ARCHIVED: In Emacs, how do I handle C-s and C-q being used for flow control?

C-s and C-q are used in the XON/XOFF flow control protocol. This can be a problem when you're using Emacs, because Emacs binds these keys to commands by default. Because Emacs won't honor them as flow control characters, too many of these characters are not passed on and overwhelm output buffers. Sometimes, intermediate software using XON/XOFF flow control will prevent Emacs from ever seeing C-s and C-q.

The following are some possible solutions:

• Disable the use of C-s and C-q for flow control: You need to determine the source of the flow control:
  • Your terminal: Your terminal may use XON/XOFF flow control to have time to display all the characters it receives. For example, VT series terminals do this. You may be able to turn this off from a setup menu. For example, on a VT220, in the setup menu, you may select No XOFF. This is also true for some terminal emulation programs on PCs.

    When you turn off flow control at the terminal, you will also need to turn it off at the other end, which might be the computer you are logged into or some terminal server in between.

    If you turn off flow control, characters may be lost; using a printer connected to the terminal may fail. You may be able to get around this problem by modifying the "termcap" entry for your terminal to include extra NUL padding characters.

  • A modem: If you are using a dial-up connection, the modems may be using XON/XOFF flow control. It's not clear how to get around this.
  • A router or terminal server: Some network server between the terminal and your computer may be using XON/XOFF flow control. It may be possible to make it use some other kind of flow control. You will probably have to ask your local network experts for help with this.
  • tty and/or pty devices: If your connection to Emacs goes through multiple tty and/or pty devices, they may be using XON/XOFF flow control even when it is not necessary.

    Eirik Fuller (eirik@theory.tn.cornell.edu) writes:

    Some versions of rlogin (and possibly telnet) do not
    pass flow control characters to the remote system to which they connect.
    On such systems, Emacs on the remote system cannot disable flow
    control on the local system.  Sometimes rlogin -8 will
    avoid this problem.
    
    One way to cure this is to disable flow control on the local host (the
    one running rlogin, not the one running rlogind)
    using the stty command, before starting the rlogin
    process.  On many systems, stty start u stop u will do this.
    
    Some versions of tcsh will prevent even this from working.
    One way around this is to start another shell before starting
    rlogin, and issue the stty command to disable
    flow control from that shell.
    
    Use stty -ixon instead of stty start u stop
    u on some systems.
    

• Make Emacs speak the XON/XOFF flow control protocol: You can make Emacs treat C-s and C-q as flow control characters by evaluating the following form to unconditionally enable flow control:

    (enable-flow-control)

  To enable flow control selectively, evaluate the following form:

    (enable-flow-control-on "vt100" "h19")

  Replace vt100 and h19 with the appropriate terminal names. These commands will automatically swap C-s and C-q to C-\ (the backslash) and C-^ (the caret). You can use variables to change the default swap keys (flow-control-c-s-replacement and flow-control-c-q-replacement).

  If you are fixing this for yourself, simply put the form in your .emacs file. If you are fixing this for your entire site, the best place to put it is in the lisp/site-start.el file. Putting this form in lisp/default.el has the disadvantage that if a user's .emacs file has an error, this will prevent lisp/default.el from being loaded and Emacs may be unusable for the user, even for correcting the .emacs file (unless the file is moved to another name).

  For further discussion of this issue, read the file PROBLEMS (in the top-level directory when you unpack the Emacs source).