Union element alignment

问题

If I have a union, C standard guarantees that the union itself will be aligned to the size of the largest element.

union U {
    long l;
    int i;
    short s;
    char c[2];
} u;

But what does it say about alignment of individual union elements inside the union? Is the following expression guaranteed to be true?

(&u.l == &u.i) && (&u.i == &u.s) && (&u.s == &u.c[0])

回答1:

The start of each element is aligned with the address of the union itself.

so the individual comparisons in the expression you ask about are true, but the expression as a whole is false unless the union is located at address 0x0001.

The deleted text applied to the following comparisons:

&u.l == &u.i == &u.s == &u.c[0]

The revised version compares distinct pointer types - the pointers should be cast to void pointers.

---

I was asked to quote the standard - or identify the section of the standard.

C99 - section 6.7.2.1 Structure and union specifiers (paragraph 14):

A pointer to a union object, suitably converted, points to each of its members (or if a member is a bitfield, then to the unit in which it resides), and vice versa.

来源：https://stackoverflow.com/questions/891471/union-element-alignment