Re: ABI Compatibility (was Re: Installation woes for the average user...)

[Richard Frith-Macdonald]

On 6 Mar 2009, at 07:05, Xavier Glattard wrote:

> Hi
>
> I dont understand this problem about ABI and binary compatibility, so
> please excuse me if this looks stupid :\
>
> The problem comes from ivars that would not be always at the same
> offset, doesn't it ? But these ivars are supposed to be private, dont
> they ? So the code that uses these ivars is always released along with
> the class interface. I can not find any official 'public' ivars. Do i
> miss something ?

There are all sorts of possible issues with symbol names when  
functions and global variables etc are added/removed/changed, but the  
main issue we are talking about is the ivar layout and subclassing.

If you add an ivar to a class, the memory used by and instance becomes  
bigger to make room for it.

If you link in some bundle or library which uses a subclass of that  
class, the bundle/library will not be expecting that ivar to be there,  
and will have stored one of the subclass ivars at the oiffset in  
memory where the new ivar is stored ... causing obvious problems.

David's solution of non-fragile ivars requires compiler and runtime  
support, but is definitely the way to go to solve the problem (except  
for when the very highest performance is needed, in which case you can  
usually use straight C and other techniques for optimisation).