Why the output of the following 3 programs is like this ?

Question

#include<stdio.h>
int add(int,int);
int main()
{
  int p=add(10,20);
  printf("%d",p);
  return 0;
}

int add(int x, int y)
{
  int sum=x+y;
}

O/P: 30

---

#include<stdio.h>
int add(int,int);
int main()
{
  int p=add(10,20);
  printf("%d",p);
  return 0;
}
int add(int x, int y)
{
  int sum=x+y;
  printf("Hello");
}

O/P: 5

---

#include<stdio.h>
int add(int,int);
int main()
{
  int p=add(10,20);
  printf("%d",p);
  return 0;
}
int add(int x, int y)
{
  int sum=x+y;
  sum++;
}

O/P: 30

Answer 1

None of the code snippets would compile because you haven't declared/defined the functions before using them.

Furthermore even if you add the declarations above main() the behaviour of the snippets would still be undefined because you are using the returned value of a non-void function having no return statement.

Answer 2

You aren't returning sum in your add function.

Answer 3

printf("%d",p); is printing some garbage that p points to. You should return sum from add:

return sum;

Answer 4

Just by luck, or rather because you compiled them without optimization. ;-)

The results are actually undefined, and depend on the compiler and compilation options. In your setup the function add() happened to leave something on the stack at the place where main() expected a return value. Try with optimization and you will see the difference.