转自：http://www.cnblogs.com/Jezze/archive/2011/12/23/2299861.html
简单归纳：fd只是一个整数，在open时产生。起到一个索引的作用，进程通过PCB中的文件描述符表找到该fd所指向的文件指针filp。
open:文件描述符的操作(如: open)返回的是一个文件描述符(int fd),内核会在每个进程空间中维护一个文件描述符表, 所有打开的文件都将通过此表中的文件描述符来引用(fd1,fd2,fd3...);
fopen:而流(如: fopen)返回的是一个FILE结构指针, FILE结构是包含有文件描述符的，FILE结构函数可以看作是对fd直接操作的系统调用的封装, 它的优点是带有I/O缓存.
Linux支持各种各样的文件系统格式，如ext2、ext3、reiserfs、FAT、NTFS、iso9660等等，不同的磁盘分区、光盘或其它存储设备都有不同的文件系统格式，然而这些文件系统都可以mount到某个目录下，使我们看到一个统一的目录树，各种文件系统上的目录和文件我们用ls命令看起来是一样的，读写操作用起来也都是一样的，这是怎么做到的呢？Linux内核在各种不同的文件系统格式之上做了一个抽象层，使得文件、目录、读写访问等概念成为抽象层的概念，因此各种文件系统看起来用起来都一样(VFS作用)，这个抽象层称为虚拟文件系统（VFS，Virtual Filesystem）,这一节我们介绍运行时文件系统在内核中的表示。
Linux内核的VFS子系统可以图示如下：
每个进程在PCB（Process Control Block）即进程控制块中都保存着一份文件描述符表，文件描述符就是这个表的索引，文件描述表中每个表项都有一个指向已打开文件的指针，现在我们明确一下：已打开的文件在内核中用file结构体表示，文件描述符表中的指针指向file结构体(理解：fd为打开文件的文件描述符，而每个进程都有一张文件描述表，fd文件描述符就是这张表的索引，同样这张表中有一表项，该表项又是指向前面提到打开文件的file结构体，file结构体才是内核中用于描述文件属性的结构体)。
struct file-----define in inlcude/linux/fs.h
struct file_operations------define in include/linux/fs.h
1 struct file { 2 union { 3 struct llist_node fu_llist; 4 struct rcu_head fu_rcuhead; 5 } f_u; 6 struct path f_path; 7 #define f_dentry f_path.dentry 8 struct inode *f_inode; /* cached value */ 9 const struct file_operations *f_op; 10 11 /* 12 * Protects f_ep_links, f_flags. 13 * Must not be taken from IRQ context. 14 */ 15 spinlock_t f_lock; 16 atomic_long_t f_count; 17 unsigned int f_flags; 18 fmode_t f_mode; 19 struct mutex f_pos_lock; 20 loff_t f_pos; 21 struct fown_struct f_owner; 22 const struct cred *f_cred; 23 struct file_ra_state f_ra; 24 25 u64 f_version; 26 #ifdef CONFIG_SECURITY 27 void *f_security; 28 #endif 29 /* needed for tty driver, and maybe others */ 30 void *private_data; 31 32 #ifdef CONFIG_EPOLL 33 /* Used by fs/eventpoll.c to link all the hooks to this file */ 34 struct list_head f_ep_links; 35 struct list_head f_tfile_llink; 36 #endif /* #ifdef CONFIG_EPOLL */ 37 struct address_space *f_mapping; 38 #ifdef CONFIG_DEBUG_WRITECOUNT 39 unsigned long f_mnt_write_state; 40 #endif 41 } __attribute__((aligned(4))); /* lest something weird decides that 2 is OK */
1 struct file_operations { 2 struct module *owner; 3 loff_t (*llseek) (struct file *, loff_t, int); 4 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *); 5 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *); 6 ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t); 7 ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t); 8 int (*iterate) (struct file *, struct dir_context *); 9 unsigned int (*poll) (struct file *, struct poll_table_struct *); 10 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long); 11 long (*compat_ioctl) (struct file *, unsigned int, unsigned long); 12 int (*mmap) (struct file *, struct vm_area_struct *); 13 int (*open) (struct inode *, struct file *); 14 int (*flush) (struct file *, fl_owner_t id); 15 int (*release) (struct inode *, struct file *); 16 int (*fsync) (struct file *, loff_t, loff_t, int datasync); 17 int (*aio_fsync) (struct kiocb *, int datasync); 18 int (*fasync) (int, struct file *, int); 19 int (*lock) (struct file *, int, struct file_lock *); 20 ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int); 21 unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long); 22 int (*check_flags)(int); 23 int (*flock) (struct file *, int, struct file_lock *); 24 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int); 25 ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int); 26 int (*setlease)(struct file *, long, struct file_lock **); 27 long (*fallocate)(struct file *file, int mode, loff_t offset, 28 loff_t len); 29 int (*show_fdinfo)(struct seq_file *m, struct file *f); 30 };