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E.8.2 Writing Module Functions

The main reason for writing an Emacs module is to make additional functions available to Lisp programs that load the module. This subsection describes how to write such module functions.

A module function has the following general form and signature:

— Function: emacs_value module_func (emacs_env *env, ptrdiff_t nargs, emacs_value *args, void *data)

The env argument provides a pointer to the API environment, needed to access Emacs objects and functions. The nargs argument is the required number of arguments, which can be zero (see make_function below for more flexible specification of the argument number), and args is a pointer to the array of the function arguments. The argument data points to additional data required by the function, which was arranged when make_function (see below) was called to create an Emacs function from module_func.

Module functions use the type emacs_value to communicate Lisp objects between Emacs and the module (see Module Values). The API, described below and in the following subsections, provides facilities for conversion between basic C data types and the corresponding emacs_value objects.

A module function always returns a value. If the function returns normally, the Lisp code which called it will see the Lisp object corresponding to the emacs_value value the function returned. However, if the user typed C-g, or if the module function or its callees signaled an error or exited nonlocally (see Module Nonlocal), Emacs will ignore the returned value and quit or throw as it does when Lisp code encounters the same situations.

After writing your C code for a module function, you should make a Lisp function object from it using the make_function function, whose pointer is provided in the environment (recall that the pointer to the environment is returned by get_environment). This is normally done in the module initialization function (see module initialization function), after verifying the API compatibility.

— Function: emacs_value make_function (emacs_env *env, ptrdiff_t min_arity, ptrdiff_t max_arity, subr func, const char *docstring, void *data)

This returns an Emacs function created from the C function func, whose signature is as described for module_func above (assumed here to be typedef'ed as subr). The arguments min_arity and max_arity specify the minimum and maximum number of arguments that func can accept. The max_arity argument can have the special value emacs_variadic_function, which makes the function accept an unlimited number of arguments, like the &rest keyword in Lisp (see Argument List).

The argument data is a way to arrange for arbitrary additional data to be passed to func when it is called. Whatever pointer is passed to make_function will be passed unaltered to func.

The argument docstring specifies the documentation string for the function. It should be either an ASCII string, or a UTF-8 encoded non-ASCII string, or a NULL pointer; in the latter case the function will have no documentation. The documentation string can end with a line that specifies the advertised calling convention, see Function Documentation.

Since every module function must accept the pointer to the environment as its first argument, the call to make_function could be made from any module function, but you will normally want to do that from the module initialization function, so that all the module functions are known to Emacs once the module is loaded.

Finally, you should bind the Lisp function to a symbol, so that Lisp code could call your function by name. For that, use the module API function intern (see intern) whose pointer is also provided in the environment that module functions can access.

Combining the above steps, code that arranges for a C function module_func to be callable as module-func from Lisp will look like this, as part of the module initialization function: 

      emacs_env *env = ert->get_environment (ert);
      emacs_value func = env->make_function (env, min_arity, max_arity,
                                             module_func, docstring, data);
      emacs_value symbol = env->intern (env, "module-func");
      emacs_value args[] = {symbol, func};
      env->funcall (env, env->intern (env, "defalias"), 2, args);

This makes the symbol module-func known to Emacs by calling env->intern, then invokes defalias from Emacs to bind the function to that symbol. Note that it is possible to use fset instead of defalias; the differences are described in defalias.

Using the module API, it is possible to define more complex function and data types: interactive functions, inline functions, macros, etc. However, the resulting C code will be cumbersome and hard to read. Therefore, we recommend that you limit the module code which creates functions and data structures to the absolute minimum, and leave the rest for a Lisp package that will accompany your module, because doing these additional tasks in Lisp is much easier, and will produce a much more readable code. For example, given a module function module-func defined as above, one way of making an interactive command module-cmd based on it is with the following simple Lisp wrapper: 

     (defun module-cmd (&rest args)
       "Documentation string for the command."
       (interactive spec)
       (apply 'module-func args))

The Lisp package which goes with your module could then load the module using the module-load primitive (see Dynamic Modules) when the package is loaded into Emacs.