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9.3 Out-Of-Band Data

The TCP/IP protocol allows users to send data asynchronously, otherwise known as out-of-band data. When using this feature, the operating system interrupts the receiving process if this process has chosen to be notified about out-of-band data. The receiver can grab this input without affecting any information currently queued on the socket. Therefore, you can use this without interfering with any current activity due to other wire and remote interfaces.

Unfortunately, most implementations of TCP/IP are broken, so use of out-of-band data is limited for safety reasons. You can only reliably send one character at a time.

The Wire package is built on top of CMUCLs networking support. In view of this, it is possible to use the routines described in section internet-oob for handling and sending out-of-band data. These all take a Unix file descriptor instead of a wire, but you can fetch a wire’s file descriptor with wire-fd.