HP Rp7410 BSD Sockets Interface Programmer's Guide - Page 81
Sending and Receiving Out-of-band
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Advanced Topics for Stream Sockets Sending and Receiving Out-of-band Data Sending and Receiving Out-of-band Data This option is not supported for UNIX Domain (AF_UNIX) sockets. If an abnormal condition occurs when a process is in the middle of sending a long stream of data, it is useful to be able to alert the other process with an urgent message. The TCP stream socket implementation includes an out-of-band data facility. Out-of-band data uses a logically independent transmission channel associated with a pair of connected stream sockets. TCP supports the reliable delivery of only one out-of-band message at a time. The message can be a maximum of one byte long. Out-of-band data arrive at the destination node in sequence and in stream, but are delivered independently of normal data. If the receiver has enabled the signalling of out-of-band data via the SIOCSPGRP socket ioctl (see the socket(7) man page), then a SIGURG is delivered when out-of-band data arrive. If the receiver is selecting for exceptional conditions on the receiving socket, it will return true to signal the arrival of out-of-band data. The receiving process can read the out-of-band message and take the appropriate action based on the message contents. A logical mark is placed in the normal data stream to indicate the point at which the out-of-band data were sent, so that data before the message can be handled differently from data following the message. Here is a data stream with an out-of-band marker: byte stream