G.5 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 Coding Systems).
One consequence of this special format-conversion of most files is
that character positions as reported by Emacs (see Position Info) 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 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 Specify Coding). 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
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
designates drive Z as an untranslated file system, and
designates directory \foo on drive Z as an untranslated file
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
remove-untranslated-filesystem. This function takes
one argument, which should be a string just like the one that was used
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 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
. t), Emacs reads and writes that file using the
no-conversion coding system (see 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
EOL conversion is determined by