emacs: Recognize Coding
22.6 Recognizing Coding Systems
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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 (
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 ‘iso-8859-1’.
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 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 (eol-mnemonic Mode Line.).
By default, the automatic detection of 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 ‘inhibit-iso-escape-detection’ is ‘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.
The variables ‘auto-coding-alist’ and ‘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
(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 ‘-*-coding:-*-’ tag.
Info Catalog
emacs: Coding Systems
emacs: International
emacs: Specify Coding