[深入理解文件系统之二] 文件描述符、inode和打开文件表

Unix文件系统学习笔记之二：文件描述符、inode和打开文件表

The procstructuredoes not record information related to file access. However the userstructurecontains a number of important file-access-related fields, namely:

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

usedduring pathname resolution when a user specifies a relative

pathname.

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

checkingfor file-access-based system calls. Similarly, u_euidand

u_egidholdthe effective user and group IDs.

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

moredetail later.

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

fromuser to kernel mode when invoking a system call.

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

datafrom 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

decrementedduring the I/O operation and the result can be passed back

tothe user.

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

orwrite operation.

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

encountered.The value of u_erroristhen passed back to the user

whenthe system call returns.

深刻理解inode就是在理解meta-data，inode是文件系统中最重要的metadata, 它的主要数据结构如下

（注意unix文件系统，它的原始数据存储在非易失性的器件上，文件系统启动之后，访问到的或者经常访问的inode被读到内存里面，这一部分被称为inodein-core。)

Eachfile in the filesystem was represented by a unique inode thatcontained fields such as:

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

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

ofthe above modes was not set, the file was assumed to be a regularfile.

Thiswould later be replaced by an explicit flag, IFREG.

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

thisfield 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 datablocks are held.

i_mtime.The time the file was last modified.

关于inode的操作，需要考虑以下方面：

Inodein core/memory

a.何时从磁盘读入到内存：打开的时候需要读入inode;

b.何时从内存写入到磁盘：如果对inode有任何更新，比如新申请了块、释放了块

c.何时可以写入到磁盘：文件已经关闭，并且没有任何进程打开了inode对应的文件

d.哪些inode需要缓存：DNLC(directory name lookup cache for vnode)

由此自然引出一个问题，如何表示进程打开的一个文件，或者说操作系统如何记录一个打开的文件呢？如果你去设计，你会怎么做？

a.它必然包含inode的部分信息（或者全部）

b.它必然包含运行时的信息（读写指针的偏移，读写buffer的位置，以及它所对应的inode指针?

当然还必须包含引用计数、打开的方式flag)

在unix系统中，正是这样实现的，下面就是unix用来记录一个打开文件的信息：

上述数据结构file_structure(内存中的就用来在unix中记录进程打开的一个文件的，而如果程序打开了多

个文件，就需要一个file_structure数组。而这恰恰是恰恰是理解文件描述符的关键，文件描述符就是前进程打开的文件列表的索引(index)。

下面这张图前面展示了文件描述符、打开文件列表和inode的关系：

（编辑：海南站长网）

【声明】本站内容均来自网络，其相关言论仅代表作者个人观点，不代表本站立场。若无意侵犯到您的权利，请及时与联系站长删除相关内容!