So long as the pointer has a valid value, we can use a pointer in a condition. Just as when we use an arithmetic value in a condition (§ 2.1.2, p. 35), if the pointer is 0, then the condition is false

int ival = 1024;
int *pi = 0; // pi is a valid, null pointer
int *pi2 = &ival; // pi2 is a valid pointer that holds the address of ival
if (pi) // pi has value 0, so condition evaluates as false
// ...
if (pi2) // pi2 points to ival, so it is not 0; the condition evaluates as true
// ...