Question

I was trying out the following C code:

void main()
{
   int i;
   for(i = 0; i< 10; i++)
   { 
      int num;
      printf("\nthe variable address is: %p", &num);

   }
   getch();
}

I had expected it to either throw an error or declare num multiple times but instead, the output shows the same value for &num, for all the iterations of the for loop.
What is the reason behind this behavior? It seems that irrespective of having the declaration in the for loop, the actual declaration/definition happens just once.

Can someone help me understand this behavior?

Answer 1

To help illustrate this, compare this:

   int i;
   int val = 0;
   for(i = 0; i< 5; i++)
   { 
      int num = val++;
      printf("\nthe variable address is: %p", &num);
      printf("\nthe value is: %d", num);
   }

This again shows that num always has the same address, but also is initialised with a distinct value each iteration.

The idea with the stack is that its layout is defined at compile time; each stack variable maps to an address on the stack with the stack frame.

Another thing to hlep you get this is to consider that if each iteration “allocated” a new variable, how would a small machine handle a large loop?

See: Call Stack