Question

What is the difference between these 2 declaration:

int operate(int (*func)(int, int), int a, int b){
    return (*func)(a, b);
}

and

int operate(int func(int, int), int a, int b){
        return func(a, b);
 }

These two also seems to be equivalent: operate(sum, 1, 1) and operate(&sum, 1, 1)

If I pass function sum as a function of 2 numbers in the place of func, the result are still the same. Why?

Answer 1

§6.7.5.3/8:

A declaration of a parameter as ‘‘function returning type’’ shall be adjusted to ‘‘pointer to function returning type’’, as in 6.3.2.1.

In other words, the two function declarations are identical.

As far as the function call goes, §6.5.2.2/3:

A postfix expression followed by parentheses () containing a possibly empty, comma-separated
list of expressions is a function call. The postfix expression denotes the called function.

Since both func(a, b); and (*func)(a, b) are postfix expressions followed by parentheses, they’re both function calls. Since func and (*func) both designate the same function, they both call the same function.