Union element alignment

Question

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]) 

Answer 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.

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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.