Whenever Emacs reads a given piece of text, it tries to recognize which coding system to use. This applies to files being read, output from subprocesses, text from X selections, etc. Emacs can select the right coding system automatically most of the time—once you have specified your preferences.
Some coding systems can be recognized or distinguished by which byte sequences appear in the data. However, there are coding systems that cannot be distinguished, not even potentially. For example, there is no way to distinguish between Latin-1 and Latin-2; they use the same byte values with different meanings.
Emacs handles this situation by means of a priority list of coding systems. Whenever Emacs reads a file, if you do not specify the coding system to use, Emacs checks the data against each coding system, starting with the first in priority and working down the list, until it finds a coding system that fits the data. Then it converts the file contents assuming that they are represented in this coding system.
The priority list of coding systems depends on the selected language environment (see Language Environments). For example, if you use French, you probably want Emacs to prefer Latin-1 to Latin-2; if you use Czech, you probably want Latin-2 to be preferred. This is one of the reasons to specify a language environment.
However, you can alter the coding system priority list in detail with the command
M-x prefer-coding-system. This command reads the name of a coding system from the minibuffer, and adds it to the front of the priority list, so that it is preferred to all others. If you use this command several times, each use adds one element to the front of the priority list.
If you use a coding system that specifies the end-of-line conversion type, such as
iso-8859-1-dos, what this means is that Emacs should attempt to recognize
iso-8859-1 with priority, and should use DOS end-of-line conversion when it does recognize
Sometimes a file name indicates which coding system to use for the file. The variable
file-coding-system-alist specifies this correspondence. There is a special function
modify-coding-system-alist for adding elements to this list. For example, to read and write all ‘
.txt’ files using the coding system
chinese-iso-8bit, you can execute this Lisp expression:
(modify-coding-system-alist 'file "\\.txt\\'" 'chinese-iso-8bit)
The first argument should be
file, the second argument should be a regular expression that determines which files this applies to, and the third argument says which coding system to use for these files.
Emacs recognizes which kind of end-of-line conversion to use based on the contents of the file: if it sees only carriage returns, or only carriage return followed by linefeed sequences, then it chooses the end-of-line conversion accordingly. You can inhibit the automatic use of end-of-line conversion by setting the variable
inhibit-eol-conversion to non-
nil. If you do that, DOS-style files will be displayed with the ‘
^M’ characters visible in the buffer; some people prefer this to the more subtle ‘
(DOS)’ end-of-line type indication near the left edge of the mode line (see eol-mnemonic).
By default, the automatic detection of the coding system is sensitive to escape sequences. If Emacs sees a sequence of characters that begin with an escape character, and the sequence is valid as an ISO-2022 code, that tells Emacs to use one of the ISO-2022 encodings to decode the file.
However, there may be cases that you want to read escape sequences in a file as is. In such a case, you can set the variable
inhibit-iso-escape-detection to non-
nil. Then the code detection ignores any escape sequences, and never uses an ISO-2022 encoding. The result is that all escape sequences become visible in the buffer.
The default value of
nil. We recommend that you not change it permanently, only for one specific operation. That’s because some Emacs Lisp source files in the Emacs distribution contain non-ASCII characters encoded in the coding system
iso-2022-7bit, and they won’t be decoded correctly when you visit those files if you suppress the escape sequence detection.
auto-coding-regexp-alist are the strongest way to specify the coding system for certain patterns of file names, or for files containing certain patterns, respectively. These variables even override ‘
-*-coding:-*-’ tags in the file itself (see Specify Coding). For example, Emacs uses
auto-coding-alist for tar and archive files, to prevent it from being confused by a ‘
-*-coding:-*-’ tag in a member of the archive and thinking it applies to the archive file as a whole.
Another way to specify a coding system is with the variable
auto-coding-functions. For example, one of the builtin
auto-coding-functions detects the encoding for XML files. Unlike the previous two, this variable does not override any ‘