[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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

From: Xavier Glattard
Subject: Re: ABI Compatibility (was Re: Installation woes for the average user...)
Date: Fri, 06 Mar 2009 10:15:04 +0100
User-agent: Thunderbird (X11/20080707)

Richard Frith-Macdonald a écrit :

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


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).

Fred, Richard : thank you for your answers.
I think I understand the 'technical' problem :)

But IMHO :
- private ivars should (can?) not be used from outside the class itself.
- protected ivars should only be used from inside the library/framework itself, and only for very good reason.
- public ivars should not exist!

Moreover ivars are never documented.

Then GNUstep users should never meet this problem. If they do then I think this is their own choice.

Do i still miss something ?

-- Regards

reply via email to

[Prev in Thread] Current Thread [Next in Thread]