Structure padding and alignment of byte-sized fields?

Question

If I have a structure such as the one below, on a 32-bit machine, will there be any padding within the structure? As I understand it, the structure will align everything to it's largest field, so what if the largest field is a byte (uint8)?

struct s {
    uint8_t a[32];
    uint8_t b[64];
};

Thanks.

The compiler is free to insert arbitrary padding between a and b. However, in practice, it's unlikely to do so here. — Oliver Charlesworth
@OliCharlesworth Do you mean that it is unlikely for the compiler to do so, or the developer? Didn't understand the last part of your comment :) — user3407764
@user3407764: As Oli didn't mention the developer, I'm gonna go with the compiler. I believe the point was that, while the spec says a compiler MAY insert padding, few would bother to in this circumstance. — Scott Hunter
If the default structure member alignment is, say 8 bytes, then fields with size that is not a multiple of 8 will be padded with extra bytes until their size is a multiple of 8. If the last field is one byte, then 7 more will be added to the size of the struct to make it match the 8-byte multiple. — DNT
@ScottHunter He might not have mentioned a developer, but I felt like the last line implied it somehow. — user3407764

this this · Accepted Answer · 2014-05-25T20:38:04

There may be padding between the members.

C standard 6.7.2.1(15): Within a structure object, the non-bit-field members and the units in which bit-fields reside have addresses that increase in the order in which they are declared. A pointer to a structure object, suitably converted, points to its initial member (or if that member is a bit-field, then to the unit in which it resides), and vice versa. There may be unnamed padding within a structure object, but not at its beginning.

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

There is probably no padding between the members on a modern machine, but C doesn't guarantee it, so don't rely on it.

You can determine if there is padding using the ofsetoff() macro and then manage that with static assert.

Structure padding and alignment of byte-sized fields?

1 Answers