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

[Jens Ayton]

On May 21, 2009, at 11:47, David Chisnall wrote:
> Only half of this is true.  Apple only ships one version of its  
> frameworks.  Each framework, however, contains different versions of  
> the libraries.  You may have noticed all of the OS X version macros  
> in the headers.  Each of those defines the parts of the interface  
> that are visible in a particular version of the framework, but they  
> are also used when compiling.  Each version of OS X has a  
> Foundation.framework containing several libFoundation.dylib files,  
> one compiled for compatibility with each previous version.

Oh, really? Have you taken the time to actually look? ;-)

The bundle structure used for frameworks in OS X is specifically  
designed to allow this (except the library binary is called  
Foundation, not libFoundation.dylib), but it isn’t used much, if at  
all. The Foundation framework, for instance, does not use this method,  
and neither does CoreFoundation (which implements the most fundamental  
Foundation classes).

In practice, the differences between different API versions is handled  
entirely using runtime checks. This can be seen in the public CFLite  
source code (see http://www.opensource.apple.com/ - no longer requires  
an account). The 10.4 versions of CFLite include the real  
implementation of the version checking function,  
_CFExecutableLinkedOnOrAfter(); 10.5 versions only have a stub.

The problem of fragile ivars is handled (in the 32-bit runtime) by not  
changing class layouts, or changing them but keeping sizes unchanged.


--
Jens Ayton