How to turn off alternative Enter with Ctrl+M in Linux

Question

Why is Ctrl+M bound to Enter in Ubuntu Jaunty? How to turn it off?

I\'m using Emacs and would like to bind Ctrl+M to some other command.

Answer 1

input-decode-map does the trick. Quote from the emacs manual:

This keymap has the same structure as other keymaps, but is used differently: it specifies translations to make while reading key sequences, rather than bindings for key sequences.

It's the same principle like I've presented above, transforming Ctrl+m to something, say Ctrl+1 and map Ctrl+1 to your command. I use Ctrl+m for backward-kill-word.

Here you go:

(global-set-key (read-kbd-macro "C-1") 'backward-kill-word)
(define-key input-decode-map "\C-m" [?\C-1])

Answer 2

I think your question is backwards. It is not C-m that is bound to Enter, it is Enter that is bound to C-m. And C-m is the same as RET.

If you run C-h k C-m, you will see something like "RET runs the command ...". C-m sends RET because it is a control code, see http://en.wikipedia.org/wiki/Control_character.

The Enter key is bound to C-m; if you run C-h k Enter, you will see something like "RET (translated from <return>) runs the command ...". See, Enter is being interpreted by emacs as <return> and then that key is getting translated to C-m.

What you want to do is first remove the translation from <return> to RET by binding it directly to what it's currently indirectly bound, e.g. (global-set-key (kbd "<return>") 'newline). Then you're free to bind C-m to whatever you want without affecting Enter.

This assumes you're using the graphical emacs. If you're running it in a terminal, this won't work, because Enter will send C-m, not <return>. You can check that using the window-system variable though.

Answer 3

Note: The issue isn't limited to Linux, it exists on Windows (and presumably Mac) as well. Read the other (non stack-overflow) source of all knowledge: Wikipedia on Carriage Return.

If you want to rebind C-m, be sure to all bind <return> otherwise you run the risk of no longer being able to use the Enter/Return key. Also, in a terminal, Emacs cannot distinguish between the two (C-m and <return>).

In a plain Emacs, the Enter/Return key is bound to <return>, which is (by default) translated to RET (same thing as C-m). If you only rebound the C-m, you'd also be affecting the Enter/Return key.

Try C-h k <return> and you'll see

RET (translated from <return>)

So, rebind both in the appropriate keymap to make sure you get the behavior you want.

It might be instructive to play with the following code:

(defun my-return ()
  (interactive)
  (message "return"))
(defun my-ret ()
  (interactive)
  (message "RET"))
(defun my-c-m ()
  (interactive)
  (message "C-m"))
(global-set-key (kbd "<return>") 'my-return)
(global-set-key (kbd "C-m") 'my-c-m)
(global-set-key (kbd "RET") 'my-ret)

Put that in your *scratch* buffer and press C-j after each line (to evaluate the sexp). Then play with the Enter/Return keys and C-m.

Answer 4

(global-set-key (kbd "C-m") 'cmd) ;

Where cmd is your command should remap control m...

As for why ctrl+m is bound to enter. I believe it had something to do with some older keyboard not having enter,tab, backspace, etc... ( I could be grossly mistaken)

For example ctrl+h is backspace, some unix operating systems will output ^H when you hit backspace on them!

Answer 5

It's unclear whether the previous answers have solved this question, so here's another spin on it:

Historically, "return" frequently meant two things: Carriage Return, and Line Feed.

Quoting wikipedia:

Originally, carriage return was the term for a mechanism or lever on a typewriter that would cause the cylinder on which the paper was held (the carriage) to return to the left side of the paper after a line of text had been typed, and would usually move the paper to the next line as well. Later it was used for the control character in Baudot code on a teletypewriter for end of line return to beginning of line and did not include line feed.

Long story short, there are two ASCII codes that are relevant to end-of-line (and, therefore, potentially to the return key): CR (ASCII decimal 13, or Ctrl-m) and LF (ASCII decimal 10, or Ctrl-J).

I think the general convention these days is for the return or enter keys to map to ASCII 13, and thus be "return" (RET in emacs messaging). But if you're running emacs from a terminal emulator, this may mean that you don't have the option of having a C-m binding that's different from RET. When I try running emacs in a terminal (GNU Emacs 23.2.1 on MacOS X in Terminal.app), and I try typing C-h c <return> (i.e. pressing my return/enter key), I get:

RET runs the command newline

If I type, instead, C-h c C-m (i.e. holding down control and pressing M), I get:

RET runs the command newline

In other words, the exact same thing. Emacs (nor any other program run from the terminal) can't tell the difference between the two. (Knowing this can sometimes be handy -- if you're logged in to a system that maps things differently than the system you're coming from, you can type C-m or C-j to get the thing you want, depending on the way the mapping is screwed up.)

And speaking of having the mapping screwed up, I'll just mention that there are some stty settings that can be relevant to such things: inlcr, inlcr, and igncr (these three are related to how input is processed -- there are also some output-related ones). I doubt that making stty changes is going to be useful to this particular problem, but I thought it worth mentioning.

In the end, I think your choices are:

1. Live with not having C-m that means anything different than return/enter

2. Run a graphical version of emacs, instead of the terminal version. This should have access to the distinct keycodes, which the terminal emulator isn't passing along (because it's emulating a terminal, which didn't have such things in the same way as modern computers do).

Answer 6

Some very good explanations here as to the nature of the problem, and I won't expound on those issues. However, I want to address the original question directly. I solved the problem for changing the (control m) sequence bound to 'newline command by the following code in my ~/.xemacs/custom.el file:

(defun my-compile-hook-for-c-and-cpp-mode ()
  "My compile hook for C and C++ mode" 
  (local-set-key [(control m)] 'compile)
  )

(add-hook 'c-mode-hook 'my-compile-hook-for-c-and-cpp-mode)
(add-hook 'c++-mode-hook 'my-compile-hook-for-c-and-cpp-mode)

In the above example, I have changed (control m) to run the 'compile command (M-x compile) when the c-mode or c++-mode is active.

Note that you can change the behavior of (control m) globally as well, without the mode related bindings. In that case, just add the following to your ~/.xemacs/custom.el file:

(global-set-key [(control m)] 'compile)