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33.9.5 Default Coding Systems

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 coding-system-for-read and coding-system-for-write (see Specifying Coding Systems).

— User Option: auto-coding-regexp-alist

This variable is an alist of text patterns and corresponding coding systems. Each element has the form (regexp . 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.

— User Option: file-coding-system-alist

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.

— User Option: auto-coding-alist

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.

— Variable: process-coding-system-alist

This variable is an alist specifying which coding systems to use for a subprocess, depending on which program is running in the subprocess. It works like 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 set-process-coding-system.

Warning: Coding systems such as undecided, which 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 latin-1-unix, rather than undecided or latin-1.

— Variable: network-coding-system-alist

This variable is an alist that specifies the coding system to use for network streams. It works much like file-coding-system-alist, 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 stream.

— Variable: default-process-coding-system

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 (input-coding . output-coding). Here input-coding applies to input from the subprocess, and output-coding applies to output to it.

— User Option: auto-coding-functions

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 undecoded 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 nil.

If a file has a ‘coding:’ tag, that takes precedence, so these functions won't be called.

— Function: find-auto-coding filename size

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, one of auto-coding-alist, auto-coding-regexp-alist, :coding, or 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). The order of looking for a matching rule is auto-coding-alist first, then auto-coding-regexp-alist, then the coding: tag, and lastly auto-coding-functions. If no matching rule was found, the function returns nil.

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 for the coding: tag is the first one or two lines of the file; in that case, size should be the size of the buffer.

— Function: set-auto-coding filename size

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 find-auto-coding.

— Function: find-operation-coding-system operation &rest arguments

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 write-region, start-process, call-process, call-process-region, insert-file-contents, or 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 target. For open-network-stream, the target is the service name or port number.

Depending on operation, this function looks up the target in file-coding-system-alist, process-coding-system-alist, or network-coding-system-alist. If the target is found in the alist, find-operation-coding-system returns its association in the alist; otherwise it returns nil.

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 file-coding-system-alist, and buffer is a buffer that contains the file's contents (not yet decoded). If 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.