How to make a function which returns a function pointer in an array of function pointers

Question

I need to call a function using an integer. I'm really new to C language and the function pointer is horribly confusing.

Goal

1. I have so many messy functions which will mess up all if I write them in the main.c.
2. So I write those messy functions into another c file messy_functs.c
3. In main.c, I need to select one of those messy functions and call it, depending on the integer obtained from my algorithm. Those functions commonly needs two arguments, int input and int *output.
4. The callee function will store the result in output which is passed as a pointer

My plan

1. In messy_functs.c, make an array function_list[] containing function pointers in the global scope.
2. In messy_functs.c. Make a function get_function_by_index(int function_index) that returns a function pointer indexed by the argument.
3. In main.c, call messy_function passing function_index and then get the result by calling the returned function passing two arguments int input and int *output.

Problem

I made my code as follows. But failed with an error error: too many arguments to function.. Please ignore some typos.

// main.c
#include "messy_functs.h"
...

void get_result(int func_idx, int input, int *output)
{
    (*get_function_by_index(func_idx))(input, output);
}

int main() {
    int result, data, func_idx;

    ...

    func_idx, data = some_algorithm_i_made(~~);
    get_result(func_idx, data, &result);
    printf("Got the result %d\n", result);

}

============
// messy_functs.c
#define FUNCTION_NUM 100
const void (*function_list[FUNCITON_NUM])(int input, int *output) = {func1, func2, func3, func_m, func_qwer, func_abab, ...(many functions) };

void (*get_function_by_index(int func_id)){
    return function_list[func_id];
}

void func1(int input, int *output) {...}
void func2(int input, int *output) {...}
(... other functions)

I've been tried to fix but got different errors such as void value not ignored as it ought to be. I know there must be other good posts that can solve my problem. I've been read many posts about function pointers and array of function pointers but couldn't make it.

Answer 1

The proper definition for the array would be:

void (* const function_list[FUNCITON_NUM])(int input, int *output) = ...

And for get_function_by_index would be:

void (*get_function_by_index(int func_id))(int, int*)

When passing or returning function pointers, it's very helpful to have a typedef for the function pointer type to make it easier to manage.

The typedef can be created in messy_functs.h for the function pointer type:

typedef void (*messy_func)(int, int *);

Then in messy_funct.c, use this type for the array:

const messy_func function_list[FUNCITON_NUM] = {func1, func2, func3, func_m, ... };

And for the return type of get_function_by_index

messy_func get_function_by_index(int func_id) {
    return function_list[func_id];
}

And you would call this function like this:

void get_result(int func_idx, int input, int *output)
{
    get_function_by_index(func_idx)(input, output);
}