When I compile the example, i get: 0x7ffd0b57ebd0 0x7ffd0b57ebd8 0 OK, so gcc reordered a & b; I'll fix this by chaning the initialisation of p and q to:

When I compile the example, i get: 0x7ffd0b57ebd0 0x7ffd0b57ebd8 0 OK, so gcc reordered a & b; I'll fix this by chaning the initialisation of p and q to: int *p = &a + 1; int *q = &b; But when I now run the example, I get: gcc -o c c.c && ./c 0x7ffe55eb0aa4 0x7ffe55eb0aa4 1 gcc -O -o c c.c && ./c 0x7ffcfeffd914 0x7ffcfeffd914 0 So, p==q only evaluates to 1 if optimisation is enabled. $ gcc --version gcc (Ubuntu 7.3.0-16ubuntu3) 7.3.0