APUE_3E - Files and Directories

2015-03-24

Overview

围绕stat函数, 详细介绍stat结构中的每一个成员, 可以使用man 2 stat查看详细信息. shell中的stat命令可以使用man stat查看
介绍unix文件系统结构 - 文件和目录在文件系统中是如何设计的?

文件访问权限

当我们用名字打开任一类型的文件时, 对该名字中包含的每一个目录, 包括它可能隐含的当前工作目录都应该具有执行权限. 例如, 为了打开文件/usr/inlcude/stdio.h, 需要对目录/, /usr, /usr/include具有执行权限.
对一个文件的读权限决定了我们是否能够打开现有的文件进行读操作. 这与open函数的O_RDONLY和O_RDWR标志相关.
对一个文件的读权限决定了我们是否能够打开现有的文件进行读操作. 这与open函数的O_WRONLY和O_RDWR标志相关.
为了在open函数中对一个文件指定O_TRUNC标志, 必须对该目录具有写权限.
为了在一个目录中创建一个新文件, 必须对包含该文件的目录具有写权限和执行权限.
为了删除一个现有文件, 必须对包含该文件的目录具有写权限和执行权限. 对该文件本身不需要有读写权限.

进程每次打开, 创建或者删除一个文件时, 内核就如下顺序进行文件访问权限测试:

If the effective user ID of the process is 0 (the superuser), access is allowed. This gives the superuser free rein throughout the entire file system.

If the effective user ID of the process equals the owner ID of the file (i.e., the process owns the file), access is allowed if the appropriate user access permission bit is set. Otherwise, permission is denied. By appropriate access permission bit, we mean that if the process is opening the file for reading, the user-read bit must be on. If the process is opening the file for writing, the user-write bit must be on. If the process is executing the file, the user-execute bit must be on.

If the effective group ID of the process or one of the supplementary group IDs of the process equals the group ID of the file, access is allowed if the appropriate group access permission bit is set. Otherwise, permission is denied.

If the appropriate other access permission bit is set, access is allowed. Otherwise, permission is denied.