Out-of-band data is data transmitted with a higher priority than ordinary data. This is usually used by either side of the connection to signal exceptional conditions. Due to the fact that most TCP/IP implementations are broken in this respect, only single characters can reliably be sent this way.
Sets the function passed in handler as a handler for the character char on the connection whose descriptor is fd. In case this character arrives, the function in handler is called without any argument.
Removes the handler for the character char from the connection with the file descriptor fd
After calling this function, the connection whose descriptor is fd will ignore any out-of-band character it receives.
Sends the character char through the connection fd out of band.