Jeremie Courreges-Anglas <j...@wxcvbn.org> writes:
> Gert Doering <g...@greenie.muc.de> writes:
>
>> Hi,
>>
>> On Sat, Jun 10, 2017 at 07:58:13PM +0200, Jeremie Courreges-Anglas wrote:
>>> OpenBSD:
>>>
>>> struct ip {
>>> #if _BYTE_ORDER == _LITTLE_ENDIAN
>>> u_int ip_hl:4, /* header length */
>>> ip_v:4; /* version */
>>> #endif
>>> #if _BYTE_ORDER == _BIG_ENDIAN
>>> u_int ip_v:4, /* version */
>>> ip_hl:4; /* header length */
>>> #endif
>>> ...
>>>
>>> u_char -> u_int. Dunno if it matters to the compiler, I can take a look
>>> later.
>>
>> I'm fairly sure it does matter. An int is likely to be at least 16 bit,
>> maybe 32 bit, so the compiler can assume "word aligned" - and *boom*.
>
> It doesn't seem to matter that much. Which aligns well with the
> "bit fields are nasty" experience people seem to have. From a quick
> glance at the c99 standard, I couldn't find language about this.Appendix J.1 mentions this: "The alignment of the addressable storage unit allocated to hold a bit-field (6.7.2.1)." as unspecified behavior. -- jca | PGP : 0x1524E7EE / 5135 92C1 AD36 5293 2BDF DDCC 0DFA 74AE 1524 E7EE
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