B.2 Aliasing and Length

When converting a pointer to a different pointer type, make sure the object it really points to is at least as long as the target of the converted pointer. For instance, suppose p has type int * and it’s cast as follows:

int *p;

struct
  {
    double d, e, f;
  } foo;

struct foo *q = (struct foo *)p;

q->f = 5.14159;

the value q->f will run past the end of the int that p points to. If p was initialized to the start of an array of type int[6], the object is long enough for three doubles. But if p points to something shorter, q->f will run on beyond the end of that, overlaying some other data. Storing that will garble that other data. Or it could extend past the end of memory space and cause a SIGSEGV signal (see Signals).