I am learning C and I am studying functions. So, I read that when I implement my own function I have to declare it before the main(). If I miss the declaration the compiler will get an error message.
As I was studying this example (finds if the number is a prime number)

#include <stdio.h> void prime(); //function prototype(declaration)  int main() {  int num,i,flag; num = input(); // No argument is passed to input() for(i=2,flag=i; i<=num/2; ++i,flag=i) { flag = i;   if(num%i==0) {  printf("%d is not prime\n",num);  ++flag;  break;  } }   if(flag==i) printf("%d is prime\n",num);   return 0; }  int input() /* Integer value is returned from input() to calling function */ {  int n; printf("\nEnter positive enter to check: ");  scanf("%d",&n);  return n; } 

I noticed that a function prime() is declared, but in the main a function input() is called and also the function input() is implemented at the bottom. Ok, I thought it was a mistake and I change the name from prime to input.
 However if I delete the declaration and I don’t put any there, the program is compiled without errors and it runs smoothly. (I compile and run it in Ubuntu)

So my question is, is it necessary to declare a void function with not arguments?

I am learning C and I am studying functions. So, I read that when I implement my own function I have to declare it before the main(). If I miss the declaration the compiler will get an error message.

As I was studying this example (finds if the number is a prime number),

#include <stdio.h> void prime(); // Function prototype(declaration) int main() { int num, i, flag; num = input(); // No argument is passed to input() for(i=2,flag=i; i<=num/2; ++i,flag=i) { flag = i; if(num%i==0) { printf("%d is not prime\n", num); ++flag; break; } } if(flag==i) printf("%d is prime\n", num); return 0; } int input() /* Integer value is returned from input() to calling function */ { int n; printf("\nEnter positive enter to check: "); scanf("%d", &n); return n; } 

I noticed that a function prime() is declared, but in the main, a function, input(), is called and also the function input() is implemented at the bottom. Ok, I thought it was a mistake and I change the name from prime to input. 

However if I delete the declaration and I don’t put any there, the program is compiled without errors and it runs smoothly. (I compile and run it on Ubuntu.)

Is it necessary to declare a void function with not arguments?