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Re: [PATCH v2] Make sure packed structs follow the gcc memory layout
- From: Mark Wielaard <mark at klomp dot org>
- To: Ulf Hermann <ulf dot hermann at theqtcompany dot com>
- Cc: elfutils-devel at sourceware dot org, Ulf Hermann <ulf dot hermann at qt dot io>
- Date: Thu, 21 Sep 2017 00:01:41 +0200
- Subject: Re: [PATCH v2] Make sure packed structs follow the gcc memory layout
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On Mon, 2017-08-21 at 12:57 +0200, Ulf Hermann wrote:
> > Wouldn't this be a general problem for
> > any struct that might be layed out differently but that we might
> > map to some on-disk data structure?
> We should generally not rely on the layout of non-packed structs. The
> compiler can insert padding in various places. I didn't find any
> places where we do, though.
I don't think the compiler can do that without breaking ABI. I believe
we are very careful about the in-memory/on-disk struct layout matching.
I am online now and found the documentation:
> If packed is used on a structure, or if bit-fields are used, it may
> be that the Microsoft ABI lays out the structure differently than the
> way GCC normally does. Particularly when moving packed data between
> functions compiled with GCC and the native Microsoft compiler (either
> via function call or as data in a file), it may be necessary to
> access either format.
> This option is enabled by default for Microsoft Windows targets.
OK, so it is x86-only and it only affects packed structs or if the
struct contains bitfields. Otherwise the layout should be the same.
So I do think your patch is correct. I don't think we ever use structs
that contain bit-fields to map from disk to memory.
Applied to master now.