[深刻理解文件系統之二] 文件描述符、inode和打開文件表

Unix文件系統學習筆記之二： 文件描述符、inode和打開文件表

• 系統盤上數據的佈局

  
  

  
  

  文件系統無非是關於數據在磁盤上的組織以及存儲空間管理的，爲此，首先須要知道磁盤上數據的整體佈局方式。以Unix爲例，最重要的一張表以下：

• Unix 進程管理中和用戶文件、io 最相關的數據結構：usr 數據結構
  

The procstructure does not record information related to file access.  However the userstructure contains a number of important  file-access-related fields, namely:

u_cdir. The inode of the current working directory is stored here. This is

used during pathname resolution when a user specifies a relative

pathname.

u_uid/u_gid. The process user ID and group ID used for permissions

checking for file-access-based system calls. Similarly, u_euidand

u_egidhold the effective user and group IDs.

u_ofile. This array holds the process file descriptors. This is described in

more detail later.

u_arg. An array of system call arguments set up during the transition

from user to kernel mode when invoking a system call.

u_base. This field holds the address of a user space buffer in which to read

data from or write data to when processing a system call such as read()

orwrite().

u_count. The number of bytes to read or write is held here. It is

decremented during the I/O operation and the result can be passed back

to the user.

u_offset. This field records the offset within the file for the current read

or write operation.

u_error. When processing a system call, this field is set if an error is

encountered. The value of u_erroris then passed back to the user

when the system call returns.

• ＩＮＯＤＥ
  

  爲了深刻理解文件inode、打開文件表、描述符三者之間的關係，首先就須要瞭解indoe。

深入理解inode就是在理解meta-data，inode是文件系統中最重要的meta data, 它的主要數據結構以下

（注意，它的原始數據存儲在非易失性的器件上，文件系統啓動以後，訪問到的或者常常訪問的inode被讀到內存裏面，這一部分被稱爲inode in-core。)

Each file in the filesystem was represented by a unique inode that contained fields such as:

i_mode. This field specifies whether the file is a directory (IFDIR), a block

special file (IFBLK), or a character special file (IFCHR). Note that if one

of the above modes was not set, the file was assumed to be a regular file.

This would later be replaced by an explicit flag, IFREG.

i_nlink. This field recorded the number of hard links to the file. When

this field reaches zero, the inode is freed.

i_uid. The file’s user ID.

i_gid. The file’s group ID.

i_size. The file size in bytes.

i_addr. This field holds block addresses on disk where the file’s data blocks are held.

i_mtime. The time the file was last modified.

關於inode的操做，須要考慮如下方面：

Inode in core/memory

a.什麼時候從磁盤讀入到內存： 打開的時候須要讀入inode;

b.什麼時候從內存寫入到磁盤：若是對inode有任何更新，好比新申請了塊、釋放了塊

c.什麼時候能夠寫入到磁盤： 文件已經關閉，而且沒有任何進程打開了inode對應的文件

d.哪些inode須要緩存：DNLC (directory name lookup cache for vnode)

• 打開文件表
  

由此天然引出一個問題，如何表示進程打開的一個文件，或者說操做系統如何記錄一個打開的文件呢？若是你去設計，你會怎麼作？

a.它必然包含inode的部分信息（或者所有）

b.它必然包含運行時的信息（讀寫指針的偏移，讀寫buffer的位置，以及它所對應的inode指針?

固然還必須包含引用計數、打開的方式flag)

在unix系統中，正是這樣實現的， 下面就是unix用來記錄一個打開文件的信息：

上述數據結構file_structure (內存中的就用來在unix中記錄進程打開的一個文件的，而若是程序打開了多

個文件，就須要一個file_structure數組。而這偏偏是偏偏是理解文件描述符的關鍵，文件描述符就是前進程打開的文件列表的索引(index)。

下面這張圖前面展現了 文件描述符、打開文件列表和inode的關係：