Function pointer to different functions with different arguments in C

Question

I have two functions with variable number and types of arguments

double my_func_one(double x, double a, double b, double c) { return x + a + b + c }
double my_func_two(double x, double p[], double c) { return x + p[0] + p[1] + c }

I want to use a pointer to a function to the functions I defined above based on a some condition getting true e.g.

if (condition_1)
   pfunc = my_func_one;
else if (condition_2)
   pfunc = my_func_two;

 // The function that will use the function I passed to it
 swap_function(a, b, pfunc);

My question is, for this scenario, Can I at all define a function pointer? If yes, how?
My understanding is that the prototype of function pointer should be the same for all those functions it CAN be pointed to.

typedef double (*pfunction)(int, int);

In my case they are not the same. Is there any other way to do this?

Language

I am developing in C and I am using gcc 4.4.3 compiler/linker

Answer 1

The cleanest way to do it is to use a union:

typedef union {
  double (*func_one)(double x, double a, double b, double c);
  double (*func_two)(double x, double p[], double c);
} func_one_two;

Then you can initialize an instance of the union, and include information to the swap_function function to say which field is valid:

func_one_two func;

if (condition_1)
   func.func_one = my_func_one;
else if (condition_2)
   func.func_two = my_func_two;

 // The function that will use the function I passed to it
 swap_function(a, b, func, condition_1);

This assumes that swap_function can know based on condition_1 being false that it should assume condition_2. Note that the union is passed by value; it's only a function pointer in size after all so that's not more expensive than passing a pointer to it.