AH.4 Text Files and Binary Files

GNU Emacs uses newline characters to separate text lines. This is the convention used on GNU and Unix.

MS-DOS and MS-Windows normally use carriage-return linefeed, a two-character sequence, to separate text lines. (Linefeed is the same character as newline.) Therefore, convenient editing of typical files with Emacs requires conversion of these end-of-line (EOL) sequences. And that is what Emacs normally does: it converts carriage-return linefeed into newline when reading files, and converts newline into carriage-return linefeed when writing files. The same mechanism that handles conversion of international character codes does this conversion also (see section Q.7 Coding Systems).

One consequence of this special format-conversion of most files is that character positions as reported by Emacs (see section D.9 Cursor Position Information) do not agree with the file size information known to the operating system.

In addition, if Emacs recognizes from a file's contents that it uses newline rather than carriage-return linefeed as its line separator, it does not perform EOL conversion when reading or writing that file. Thus, you can read and edit files from GNU and Unix systems on MS-DOS with no special effort, and they will retain their Unix-style end-of-line convention after you edit them.

The mode line indicates whether end-of-line translation was used for the current buffer. If MS-DOS end-of-line translation is in use for the buffer, a backslash `\' is displayed after the coding system mnemonic near the beginning of the mode line (see section B.3 The Mode Line). If no EOL translation was performed, the string `(Unix)' is displayed instead of the backslash, to alert you that the file's EOL format is not the usual carriage-return linefeed.

To visit a file and specify whether it uses DOS-style or Unix-style end-of-line, specify a coding system (see section Q.9 Specifying a Coding System). For example, C-x RET c unix RET C-x C-f foobar.txt visits the file `foobar.txt' without converting the EOLs; if some line ends with a carriage-return linefeed pair, Emacs will display `^M' at the end of that line. Similarly, you can direct Emacs to save a buffer in a specified EOL format with the C-x RET f command. For example, to save a buffer with Unix EOL format, type C-x RET f unix RET C-x C-s. If you visit a file with DOS EOL conversion, then save it with Unix EOL format, that effectively converts the file to Unix EOL style, like dos2unix.

When you use NFS or Samba to access file systems that reside on computers using GNU or Unix systems, Emacs should not perform end-of-line translation on any files in these file systems--not even when you create a new file. To request this, designate these file systems as untranslated file systems by calling the function add-untranslated-filesystem. It takes one argument: the file system name, including a drive letter and optionally a directory. For example,

(add-untranslated-filesystem "Z:")

designates drive Z as an untranslated file system, and

(add-untranslated-filesystem "Z:\\foo")

designates directory `\foo' on drive Z as an untranslated file system.

Most often you would use add-untranslated-filesystem in your `_emacs' file, or in `site-start.el' so that all the users at your site get the benefit of it.

To countermand the effect of add-untranslated-filesystem, use the function remove-untranslated-filesystem. This function takes one argument, which should be a string just like the one that was used previously with add-untranslated-filesystem.

Designating a file system as untranslated does not affect character set conversion, only end-of-line conversion. Essentially, it directs Emacs to create new files with the Unix-style convention of using newline at the end of a line. See section Q.7 Coding Systems.

Some kinds of files should not be converted at all, because their contents are not really text. Therefore, Emacs on MS-DOS distinguishes certain files as binary files. (This distinction is not part of MS-DOS; it is made by Emacs only.) Binary files include executable programs, compressed archives, etc. Emacs uses the file name to decide whether to treat a file as binary: the variable file-name-buffer-file-type-alist defines the file-name patterns that indicate binary files. If a file name matches one of the patterns for binary files (those whose associations are of the type (pattern . t), Emacs reads and writes that file using the no-conversion coding system (see section Q.7 Coding Systems) which turns off all coding-system conversions, not only the EOL conversion. file-name-buffer-file-type-alist also includes file-name patterns for files which are known to be DOS-style text files with carriage-return linefeed EOL format, such as `CONFIG.SYS'; Emacs always writes those files with DOS-style EOLs.

If a file which belongs to an untranslated file system matches one of the file-name patterns in file-name-buffer-file-type-alist, the EOL conversion is determined by file-name-buffer-file-type-alist.