【Real and Effective IDs】
Real User ID：登陆用户的ID。
Effective User ID：有效用户ID。如设置了suid，则Effective User ID为文件主ID。
Saved User ID：The saved user ID (suid) is used when a program running with elevated privileges needs to temporarily do some unprivileged work: it changes its effective user ID from a privileged value (typically root) to some unprivileged one, and this triggers a copy of the privileged user ID to the saved user ID slot.[2] Later, it can set its effective user ID back to the saved user ID (an unprivileged process can only set its effective user ID to three values: its real user ID, its saved user ID, and its effective user ID—i.e., unchanged) to resume its privileges.
参考：http://en.wikipedia.org/wiki/User_identifier#Saved_user_ID
