解决方法
简单形式：当TCP套接字接收带外数据时会出现异常情况.
如果您阅读了选择manual page,可以参考另一个名为select_tut的补充手册页,其中说明如下：
exceptfds
This set is watched for “exceptional conditions”. In practice,only one such exceptional condition is common: the availability of out-of-band (OOB) data for reading from a TCP socket. See recv(2),send(2),and tcp(7) for more details about OOB data. (One other less common case where select(2) indicates an exceptional condition occurs with pseudo-terminals in packet mode; see tty_ioctl(4).) After select() has returned,exceptfds will be cleared of all file descriptors except for those for which an exceptional condition has occurred.
