run function with n params - c

Question

I need to solve one problem in a virgin ANSI C (1989).
I have some pointer (void *func) to function with (int n) double parameters, and have array double values[] with n numbers. So I want to run my function with n param that are located in value.
For example, I have function:
double hypotenuse(double x, double y, double z);
so
void *func = (void *)hypotenuse; double values[3] = {5, 4, 3}; int n = 3;
and I want to do something like this:
func(n, values);
The problem is that I can't change the prototypes of the functions,
so I need to do this somehow (maybe some macros?).

Answer 1

The main problem is that you have to cast the pointer differently depending on the number of argument (i.e. depending on the n variable).
One way is to use a wrapper-function containing a switch statement for the argument number:
double wrapper(void *func, double args[], int n)
{
switch (n)
{
case 0:
return ((double (*)(void)) func)();
case 1:
return ((double (*)(double)) func)(args[0]);
case 2:
return ((double (*)(double, double)) func)(args[0], args[1]);
case 3:
return ((double (*)(double, double, double)) func)(args[0], args[1], args[2]);
default:
printf("Error: wrapper called with %d arguments\n", n)
break;
}
return 0.0;
}

Answer 2

Written late last night — but my Internet connection went down as I tried to post it. I see Joachim has written essentially the same answer.
Within limits, this will work:
#include <assert.h>
extern double function_invoker(void *func, int n, double *values);
double function_invoker(void *func, int n, double *values)
{
switch (n)
{
case 0:
return (*(double (*)(void))func)();
case 1:
return (*(double (*)(double))func)(values[0]);
case 2:
return (*(double (*)(double, double))func)(values[0], values[1]);
case 3:
return (*(double (*)(double, double, double))func)(values[0], values[1], values[2]);
default:
assert("Need more entries in the switch in function_invoker()" == 0);
return(0.0);
}
}
The obvious limits are how many entries you want to make in the switch. I've seen loosely analogous code go up to over 100 arguments; I'm not sure why that was considered necessary.
That code compiles without warnings under GCC 4.6.0 on Mac OS X 10.8.2:
$ gcc -g -std=c99 -Wall -Wextra -Wmissing-prototypes -Wstrict-prototypes \
-Wold-style-definition -c x.c
$
But if you go with double (*)() in place of void *, you get:
$ gcc -g -std=c99 -Wall -Wextra -Wmissing-prototypes -Wstrict-prototypes \
-Wold-style-definition -c x.c
x.c:3:1: warning: function declaration isn’t a prototype [-Wstrict-prototypes]
x.c:5:1: warning: function declaration isn’t a prototype [-Wstrict-prototypes]
$