How do the C compiler deal with the local var in stack during recursion?

Question

I write a simple binary search program and meet a problem, see my three confusions below:

• If I choose icc -g main.c or icc -O0 main.c the code return wrong answer, but if I use icc -O2 main.c I can have the correct answer.

• When the code mid = (beg + end) / 2; be defined before the if(beg <= end) { the answer is right no matter what flag I use to compile it.

• I think the problem is the local var mid in the stack during the recursion.

Help me, thank you!

#include <stdio.h>
#define MAX 6
int a[MAX] = {1,2,3,4,5,6};
int improved_binary_search(int key, int beg, int end) {
    int mid, temp;
    //mid = (beg + end) / 2;
    if(beg <= end) {
             mid = (beg + end ) /2;
             if(a[mid] == key)
                     return mid;
             if(a[mid] < key)
                     return improved_binary_search(key, mid + 1, end);
              if(a[mid] > key)
                      return improved_binary_search(key, beg, mid - 1);
              //printf("test is %d\n", mid);
      }
      if(mid == 0)
              return 0;
      if(mid == MAX - 1)
              return MAX - 1;
      if(a[mid] < key)
              return a[mid+1] - key < key - a[mid] ? mid+1 : mid;
      if(a[mid] > key)
              return a[mid] - key < key - a[mid-1] ? mid : mid-1;
      return -1;
  }

int main(void)
{
     printf("%d\n", improved_binary_search(100, 0, MAX - 1));
     return 0;
}

Answer 1

You just declared mid but did not initialize it, it might happen that if the first if condition( if(beg <= end) )is not hit then the value of mid is Indeterminate .

That would result in Undefined Behavior . The behavior you see is because of this UB and not because of compiler configurations.

You should always initialize your variables to meaningful values.

Answer 2

One problem is that you don't initialize mid unless beg <= end . The (potentially uninitialized) variable is then used in the second half of improved_binary_search() . This is undefined behaviour.

To fix, make sure to always initialize mid .