c - 什么是文件描述符的 "exception"？

Question

当调用 select() 询问哪些文件描述符有“异常”等待时，这是什么意思？

如何触发这些“异常”之一？

如果有人能给我指出一个很好的解释，那就太棒了。我一直在谷歌搜索，但找不到任何东西。

Answer 1

简短形式:当 TCP 套接字接收到带外数据时会出现异常情况。

如果您阅读选择 manual page ，您会找到对另一个名为 select_tut 的补充手册页的引用，其中包含解释:

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.