Subtracting two pointers giving unexpected result

Question

#include 

int main() {
    int *p = 100;
    int *q = 92;
    printf(\"%d\\n\", p - q);  //prints 2
}

Shouldn\'t the output of above pr

Answer 1

These are integer pointers, sizeof(int) is 4. Pointer arithmetic is done in units of the size of the thing pointed to. Therefore the "raw" difference in bytes is divided by 4. Also, the result is a ptrdiff_t so %d is unlikely to cut it.

But please note, what you are doing is technically undefined behaviour as Sourav points out. It works in the most common environments almost by accident. However, if p and q point into the same array, the behaviour is defined.

int a[100];
int *p = a + 23;
int *q = a + 25;

printf("0x%" PRIXPTR "\n", (uintptr_t)a); // some number
printf("0x%" PRIXPTR "\n", (uintptr_t)p); // some number + 92
printf("0x%" PRIXPTR "\n", (uintptr_t)q); // some number + 100
printf("%ld\n", q - p); // 2