Hi Guennadi, On Saturday 26 April 2014 17:28:24 Guennadi Liakhovetski wrote: > On Fri, 28 Mar 2014, Laurent Pinchart wrote: > > On Friday 28 March 2014 18:44:04 Guennadi Liakhovetski wrote: > > > On Fri, 28 Mar 2014, Laurent Pinchart wrote: > > > > On Thursday 27 March 2014 22:34:07 Guennadi Liakhovetski wrote: > > > > > It turns out, that 64-bit compilations sometimes align structs > > > > > within other structs on 32-bit boundaries, but in other cases > > > > > alignment is done on 64-bit boundaries, adding padding if necessary. > > > > > > > > You make it sound like the behaviour is random, I'm pretty sure it > > > > isn't > > > > > > > > :-) > > > > > > I didn't mean it was random, I just meant it is not be as simple as > > > "align always." E.g. if there are only 32-bit fields in the embedded > > > struct, it won't be aligned, below I explain a bit with pointers. I just > > > don't know the exact logic, that's used there. > > > > The logic is basically that fields are aligned within structures to a > > multiple of their native access size, and structures are aligned to a > > multiple of the access size of the largest field. If a structure on a > > 64-bit systems contains a pointer the pointer field will be aligned to a > > multiple of 8 bytes within the structure, and instances of the structure > > will be aligned to multiples of 8 bytes as well. If that structure is > > embedded inside another structure, it will be placed on an 8 bytes > > boundary, possibly creating a gap if the fields before the structure > > don't add up to a multiple of 8 bytes. This is what happens here. > > Yes, that's what I thought too, but I didn't have a documented > confirmation at hand, so, I left it a bit vague :) Have you got a pointer > to this? AFAIK how data are aligned in memory isn't part of the C standard but is defined by the platform ABI. For instance see the ARM Procedure Call Standard (AAPCS - http://infocenter.arm.com/help/topic/com.arm.doc.ihi0042e/IHI0042E_aapcs.pdf). There's probably a similar document for x86. -- Regards, Laurent Pinchart -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
