The channel functions provide interfaces to the Berkeley socket library, which allows network applications to be written. The open functions return channel identifiers that may also be used with most file identifier functions (for example, read, write, and close). Several open_ routines return a channel identifier that may be used by subsequent I/O functions. A channel identifier is a small integer file identifier that is returned by open and can be used with the file identifier routines getline, put, close, read, and write. These channel functions allow binary data to be transmitted.