Wireshark · Wireshark-dev: Re: [Wireshark-dev] C Bitfield types and alignment

[Jeff Morriss <jeff.morriss.ws@xxxxxxxxx>]

On 12/05/2011 04:41 PM, Bill Meier wrote:
> For example: Given
> struct radiotap_override {
> guint8 field;
> guint align:4, size:4;
> } aaa;
>
> struct radiotap_override {
> guint8 field;
> guint8 align:4, size:4;
> } bbb;
>
> It turns out that:
> sizeof(aaa) = 8
> sizeof(bbb) = 2
>
> Apparently the alignment of the start of the bitfield depends upon the
> (largest ?) bitfield type;

In the 'aaa' structure the compiler will (normally) insert 3 bytes of 
padding between 'field' and the bitfields so that the guint is 4-byte 
aligned; improper alignment isn't allowed on some CPUs (like SPARC) and 
is slow (often/always? requiring 2 memory access cycles to read) on others.