Re: Topics about GNUstep/libobjc2

[David Chisnall]

Hi Rolf,

From your code snippets, it looks as if it should work and you’ve probably 
found either a compiler or runtime bug.  If you can put together a reduced test 
case (ideally something that doesn’t depend on anything other than libobjc) 
I’ll take a look.

David

> On 24 Oct 2016, at 23:38, Dr. Rolf Jansen <rj@obsigna.com> wrote:
> 
> Hi David,
> 
> many thanks for the reply.
> 
> Does this mean, that it is not advised to use arbitrary C structs as 
> Objective-C instance variables?
> 
> The code is more than 10 years old, and at that time I started learning 
> Objective-C, and I heavily intermixed C data structures and algorithms into a 
> hollow OOP construct. It is a calculator engine with more than 10000 lines of 
> code.
> 
> The code (without #pragma pack, and -O1,-O2,-O3, or -Ofast) crashes 
> consistently on FreeBSD when accessing one instance variable indicated below 
> from within the respective object.
> 
>  ...
> 
>  typedef struct
>  {
>     ErrorFlags   flag;
>     unsigned     begin, count;
>     char        *field;
>  } ErrorRecord;
> 
>  typedef struct ResultType
>  {
>     ResultTypeFlags     f;
>     unsigned            n;
>     boolean             b;
>     long long           i;
>     long double         r;
>     long double complex z;
>     struct ResultType  *v;
>     char               *s;
>     ErrorRecord         e;
>  } ResultType;
> 
>  @interface CalcObject : MyObject
>  {
>     boolean     isPercent;
>     TokenPtr    itsToken;
>     Precedence  itsPrecedence;
>     OpKind      itsOpKind;
>     CalcObject *itsParent;
>     Calculator *itsCalculator;
>  }
> 
>  ...
>  - many instance methods;
>  ...
> 
>  @end
> 
> 
>  @interface CalcFactor : CalcObject
>  {
>     ResultType itsValue;    <<<<<<<<<<<< accessing this one gives Bus Error
>  }
> 
>  ...
>  - some more instance methods;
>  ...
> 
>  @end
> 
> 
> Questions:
> 
> As said already, on Mac OS X this code does not crash with Apple's 
> non-fragile ABI. Is this only by coincidence and other similar constructs may 
> crash? Or, may I continue to use my 10000 line of code, without a major 
> rewrite?
> 
> Likewise on FreeBSD, is it safe to rely on the magic of #pragma pack(8) or is 
> my code too fragil for the non-fragil ABI?
> 
> AFAIK, 8 byte struct packing/padding is standard for 64bit architectures and 
> 4 byte packing/padding for 32bit architectures. Isn't it possible to do this 
> reliable for C structs included as Objective-C instance variables?
> 
> Best regards
> 
> Rolf
> 
>> Am 24.10.2016 um 17:09 schrieb David Chisnall <David.Chisnall@cl.cam.ac.uk>:
>> 
>> 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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