needed for tty driver, and maybe others */

　　void *private_data;//使用这个成员来指向分配的数据

　　#ifdef CONFIG_EPOLL

　　/* Used by fs/eventpoll.c to link all the hooks to this file */

　　    struct list_head f_ep_links;

　　    spinlock_t f_ep_lock;

　　#endif /* #ifdef CONFIG_EPOLL */

　　struct address_space *f_mapping;

　　};

我们使用open函数的时候，传入的参数就会被记录在内核中，

int open(const char *pathname, int flags, mode_t mode);

返回int就是对应的file在数组中的位置，struct file[]，

flags --》unsigned int f_flags; //对应于open时指定的flag

mode --》mode_t f_mode; //读写模式：open的mod_t mode参数

loff_t f_pos;//当前文件指针位置，文件读写的时候的偏移地址

我们看下file_operations结构体，应用层的代码要和这个结构体的相映射。

struct file_operations {
	struct module *owner;
	loff_t (*llseek) (struct file *, loff_t, int);
	ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
	ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
	ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
	ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
	int (*readdir) (struct file *, void *, filldir_t);
	unsigned int (*poll) (struct file *, struct poll_table_struct *);
	int (*ioctl) (struct inode *, struct file *, unsigned int, unsigned long);
	long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
	long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
	int (*mmap) (struct file *, struct vm_area_struct *);
	int (*open) (struct inode *, struct file *);
	int (*flush) (struct file *, fl_owner_t id);
	int (*release) (struct inode *, struct file *);
	int (*fsync) (struct file *, int datasync);
	int (*aio_fsync) (struct kiocb *, int datasync);
	int (*fasync) (int, struct file *, int);
	int (*lock) (struct file *, int, struct file_lock *);
	ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
	unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
	int (*check_flags)(int);
	int (*flock) (struct file *, int, struct file_lock *);
	ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
	ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
	int (*setlease)(struct file *, long, struct file_lock **);
};