Re: Topics about GNUstep/libobjc2

[David Chisnall]

Hi Rolf,

With the non-fragile ABI, it isn’t safe to assume that classes have the same 
layout as C structs.  This works with the fragile ABI, because the compiler is 
entirely responsible for layout there.  It sounds as if #pragma pack is 
removing some padding that the ABI mandates for C structs, but which the 
Objective-C runtime is not adding.

David

> On 24 Oct 2016, at 17:52, Dr. Rolf Jansen <rj@obsigna.com> wrote:
> 
> Would it be OK to discuss on this list topics about Objective-C development 
> involving clang + GNUstep/libobjc2, but almost nothing else from GNUstep?
> 
> I constructed my own root class having only the bare basic methods that are 
> needed by a quite old project, that I revamped for inclusion into a new plain 
> C project. My class/code is working well on FreeBSD 11.0 RELEASE-p1 (amd64) 
> when compiled with clang 3.8 (system) with -O0 or -Os and linked against 
> libobjc2 1.8.1 (ports). Once compiled with any other optimization mode, it 
> crashes (Bus Error) when accessing a certain instance variable struct.
> 
> It is working well on Mac OS X 11.12 when compiled in any -O mode with my 
> root class and linked against the native ObjC runtime. It is almost always 
> working on FreeBSD 11 in any -O mode when using NSObject as the root class 
> and linked against libobjc2 1.8.1 and gnustep-base (ports), although, I saw 2 
> random crashes.
> 
> After some debugging I found a workaround. Once I add a #pragma pack(8) 
> directive at the top of the headers that declare my class hierarchy + all the 
> C structs that are used throughout, the code with my root class works well on 
> FreeBSD 11 when compiled in any -O mode -- a pack(4) does work as well.
> 
> It is still possible that something is wrong with my code. However, after 
> these many experiments, I tend to assume that the Objective-C runtime and the 
> compiler sometimes disagree on correct packing/alignment of instance 
> variables in instantiated objects. I can't tell, though, if the runtime or 
> the compiler is responsible for this.
> 
> In the case that it is appropriate to discuss this further on this list, I am 
> ready to send more information. If this is not the appropriate list, then 
> please may I ask for advise on which location this topic may be discussed.
> 
> The #pragma pack(8) does neither cost anything nor is it a very ugly hack, so 
> perhaps, it isn't even worth to discuss this issue furthermore, given that 
> clang 4.0 may be out soon.
> 
> Best regards
> 
> Rolf
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