c - setuid 与 seteuid 函数

Question

setuid 和seteuid 函数有什么区别。在手册页中，这两个函数都有相似的描述。

设置标识符:

DESCRIPTION

   setuid()  sets  the  effective user ID of the calling process.  If the effective UID of the caller is root, the real UID and saved
   set-user-ID are also set.

seteuid:

DESCRIPTION

   seteuid()  sets  the  effective user ID of the calling process.  Unprivileged user processes may only set the effective user ID to
   the real user ID, the effective user ID or the saved set-user-ID.

在这两个描述中都包含设置调用进程的有效用户ID。那么有什么区别这两个以及这些函数之间的功能差异。

还有一个疑问是，使用 chmod(chmod u+s ) 只有我们可以给文件设置set user id权限。然后只有在程序运行期间，进程才有权限设置谁用户身份。除了这些功能之外，这些功能如何将有效用户标识设置为进程。

Answer 1

来自man page :

   Thus, a set-user-ID-root program wishing to temporarily drop root
   privileges, assume the identity of an unprivileged user, and then
   regain root privileges afterward cannot use setuid().  You can
   accomplish this with seteuid(2).