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<title>EMACSulation LG #29</title>
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<meta name="description"
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for Emacs, allowing programs to send commands to an Emacs process on the
local machine or across the network.">
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<H4>
"Linux Gazette...<I>making Linux just a little more fun!</I>"
</H4>
<P> <HR> <P>
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<center>
<H1><font color="maroon">EMACSulation</font></H1>
<h4>by <a href="mailto:emarsden@mail.dotcom.fr">Eric Marsden</a>
</h4>
</center>
<blockquote><small>
This column is devoted to making the best use of Emacs, text editor
extraordinaire. Each issue I plan to present an Emacs extension
which can improve your productivity, make the sun shine more brightly
and the grass greener.
</small></blockquote>
<p><HR> <P>
<h1>Emacs as a server</h1>
<p> Many people meet Emacs as the default program launched by Elm to edit
an email message. Starting up Emacs in this way is a bit of a pain,
because of the delay and memory use, but also because the fresh Emacs
doesn't share buffers and a kill ring (storage for cut/copied text)
with other invocations. Rather than starting up a new Emacs for each
letter, you can set the environment variable <tt>EDITOR</tt> to
<tt>emacsclient</tt>.
<p> The Emacs server mechanism allows one Emacs process to service editing
requests from other applications. To enable it you need to add a line
like <tt>(server-start)</tt> in <tt>~/.emacs</tt>. When you say
<tt>emacsclient filename</tt>, the program searches for an Emacs
process (launching one if necessary) and sends it a message asking it
to edit <tt>filename</tt>. The caller process (the shell for example)
is blocked while the file is being edited. When you're finished type
<tt>C-x #</tt> and the client will be unblocked.
<h2>Gnuserv</h2>
<p> Gnuserv is a more sophisticated remote invocation mechanism for Emacs,
written by <a href="http://hplbwww.hpl.hp.com/people/ange/">Andy
Norman</a> (also the author of <a
href="http://www.linuxgazette.com/issue26/marsden.html">ange-ftp</a>).
It allows arbitrary Emacs Lisp commands to be sent to an Emacs process
running either on the local machine or on another machine on the
network.
<ol>
<li> If you're using XEmacs everything is already set up; you will only
need to add a <tt>(gnuserv-start)</tt> to your <tt>~/.emacs</tt>.
<li> Obtain the gnuserv-2.1alpha RPM from Red Hat contrib, or <a
href="http://cgi.debian.org/www-master/debian.org/Packages/unstable/editors/gnuserv.html">
the .deb</a> for Debian Hamm and skip to 6 ;
<li> Download the <a
href="http://hplbwww.hpl.hp.com/people/ange/gnuserv/">source</a> ;
<li> Edit <tt>config.h</tt> (I suggest using <tt>#define
DONT_USE_LITOUT</tt>) and <tt>gnuserv.h</tt>, where you choose the
communication method (the default is Internet domain <a
href="http://www.lowtek.com/sockets/">sockets</a>, which is necessary
if you want to be able to do the remote stuff) ;
<li> Put the file <tt>gnuserv.el</tt> somewhere in Emacs' load path. Let's
assume you have an <tt>~/elisp/</tt> directory where you put your
favorite Emacs Lisp extensions; an alternative is to copy gnuserv.el
to a site-wide Emacs Lisp directory such as
<tt>/usr/lib/emacs/site-lisp</tt> (type <tt>C-h v load-path</tt> in
Emacs to see a list of possiblities) ;
<li> Add something like this to your <tt>~/.emacs</tt> :
<pre>
(setq load-path (cons (expand-file-name "~/elisp") load-path))
(autoload 'gnuserv-start "gnuserv" "Better Emacs server support")
(setq gnuserv-frame (current-frame))
(gnuserv-start)
</pre>
<p> The second line tells Emacs that the function <tt>gnuserv-start</tt> is
defined in a file called gnuserv.el, which Emacs will load on demand.
The third line inhibits the default behaviour of opeing a new frame for
each new file edited (leave this out if you prefer). The last line
fires up the server.
</ol>
<p> To check that everything is working, type
<pre><tt>
~$ gnuclient <filename>
</tt></pre>
<p> which asks Emacs to open <tt><filename></tt>, just like
emacsclient. If this doesn't work (with a message like ``Refused
connection'' or ``Broken pipe''), skip down to the <a
href="#security">Security section</a>. You can also send off an
arbitrary bit of Emacs Lisp :
<pre>
~$ gnudoit '(message "Hi there, %s!" (user-full-name))'
</pre>
<h2>Applications</h2>
<p> Now you can do all sorts of silly things. You could get two Emacsen on
networked machines to play ping pong, echoing messages back and forth.
You could use Emacs as a CGI script server, profiting from its powerful
library without incurring the overhead of launching an interpreter for
each script (a little like the <a
href="http://www.FastCGI.com/">FastCGI</a> mechanism). For example,
let's hack together an external interface to Emacs' builtin
psychologist :
<pre>
(defun eliza-start ()
"Fire up the doctor."
(interactive)
(doctor)
;; We only have to type return once under this interface.
(re-search-backward " twice" nil t)
(replace-match "")
(goto-char (point-max))
(buffer-substring (point-min) (point-max)))
(defun eliza-continue (str)
"Send a string to the doctor and return her response."
(interactive)
(switch-to-buffer "*doctor*")
(insert "\n" str "\n")
(doctor-read-print)
(save-excursion
(re-search-backward "\n\n\\(\\(.+\n?\\)+\\)\n\n")
(match-string 1)))
(defun eliza-cleanup ()
"Pay the bill and leave."
(interactive)
(let ((buf (get-buffer "*doctor*")))
(if buf (kill-buffer buf))))
</pre>
<p> This can be used from the command line (for CGI use you'd have to
think about the trickier question of concurrent accesses) with a script
such as
<pre>
#! /bin/sh
gnudoit '(eliza-start)'
while read line
do
gnudoit "(eliza-continue \"$line\")"
done
gnudoit '(eliza-cleanup)'
</pre>
<p> You might even find constructive uses for the gnuserv technology, like
starting <a href="http://www.Gnus.org/">Gnus</a> (an Emacs news/email
client) and ange-ftp transfers from a ``network Emacs'', so that your
primary Emacs isn't affected by networking delays. You could also
communicate with Emacs from a crontab, telling it to fetch some web
pages with Emacs/w3, or send someone an email. You could use
Netscape's third party <a
href="http://developer.netscape.com/software/sdks/index.html?content=mailnews.html">
email/news API</a> to invoke Emacs instead of the builtin mail and news
clients. It might also be useful for sending commands to Emacs from a
<a href="http://www.plig.org/xwinman/">window manager</a> menu.
<h2>Security considerations<a name="security"></A></h2>
<p> More and more Linux distributions use good <a
href="http://ciac.llnl.gov/ciac/documents/ciac2316.html">X security</a>
as set up out of the box. You will notice for example that when you su
to root on a modern system, you will no longer be able to launch X
clients, because the X server is protected by an <a
href="http://www.xs4all.nl/~zweije/xauth.html">xauth</a> cookie.
<p> While allowing access to your X display is bad enough (someone could
capture all your keystrokes, for example), giving remote access to your
Emacs process is much more worrying, since Emacs can execute arbitrary
commands under your id, delete files, send insulting email to the
President of the United States, etc.
<p> Since release 2.1, gnuserv is able to use MIT-MAGIC-COOKIE-1
authentication for remote requests. This protocol uses the contents of
your <tt>~/.Xauthority</tt> file, as described in the xauth(1) man page.
Gnuserv requires a cookie for display number 999, which you can create
as follows (blade is the name of the machine) :
<pre>
~$ xauth add blade:999 . `cat /etc/passwd | md5sum`
~$ xauth list
blade/unix:0 MIT-MAGIC-COOKIE-1 bc1d627babdbabe9d1f288d2b57c348f
blade:999 MIT-MAGIC-COOKIE-1 d89570b20925d401c05a79be67159cae
</pre>
<p> (<tt>`cat /etc/passwd | md5sum`</tt> is just a convenient way of
generating a cookie; on most Linux systems you will be able to use the
<a
href="http://hegel.ittc.ukans.edu/topics/linux/man-pages/man1/mcookie.1.html">mcookie</a>
command, or you can bake a cookie by hand). Now you should be able to
use gnuclient/gnudoit on the local machine. The next step is to
transfer the cookie to each remote machine from which you plan to
access Emacs, with a command such as :
<pre>
~$ xauth extract - blade:999 | rsh remotehost.edu xauth merge -
</pre>
<p> If you don't run X you will have to fall back to a host-based access
control system : the environment variable <tt>GNU_SECURE</tt> is
assumed to point to a file which contains a list of machines which are
authorized to open connections to your Emacs process. Finally, if your
machine isn't networked, you've probably already skipped to the next
section.
<h2>How does it work?</h2>
<p> Your command takes a fairly convoluted route to reach Emacs. There are
four parties involved in a transaction : the ``client'', or
program which wants a service from Emacs (Elm for example), the
gnuclient program (which runs on the requesting machine), the gnuserv
process (which runs on the machine running Emacs), and of course the
Emacs process itself. They communicate as indicated by the following
diagram :
<p> <img src="./gx/marsden/gnuserv-arch.gif" alt="Communication diagram (3 kB)">
<p> The fragile aspect of the system is that if the gnuserv program dies
for some reason, everything grinds to a halt. An alternative
communication method which would involve fewer working parts could be
inspired from the Netscape remote invocation protocol. The gnuclient
capabilities would be added directly to Emacs, and a gnudoit request
would look something like
<pre>
emacs -remote -lisp '(message "Hi")'
</pre>
<p> The new Emacs process would search for an existing Emacs process to
which it could dispatch the request, or service it directly. The
disadvantage is that each request would be slower, since an Emacs needs
to be forked each time. The image is most often in the disk cache, so
this wouldn't be catastrophically slow (it works OK with Mozilla, which
is <a
href="http://x5.dejanews.com/getdoc.xp?AN=202452924&CONTEXT=896094230.406519988&hitnum=1">far
bigger</a> than Emacs).
<h2>Next time ...</h2>
<p> I've received a fair bit of email asking how to customize various
aspects of Emacs, so I'll try to scratch the surface of this vast topic
next month, and discuss the Customize package. Don't hesitate to
contact me at <tt><emarsden@mail.dotcom.fr></tt> with comments,
corrections or suggestions (what's <em>your</em> favorite
couldn't-do-without Emacs extension package?). <code>C-u 1000 M-x
hail-emacs</code> !
<p> <b>PS</b> : Emacs isn't in any way limited to Linux, since
implementations exist for many other operating systems (and some
systems which only halfway operate). However, as one of the leading
bits of free software, one of the most powerful, complex and
customizable, I feel it has its place in the <i>Linux Gazette</i>.
<!--===================================================================-->
<P> <hr> <P>
<A HREF="../issue25/marsden.html">EMACSulation #1, February 1998</A><br>
<A HREF="../issue26/marsden.html">EMACSulation #2, March 1998</A><br>
<A HREF="../issue27/marsden.html">EMACSulation #3, April 1998</A>
<P> <hr> <P>
<center><H5>Copyright © 1998, Eric Marsden <BR>
Published in Issue 29 of <i>Linux Gazette</i>, June 1998</H5></center>
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