如果没有找到处理请求的处理程序，服务器将返回一个501错误。
If no handlers are found to handle the request, the server will return a 501 error.
如果在表中没有找到这个协议序列，这个例程返回错误。
If the protocol sequence is not found in the table, an error is returned by this routine.
返回以下错误，表示没有找到具有这个名称的过程。
The following error is returned, indicating that no procedure with this name was found.
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