Function pointer to different functions with different arguments in C

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.

My question is, for this scenario, Can I at all define a function pointer?

No. (Other than by dirty typecasting.)

Is there any other way to do this?

Your best bet is to create a wrapper function for one of your existing functions. For example:

double my_func_one_wrapper(double x, double p[], double c) {
    return my_func_one(x, p[0], p[1], c);
}

That way, you have two functions with the same signature, and therefore the same function-pointer type.

An old topic but I am facing same issue and finally came with this idea.

If you want the same prototype for each functions you can wrap the parameters in structure like this :

 typedef struct {
     double a,
            b,
            c; 
 }chunk1_t;

  typedef struct {
     double p[];
     double c; 
 }chunk2_t;

Then your function pointer becomes:

 double (*pfunc) (double x, void *args);

which leads to something like this :

 pfunc cb;

 double swap_function(double x, pfunc cb, void *args);

 double my_func_one(double x, void *args) { 
   chunk1_t *chunk = (chunk1_t*) args;
   return x + chunk->a + chunk->b + chunk->c; 
 }

 double my_func_two(double x, void *args) {
   chunk2_t *chunk = (chunk2_t*) args;
   return x + chunk->p[0] + chunk->p[1] + chunk->c ;
 }

 int main(){
   // declare a, b,...
   double a = 1.0f;
   //...
   // cast for safety
   chunk1_t myChunk1 = {(double)a, (double)b, (double)c};
   // don't if you like risk
   chunk2_t myChunk2 = {p, c};

   swap_function(x, cb, &myChunk1); 
   swap_function(x, cb, &myChunk2);
 }

Using function pointer stored in structure:

 #define FUNC_ONE_METHOD 1
 #define FUNC_TWO_METHOD 2

 typedef struct chunk{
     double a, b, c;
     double p[];
     int method;
     double (*pfunc) (double x, struct chunk *self);
 }chunk_t;

 double swap_function(double x, chunk_t *ch){
    switch (ch->method)
    {
        case FUNC_TWO_METHOD:
            ch->pfunc = my_func_two;
            break;
        case FUNC_ONE_METHOD:
            ch->pfunc = my_func_one;
            break;
        default:
            return -1; // or throw error
    return ch->pfunc(x, ch);
 }


 chunk c = {.a= 1, .b=3, .c=1, .method=1};
 swap_function(x, &c);