66.5 Coding Systems

A coding system is an object that defines how text containing multiple character sets is encoded into a stream of (typically 8-bit) bytes. The coding system is used to decode the stream into a series of characters (which may be from multiple charsets) when the text is read from a file or process, and is used to encode the text back into the same format when it is written out to a file or process.

For example, many ISO-2022-compliant coding systems (such as Compound Text, which is used for inter-client data under the X Window System) use escape sequences to switch between different charsets – Japanese Kanji, for example, is invoked with ‘ESC $ ( B’; ASCII is invoked with ‘ESC ( B’; and Cyrillic is invoked with ‘ESC - L’. See make-coding-system for more information.

Coding systems are normally identified using a symbol, and the symbol is accepted in place of the actual coding system object whenever a coding system is called for. (This is similar to how faces and charsets work.)

Function: coding-system-p object

This function returns non-nil if object is a coding system.