What's a file descriptor's "exception"?

c unix io file-descriptor multiplexing
Verdagon picture Verdagon · Mar 30, 2013 · Viewed 7.6k times · Source

When one calls select() asking which file descriptors have "exceptions" waiting, what does that mean?

How does one trigger one of these "exceptions"?

If anyone can point me to a nice explanation, that'd be awesome. I've been googling and can't find a thing.

Answer

jmkeyes picture jmkeyes · Mar 30, 2013

Short form: exceptional situations occur when a TCP socket recieves out of band data.

If you read the select manual page, you will find a reference to another supplementary manual page called select_tut with the explanation:

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.

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