Declaring function inside function in C

Question

I found this question on an online exam. This is the code:

#include <stdio.h>

int main(void) {
    int demo();
    demo();
    (*demo)();
    return 0;
}

int demo(){
    printf("Morning");
}

I saw the answer after the test. This is the answer:

MorningMorning

I read the explanation, but can't understand why this is the answer. I mean, shouldn't the line int demo(); cause any "problem"?

Any explanation is helpful. Thanks.

Answer 1

It is not a problem because that int demo(); is not a function definition, it is just an external declaration, saying (declaring) that a function of such name exists.

In C you cannot define a nested function:

int main(void) {
    int demo() {} //Error: nested function!!!
}

But you can declare a function just fine. It is actually equivalent to:

#include <stdio.h>

int demo(); //function declaration, not definition

int main(void) {
    demo();
    (*demo)();
    return 0;
}

int demo(){
    printf("Morning");
}

except that in your code the external forward demo() declaration is only visible inside main.

Answer 2

You can declare a function inside another function. This is allowed. The only thing is it is visible only inside the function in which it is declared.