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33.10.7 Explicit Encoding and Decoding

All the operations that transfer text in and out of Emacs have the ability to use a coding system to encode or decode the text. You can also explicitly encode and decode text using the functions in this section.

The result of encoding, and the input to decoding, are not ordinary text. They logically consist of a series of byte values; that is, a series of ASCII and eight-bit characters. In unibyte buffers and strings, these characters have codes in the range 0 through #xFF (255). In a multibyte buffer or string, eight-bit characters have character codes higher than #xFF (see Text Representations), but Emacs transparently converts them to their single-byte values when you encode or decode such text.

The usual way to read a file into a buffer as a sequence of bytes, so you can decode the contents explicitly, is with insert-file-contents-literally (see Reading from Files); alternatively, specify a non-nil rawfile argument when visiting a file with find-file-noselect. These methods result in a unibyte buffer.

The usual way to use the byte sequence that results from explicitly encoding text is to copy it to a file or process—for example, to write it with write-region (see Writing to Files), and suppress encoding by binding coding-system-for-write to no-conversion.

Here are the functions to perform explicit encoding or decoding. The encoding functions produce sequences of bytes; the decoding functions are meant to operate on sequences of bytes. All of these functions discard text properties. They also set last-coding-system-used to the precise coding system they used.

— Command: encode-coding-region start end coding-system &optional destination

This command encodes the text from start to end according to coding system coding-system. Normally, the encoded text replaces the original text in the buffer, but the optional argument destination can change that. If destination is a buffer, the encoded text is inserted in that buffer after point (point does not move); if it is t, the command returns the encoded text as a unibyte string without inserting it.

If encoded text is inserted in some buffer, this command returns the length of the encoded text.

The result of encoding is logically a sequence of bytes, but the buffer remains multibyte if it was multibyte before, and any 8-bit bytes are converted to their multibyte representation (see Text Representations).

Do not use undecided for coding-system when encoding text, since that may lead to unexpected results. Instead, use select-safe-coding-system (see select-safe-coding-system) to suggest a suitable encoding, if there's no obvious pertinent value for coding-system.

— Function: encode-coding-string string coding-system &optional nocopy buffer

This function encodes the text in string according to coding system coding-system. It returns a new string containing the encoded text, except when nocopy is non-nil, in which case the function may return string itself if the encoding operation is trivial. The result of encoding is a unibyte string.

— Command: decode-coding-region start end coding-system &optional destination

This command decodes the text from start to end according to coding system coding-system. To make explicit decoding useful, the text before decoding ought to be a sequence of byte values, but both multibyte and unibyte buffers are acceptable (in the multibyte case, the raw byte values should be represented as eight-bit characters). Normally, the decoded text replaces the original text in the buffer, but the optional argument destination can change that. If destination is a buffer, the decoded text is inserted in that buffer after point (point does not move); if it is t, the command returns the decoded text as a multibyte string without inserting it.

If decoded text is inserted in some buffer, this command returns the length of the decoded text. If that buffer is a unibyte buffer (see Selecting a Representation), the internal representation of the decoded text (see Text Representations) is inserted into the buffer as individual bytes.

This command puts a charset text property on the decoded text. The value of the property states the character set used to decode the original text.

— Function: decode-coding-string string coding-system &optional nocopy buffer

This function decodes the text in string according to coding-system. It returns a new string containing the decoded text, except when nocopy is non-nil, in which case the function may return string itself if the decoding operation is trivial. To make explicit decoding useful, the contents of string ought to be a unibyte string with a sequence of byte values, but a multibyte string is also acceptable (assuming it contains 8-bit bytes in their multibyte form).

If optional argument buffer specifies a buffer, the decoded text is inserted in that buffer after point (point does not move). In this case, the return value is the length of the decoded text. If that buffer is a unibyte buffer, the internal representation of the decoded text is inserted into it as individual bytes.

This function puts a charset text property on the decoded text. The value of the property states the character set used to decode the original text: 

          (decode-coding-string "Gr\374ss Gott" 'latin-1)
               ⇒ #("Grüss Gott" 0 9 (charset iso-8859-1))
— Function: decode-coding-inserted-region from to filename &optional visit beg end replace

This function decodes the text from from to to as if it were being read from file filename using insert-file-contents using the rest of the arguments provided.

The normal way to use this function is after reading text from a file without decoding, if you decide you would rather have decoded it. Instead of deleting the text and reading it again, this time with decoding, you can call this function.