### 14.9 Port Primitives

This section describes the low-level operations that can be used to build and manipulate I/O ports.

The purpose of these operations is twofold: to allow programmers to construct new kinds of I/O ports, and to provide faster I/O operations than those supplied by the standard high level procedures. The latter is useful because the standard I/O operations provide defaulting and error checking, and sometimes other features, which are often unnecessary. This interface provides the means to bypass such features, thus improving performance.

The abstract model of an I/O port, as implemented here, is a combination of a set of named operations and a state. The state is an arbitrary object, the meaning of which is determined by the operations. The operations are defined by a mapping from names to procedures.

The set of named operations is represented by an object called a port type. A port type is constructed from a set of named operations, and is subsequently used to construct a port. The port type completely specifies the behavior of the port. Port types also support a simple form of inheritance, allowing you to create new ports that are similar to existing ports.

The port operations are divided into two classes:

Standard operations

  There is a specific set of standard operations for input ports, and a different set for output ports. Applications can assume that the standard input operations are implemented for all input ports, and likewise the standard output operations are implemented for all output ports.

Custom operations

  Some ports support additional operations. For example, ports that implement output to terminals (or windows) may define an operation named `y-size` that returns the height of the terminal in characters. Because only some ports will implement these operations, programs that use custom operations must test each port for their existence, and be prepared to deal with ports that do not implement them.