This section describes variables that specify the default coding system for certain files or when running certain subprograms, and the function that I/O operations use to access them.
The idea of these variables is that you set them once and for all to the
defaults you want, and then do not change them again. To specify a
particular coding system for a particular operation in a Lisp program,
don’t change these variables; instead, override them using
(see Specifying Coding Systems).
This variable is an alist of text patterns and corresponding coding
systems. Each element has the form
. coding-system); a file whose first few kilobytes match
regexp is decoded with coding-system when its contents are
read into a buffer. The settings in this alist take priority over
coding: tags in the files and the contents of
file-coding-system-alist (see below). The default value is set
so that Emacs automatically recognizes mail files in Babyl format and
reads them with no code conversions.
This variable is an alist that specifies the coding systems to use for
reading and writing particular files. Each element has the form
(pattern . coding), where pattern is a regular
expression that matches certain file names. The element applies to file
names that match pattern.
The CDR of the element, coding, should be either a coding system, a cons cell containing two coding systems, or a function name (a symbol with a function definition). If coding is a coding system, that coding system is used for both reading the file and writing it. If coding is a cons cell containing two coding systems, its CAR specifies the coding system for decoding, and its CDR specifies the coding system for encoding.
If coding is a function name, the function should take one
argument, a list of all arguments passed to
find-operation-coding-system. It must return a coding system
or a cons cell containing two coding systems. This value has the same
meaning as described above.
If coding (or what returned by the above function) is
undecided, the normal code-detection is performed.
This variable is an alist that specifies the coding systems to use for
reading and writing particular files. Its form is like that of
file-coding-system-alist, but, unlike the latter, this variable
takes priority over any
coding: tags in the file.
This variable is an alist specifying which coding systems to use for a
subprocess, depending on which program is running in the subprocess. It
file-coding-system-alist, except that pattern is
matched against the program name used to start the subprocess. The coding
system or systems specified in this alist are used to initialize the
coding systems used for I/O to the subprocess, but you can specify
other coding systems later using
Warning: Coding systems such as
determine the coding system from the data, do not work entirely reliably
with asynchronous subprocess output. This is because Emacs handles
asynchronous subprocess output in batches, as it arrives. If the coding
system leaves the character code conversion unspecified, or leaves the
end-of-line conversion unspecified, Emacs must try to detect the proper
conversion from one batch at a time, and this does not always work.
Therefore, with an asynchronous subprocess, if at all possible, use a
coding system which determines both the character code conversion and
the end of line conversion—that is, one like
This variable is an alist that specifies the coding system to use for
network streams. It works much like
with the difference that the pattern in an element may be either a
port number or a regular expression. If it is a regular expression, it
is matched against the network service name used to open the network
This variable specifies the coding systems to use for subprocess (and network stream) input and output, when nothing else specifies what to do.
The value should be a cons cell of the form
. output-coding). Here input-coding applies to input from
the subprocess, and output-coding applies to output to it.
This variable holds a list of functions that try to determine a coding system for a file based on its undecoded contents.
Each function in this list should be written to look at text in the
current buffer, but should not modify it in any way. The buffer will
contain the text of parts of the file. Each function should take one
argument, size, which tells it how many characters to look at,
starting from point. If the function succeeds in determining a coding
system for the file, it should return that coding system. Otherwise,
it should return
The functions in this list could be called either when the file is visited and Emacs wants to decode its contents, and/or when the file’s buffer is about to be saved and Emacs wants to determine how to encode its contents.
If a file has a ‘coding:’ tag, that takes precedence, so these functions won’t be called.
This function tries to determine a suitable coding system for
filename. It examines the buffer visiting the named file, using
the variables documented above in sequence, until it finds a match for
one of the rules specified by these variables. It then returns a cons
cell of the form
(coding . source), where
coding is the coding system to use and source is a symbol,
auto-coding-functions, indicating which one
supplied the matching rule. The value
:coding means the coding
system was specified by the
coding: tag in the file
(see coding tag in The GNU Emacs Manual).
The order of looking for a matching rule is
auto-coding-regexp-alist, then the
tag, and lastly
auto-coding-functions. If no matching rule was
found, the function returns
The second argument size is the size of text, in characters,
following point. The function examines text only within size
characters after point. Normally, the buffer should be positioned at
the beginning when this function is called, because one of the places
coding: tag is the first one or two lines of the file;
in that case, size should be the size of the buffer.
This function returns a suitable coding system for file
filename. It uses
find-auto-coding to find the coding
system. If no coding system could be determined, the function returns
nil. The meaning of the argument size is like in
This function returns the coding system to use (by default) for performing operation with arguments. The value has this form:
(decoding-system . encoding-system)
The first element, decoding-system, is the coding system to use for decoding (in case operation does decoding), and encoding-system is the coding system for encoding (in case operation does encoding).
The argument operation is a symbol; it should be one of
open-network-stream. These are the names of the Emacs I/O
primitives that can do character code and eol conversion.
The remaining arguments should be the same arguments that might be given
to the corresponding I/O primitive. Depending on the primitive, one
of those arguments is selected as the target. For example, if
operation does file I/O, whichever argument specifies the file
name is the target. For subprocess primitives, the process name is the
open-network-stream, the target is the service name
or port number.
Depending on operation, this function looks up the target in
network-coding-system-alist. If the target is found in the
find-operation-coding-system returns its association in
the alist; otherwise it returns
If operation is
insert-file-contents, the argument
corresponding to the target may be a cons cell of the form
(filename . buffer). In that case, filename
is a file name to look up in
buffer is a buffer that contains the file’s contents (not yet
file-coding-system-alist specifies a function to
call for this file, and that function needs to examine the file’s
contents (as it usually does), it should examine the contents of
buffer instead of reading the file.