This document explains how to add efunctions to the MudOS driver.  Here are the
steps:

1) Add a prototype for the efunction to func_spec.c.  This is not an ANSI
   C prototype but a prototype in the same form as the other prototypes in
   that file.  For example, the prototype for the cat() efun is as follows:

   int cat(string, void|int, void|int);

   The second and third arguments of cat can either be void or int.  Here
   'void' means optional.  Thus cat("xyz") is allowed as is cat("xyz",5,8);
   The return type of cat() is int.

   If a function is to allow a variable number of arguments (vararg), you can
   declare with trailing elipses:

   void call_out(string, int, ...);

   call_out() does not return a value.

2) Add a function of the form f_efunction_name to efunctions.c or to
   a C source file created for the purpose of adding your own local efunctions.
   Creating your own C file for local efunctions can be the best choice because
   doing so makes it easy for you to integrate your changes into future
   releases of MudOS.  If you do create a separate C file for your efunctions
   (e.g. local_efuns.c), then you will need to #include "efuns.h" which gives
   your file the extern, and static declarations that efuns are likely to
   need.  For examples of how to structure the f_efunction_name
   function, look at the efunctions already defined in efunctions.c.  In
   addition, you should read the file ./doc/driver/stackmachine.  This
   file is fairly old but is still useful toward understanding the general
   structure of the driver stack machine.  The main points to know are
   that the arguments passed to an efunction are stored on a stack of svalues
   named sp.  For example, if the efunction is passed three arguments, then the
   first argument will be at (sp - 2), the second at (sp - 1), and the
   third at sp.  If a function allows a variable number of arguments, then
   the num_arg parameter of the efun will be set to indicate the number of
   arguments actually passed (on the sp stack).  Each efunction is responsible
   for leaving a single svalue on the sp stack at the time the efunction
   exits (even if the efunction is defined to return void).  The usual
   strategy is to use the stack values that you need (sp - x),
   pop_n_elems(num_args) and then push a result onto the stack.  Note,
   if an efunction takes a single argument and is defined to return void,
   then you needn't pop anything from the stack or push anything on (the
   single argument can serve as the return value).