[Pyobjc-dev] Fwd: Hard Objc-Runtime question: objc_getClassList lies to me...

[Ronald O. <ous...@ci...>]

The message below has some technical information on ZeroLinking. On a 
first reading, Zerolinking cannot be used with PyObjC as the latter 
assumes that objc_getClassList returns all classes.

Begin forwarded message:

> From: Greg Parker <gp...@ap...>
> Date: Sun, 27 Jul 2003 19:13:27 -0700
> To: mha...@ma...
> Cc: coc...@li...
> Subject: Re: Hard Objc-Runtime question: objc_getClassList lies to 
> me...
>
> Martin wrote:
> > The first time I call -allClasses I don't get "DWTClassListerTest
> > included in the results, though it works after the call to
> > objc_lookUpClass. :(
> >
> > Well... The Runtime obviously knows about the class I am asking it,
> > because I can look it up via "objc_lookupClass", but how can I get
> > all of these classes without knowing their name beforehand?
>
> As you've already discovered, this behavior is a side effect of
> ZeroLink. Here's some more details about what's going on.
>
> (Terminology note: an `image` is any Mach-O file with binary code in 
> it,
> like an application, library, or bundle. Mach-O images are subdivided
> into `modules`. Each module typically corresponds to a single source
> code file.)
>
> objc_getClassList() returns all of the classes that have been
> registered with the Objective-C runtime. Ordinarily, all classes in
> an image are loaded together when that image is loaded into the 
> process.
> Thus objc_getClassList() will show all classes in all of Foundation
> once Foundation is loaded.
>
> When running with ZeroLink, the Objective-C runtime (as well as C
> and C++) is lazier. Instead of loading all classes in an image
> together, the runtime only loads all classes in an individual
> module; other classes in other modules in the same image are left
> alone.
>
> objc_lookUpClass() used to simply check whether a class had been
> loaded yet. It now has a side-effect when running with ZeroLink:
> if the class has not been loaded yet but ZeroLink can find it,
> then ZeroLink will bring it in.
>
> In your code, the "missing" class is presumably in a different
> source file and thus in a different Mach-O module. The class is
> not yet loaded when objc_getClassList() is called the first time,
> and objc_lookUpClass() forces ZeroLink to load it before the second
> objc_getClassList() call.
>
> If you need objc_getClassList() to return all of your classes, you can
> * explicitly use them early on in your code
> * link your program with -single_module
> * turn off ZeroLink in your project
>
>
> --
> Greg Parker     gp...@ap...     Java & Objective-C
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