如何通过文件描述符找到文件？

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#文件描述符表

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内核维护了进程级的文件描述符表、系统级的打开文件表和文件系统的i-node表。文件描述符用于索引文件表，其中包含文件的相关信息，如打开文件句柄和文件元数据。结构体`struct file`和`struct inode`分别保存文件描述符和磁盘文件位置的详细信息，使得进程能进行文件读写操作。

内核维护了3 个数据结构：
• 进程级的文件描述符表。（为单个进程进行维护）
• 系统级的打开文件表。（为所有打开的文件维护，表名：open file descri）
• 文件系统的i-node 表。（inode---index node）

内核对所有打开的文件维护有一个系统级的描述表格(open file description table)。

有时，也称之为打开文件表(open file table), 并将表中各条目称为打开文件句柄(open file handle)。

内核为每个进程维护一个打开文件的列表，该表被称为文件表(file table)；

该表的每一条目都记录了单个文件描述符的相关信息；

• 控制文件描述符操作的一组标志；
• 对打开文件句柄的引用；

文件表通过非负整数fds(file descriptors)进行索引；

https://elixir.bootlin.com/linux/v5.7.8/source/include/linux/fdtable.h#L48

/*
 * Open file table structure 描述打开文件信息的结构体变量
 */
struct files_struct {
  /*
   * read mostly part
   */
	atomic_t count; 
	bool resize_in_progress;
	wait_queue_head_t resize_wait;

	struct fdtable __rcu *fdt;
	struct fdtable fdtab;
  /*
   * written part on a separate cache line in SMP
   */
	spinlock_t file_lock ____cacheline_aligned_in_smp;
	unsigned int next_fd;
	unsigned long close_on_exec_init[1];
	unsigned long open_fds_init[1];
	unsigned long full_fds_bits_init[1];
	struct file __rcu * fd_array[NR_OPEN_DEFAULT];//表示内核为进程维护的文件表；NR_OPEN_DEFAULT表示一个进程可以打开文件的最大个数；该数组的索引项对应fd(file descriptors),通过fd索引对应的打开的文件的信息；所谓的表本质是个结构体指针数组
};

为什么进程能够通过file结构体来找到磁盘上相应文件的位置来进行文件读写？

文件表中每项---文件表项(“struct file”指针)均包含一个打开文件的信息，其中包括一个指向文件备份inode内存拷贝的指针和元数据；

进程每打开一个文件(这里指不同的文件)，内存中均会有一个“struct file”结构体生成；

内核为每个进程维护的文件表本质是一个“struct file”指针数组，“struct file”见https://elixir.bootlin.com/linux/v5.7.8/source/include/linux/fs.h#L941

struct file {
	union {
		struct llist_node	fu_llist;
		struct rcu_head 	fu_rcuhead;
	} f_u;
	struct path		f_path;   //文件路径
	struct inode		*f_inode;	/* cached value */指向文件备份inode内存拷贝的指针和元数据

	const struct file_operations	*f_op; //对文件的操作方法

	/*
	 * Protects f_ep_links, f_flags.
	 * Must not be taken from IRQ context.
	 */
	spinlock_t		f_lock;
	enum rw_hint		f_write_hint;
	atomic_long_t		f_count;
	unsigned int 		f_flags;
	fmode_t			f_mode;
	struct mutex		f_pos_lock;
	loff_t			f_pos;
	struct fown_struct	f_owner;
	const struct cred	*f_cred;
	struct file_ra_state	f_ra;

	u64			f_version;
#ifdef CONFIG_SECURITY
	void			*f_security;
#endif
	/* needed for tty driver, and maybe others */
	void			*private_data;

#ifdef CONFIG_EPOLL
	/* Used by fs/eventpoll.c to link all the hooks to this file */
	struct list_head	f_ep_links;
	struct list_head	f_tfile_llink;
#endif /* #ifdef CONFIG_EPOLL */
	struct address_space	*f_mapping;
	errseq_t		f_wb_err;
} __randomize_layout
  __attribute__((aligned(4)));	/* lest something weird decides that 2 is OK */

“struct inode”结构体，https://elixir.bootlin.com/linux/v5.7.8/source/include/linux/fs.h#L633

通过“struct inode”结构体，获取文件数据块的在磁盘上的位置以及文件大小等信息；

/*
 * Keep mostly read-only and often accessed (especially for
 * the RCU path lookup and 'stat' data) fields at the beginning
 * of the 'struct inode'
 */
struct inode {
	umode_t			i_mode;
	unsigned short		i_opflags;
	kuid_t			i_uid;
	kgid_t			i_gid;
	unsigned int		i_flags;

#ifdef CONFIG_FS_POSIX_ACL
	struct posix_acl	*i_acl;
	struct posix_acl	*i_default_acl;
#endif

	const struct inode_operations	*i_op;
	struct super_block	*i_sb;
	struct address_space	*i_mapping;

#ifdef CONFIG_SECURITY
	void			*i_security;
#endif

	/* Stat data, not accessed from path walking */
	unsigned long		i_ino;
	/*
	 * Filesystems may only read i_nlink directly.  They shall use the
	 * following functions for modification:
	 *
	 *    (set|clear|inc|drop)_nlink
	 *    inode_(inc|dec)_link_count
	 */
	union {
		const unsigned int i_nlink;
		unsigned int __i_nlink;
	};
	dev_t			i_rdev;
	loff_t			i_size;
	struct timespec64	i_atime;
	struct timespec64	i_mtime;
	struct timespec64	i_ctime;
	spinlock_t		i_lock;	/* i_blocks, i_bytes, maybe i_size */
	unsigned short          i_bytes;
	u8			i_blkbits;
	u8			i_write_hint;
	blkcnt_t		i_blocks;

#ifdef __NEED_I_SIZE_ORDERED
	seqcount_t		i_size_seqcount;
#endif

	/* Misc */
	unsigned long		i_state;
	struct rw_semaphore	i_rwsem;

	unsigned long		dirtied_when;	/* jiffies of first dirtying */
	unsigned long		dirtied_time_when;

	struct hlist_node	i_hash;
	struct list_head	i_io_list;	/* backing dev IO list */
#ifdef CONFIG_CGROUP_WRITEBACK
	struct bdi_writeback	*i_wb;		/* the associated cgroup wb */

	/* foreign inode detection, see wbc_detach_inode() */
	int			i_wb_frn_winner;
	u16			i_wb_frn_avg_time;
	u16			i_wb_frn_history;
#endif
	struct list_head	i_lru;		/* inode LRU list */
	struct list_head	i_sb_list;
	struct list_head	i_wb_list;	/* backing dev writeback list */
	union {
		struct hlist_head	i_dentry;
		struct rcu_head		i_rcu;
	};
	atomic64_t		i_version;
	atomic64_t		i_sequence; /* see futex */
	atomic_t		i_count;
	atomic_t		i_dio_count;
	atomic_t		i_writecount;
#if defined(CONFIG_IMA) || defined(CONFIG_FILE_LOCKING)
	atomic_t		i_readcount; /* struct files open RO */
#endif
	union {
		const struct file_operations	*i_fop;	/* former ->i_op->default_file_ops */
		void (*free_inode)(struct inode *);
	};//对inode的操作
	struct file_lock_context	*i_flctx;
	struct address_space	i_data;
	struct list_head	i_devices;
	union {
		struct pipe_inode_info	*i_pipe;
		struct block_device	*i_bdev;
		struct cdev		*i_cdev;
		char			*i_link;
		unsigned		i_dir_seq;
	};

	__u32			i_generation;

#ifdef CONFIG_FSNOTIFY
	__u32			i_fsnotify_mask; /* all events this inode cares about */
	struct fsnotify_mark_connector __rcu	*i_fsnotify_marks;
#endif

#ifdef CONFIG_FS_ENCRYPTION
	struct fscrypt_info	*i_crypt_info;
#endif

#ifdef CONFIG_FS_VERITY
	struct fsverity_info	*i_verity_info;
#endif

	void			*i_private; /* fs or device private pointer */
} __randomize_layout;

参考书籍：Linux/Unix系统编程手册