From: Stefan Wiermann <wiers000 AT mail DOT Uni-Mainz DOT DE>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: function pointers
Date: Mon, 3 Jul 2000 15:56:08 +0200
Organization: Johannes Gutenberg-Universitaet Mainz, Germany
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On Mon, 3 Jul 2000, Sean Proctor wrote:

o> okay, I posted this on comp.lang.os  I don't think there is a way to
> do this well... what I'd basically like to do is have a pointer to a
> function, and a pointer to an argument list which I can put anything
> into...  I'd like to call library functions, but I don't know which
> I'm going to call, or what the arguments are going to be at
> compilation.  here's what I'd like to do:
> 
> void *args;
> int (*general_fun)();
> int fun1(int);
> int fun2(float, int);

You may not pass different types/argument lists to a (*myfunc) () call.
You declared (*general_fun)() implicitely as (*general_fun)(void).
So you are allowed to call it without varlist only.

My first suggestion: If you want to pass only a limited type of different
var lists like here either int or float,int, pass it as structure:

typedef struct mystruct <brace> float foo; int bar; <brace>;

or (better) as pointer to mystruct: (*general_fun)(mystruct*)....
May be you want to have a union instead of a struct.
But in both cases you have to provide information how the varlist is to be
read:
e.g 

---SNIP---

struct bla <brace> count_f; // number of passed floats 
                   count_i; // number of passed ints
	           float *float_pointer //points to an array of floats
	           int   *int_pointer   // the same for ints
	   <brace> 

---SNIP---

Alternatively, you can use the stdarg-lib which provides handling of
variable var lists.

Me, personally, I prefer the first method, because I can not help, the
stdarg-thingies look kinda weird and tricky ;-)


> 
> int main(void)
> {
> 	int test, fun1_arg1, fun2_arg2;
> 	float fun2_arg1;
> 	...
> 	if(test) {
> 		general_fun = fun1;
> 		args = fun1_arg1;
> 	}
> 	else {
> 		general_fun = fun2;
> 		args = malloc(sieof(float) + sizeof(int));
> 		*args = fun2_arg1;
> 		*(args + sizeof(float)) = fun2_arg2;
> 		/* I know the above 3 lines don't work at all, but I'm
> hoping someone will understand what I hope to accomplish by them and
> suggest a solution */

-----SNIP----

Uhoh, I think it's clear what <brace> means. My stone-aged MWM provides
some very weird xterm restrictions.

WBR

Stefan Wiermann