Question

Please consider the following code:

typedef struct {
  int type;
} object_t;

typedef struct {
  object_t object;
  int age;
} person_t;

int age(object_t *object) {
  if (object->type == PERSON) {
    return ((person_t *)object)->age;
  } else {
    return 0;
  }
}

Is this legal code or is it violating the C99 strict aliasing rule? Please explain why it is legal/illegal.

Answer 1

Strict aliasing rule is about two pointers of different types referencing the same location in memory (ISO/IEC9899/TC2). Although your example reinterprets the address of object_t object as an address of person_t, it does not reference memory location inside object_t through the reinterpreted pointer, because age is located past the boundary of object_t. Since memory locations referenced through pointers are not the same, I’d say that it is not in violation of the strict aliasing rule. FWIW, gcc -fstrict-aliasing -Wstrict-aliasing=2 -O3 -std=c99 seems to agree with that assessment, and does not produce a warning.

This is not enough to decide that it’s legal code, though: your example makes an assumption that the address of a nested structure is the same as that of its outer structure. Incidentally, this is a safe assumption to make according to the C99 standard:

6.7.2.1-13. A pointer to a structure object, suitably converted, points to its initial member

The two considerations above make me think that your code is legal.