服务只有被消费了才有用；所以，要规定服务的效用，最好的方式就是描述消费者对它们有何期望。
Services are useful only insofar as they are consumed, and so the best way of specifying their usefulness is to describe what consumers expect of them.
有用性可能会有益于效用的增加，但是风格、潮流、想法同样可以。
Usefulness might contribute to utility, but so does style, fashion, or even whimsy.
这使 set-user-ID（非根）程序能够放弃它的所有用户特权，执行一些非特权操作，然后恢复原来的有效用户 ID。
This allows a set-user-ID (other than root) program to drop all of its user privileges, do some un-privileged work, and then re-engage the original effective user ID in a secure manner.
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