[Ecls-list] Declaring functions with variable arguments from C

[Juan Jose Garcia Ripoll]

On Monday 09 June 2003 18:42, Julian St. wrote:
> Hello,
>
> how would I declare a C function (and use the arguments) that would look
> from Lisp somewhat like
>
> (defun emit-print (event &rest cmds) ...
>
> I hope I am correct with my approach using cl_def_c_function_va() ?

If your function (Let us call it "my_c_fun()") has a prototype like
	cl_object my_c_fun(int narg, ...)
and it expects only to receive values of type "cl_object", then you can use
	cl_def_c_function_va(c_string_to_object("MY-C-FUN"), my_c_fun)
to assign the function to the lisp symbol MY-C-FUN. After the call to 
"cl_def..." the function may be used in the Lisp world. The argument NARG 
will tell the function how many values it should retrieve.

For retrieving optional arguments in a safe way, you should use the cl_va_* 
interface. This works as follows:

	cl_object a_test_function(int narg, ...) {
		cl_va_list args;
		cl_va_start(args, narg, narg, 0);
		cl_object output = Cnil;

		for (i = 0; i<narg; i++)
			output = CONS(output, cl_va_arg(args));
		return1(output);
	}

The whole interface for defining functions, etc, is one of the most "unstable" 
features of ECL, in the sense that it is prone to change in the near future. 
Since I use it very little, I will welcome (and probably implement :-) any 
suggestions to improve its usability.

Best regards,

Juanjo

-- 
Max-Planck-Institut fuer Quantenoptik	+49/(0)89/32905-345
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Garching b. Muenchen, Germany		Juan.Ripoll at mpq.mpg.de