adding_efuns

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

1. Creating a directory under src/packages/ with name mypkg.

2. inside src/packages/mypkg directory, create LPC function prototype file called mypkg.spec.

   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.

3. Create a file mypkg.cc and create an CMakeLists.txt to compile it, add an library target called PACKAGE_MYPKG.

4. In your mypkg.cc, start by including the pkg API header,

   #include "base/package_api.h"

   Then add a function of the form f_efunction_name() {}

   For examples of how to structure the f_efunction_name function, look at the other efun already defined in the driver.

   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).