alignment faults in 3.6

Mans Rullgard mans.rullgard at
Fri Oct 5 04:20:56 EDT 2012

On 5 October 2012 08:12, Russell King - ARM Linux
<linux at> wrote:
> On Fri, Oct 05, 2012 at 03:25:16AM +0100, Mans Rullgard wrote:
>> On 5 October 2012 02:56, Rob Herring <robherring2 at> wrote:
>> > This struct is the IP header, so a struct ptr is just set to the
>> > beginning of the received data. Since ethernet headers are 14 bytes,
>> > often the IP header is not aligned unless the NIC can place the frame at
>> > a 2 byte offset (which is something I need to investigate). So this
>> > function cannot make any assumptions about the alignment. Does the ABI
>> > define structs have some minimum alignment? Does the struct need to be
>> > declared as packed or something?
>> The ABI defines the alignment of structs as the maximum alignment of its
>> members.  Since this struct contains 32-bit members, the alignment for the
>> whole struct becomes 32 bits as well.  Declaring it as packed tells gcc it
>> might be unaligned (in addition to removing any holes within).
> This has come up before in the past.
> The Linux network folk will _not_ allow - in any shape or form - for
> this struct to be marked packed (it's the struct which needs to be
> marked packed) because by doing so, it causes GCC to issue byte loads/
> stores on architectures where there isn't a problem, and that decreases
> the performance of the Linux IP stack unnecessarily.

Which architectures?  I have never seen anything like that.

Mans Rullgard / mru

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