About Pointers To Functions in function declarations

#include<stdio.h>
#include<stdlib.h>

int fun1()
{
    printf("I am fun1.");
    return 0;
}

int fun2(int fun())
{
    fun();
    return 0;
}

int main()
{
    fun2(fun1);
    return 0;
}

The above program can run. As far as I am concerned, I can understand int fun2(int (*fun)()), but I do not know how int fun2(int fun()) works. Thank you.

Nawaz

When you write int fun2(int fun()), the parameter int fun() converts into int (*fun)(), it becomes exactly equivalent to this:

int fun2(int (*fun)());

A more famiiar conversion happens in case of array when you declare it as function parameter. For example, if you've this:

int f(int a[100]);

Even here the parameter type converts into int*, and it becomes this:

int f(int *a);

The reason why function type and array type converts into function pointer type, and pointer type, respectively, is because the Standard doesn't allow function and array to be passed to a function, neither can you return function and array from a function. In both cases, they decay into their pointer version.

The C++03 Standard says in §13.1/3 (and it is same in C++11 also),

Parameter declarations that differ only in that one is a function type and the other is a pointer to the same function type are equivalent. That is, the function type is adjusted to become a pointer to function type (8.3.5).

And a more interesting discussion is here:

Reference to Function syntax - with and without &

asaelr

int fun2(int (*fun)()) and int fun2(int fun()) are exactly the same. When you declare a function argument from a function type, the compiler uses it as if it were a pointer to the same function type.

These two function definitions are equivalent in C:

 int fun2(int fun()) { ... }

and

 int fun2(int (*fun)()) { ... }

In the first function the parameter is adjusted to a function pointer. See C Standard paragraph:

(C99, 6.7.5.3p8) "A declaration of a parameter as ‘‘function returning type’’ shall be adjusted to ‘‘pointer to function returning type’’, as in 6.3.2.1."

Looking to it in a lower level (and in a x86-based architecture):