Re: Topics about GNUstep/libobjc2

[Dr . Rolf Jansen]

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