C struct elements alignment (ansi)

Question

just a simple question... what the standard says about the structure members alignment? for example with this one:

struct
{
    uint8_t a;
    uint8_t b;
    /* other members */
} test;

It is guarateed that b is at offset 1 from the struct start? Thanks

Answer 1

The standard (as of C99) doesn't really say anything.

The only real guarantees are that (void *)&test == (void *)&a, and that a is at a lower address than b. Everything else is up to the implementation.

Answer 2

C11 6.7.2.1 Structure and union specifiers p14 says

Each non-bit-field member of a structure or union object is aligned in an implementation- defined manner appropriate to its type.

meaning that you can't make any portable assumptions about the difference between the addresses of a and b.