Re: CoreFoundation

[David Chisnall]

Hi Stef,

Having an implementation of Core Foundation would be nice, but I  
believe your approach is wrong.

First, please do not use the info field in the class structure for  
anything.  That field is for the runtime to use.  The GCC runtime uses  
several of them, the GNUstep runtime uses a few more.  It should not  
be used; there is no guarantee that future runtime versions won't use  
them all.

With regard to the CFTypeID, this should be the address of the class  
structure.  For those unfamiliar with how CoreFoundation works:

Each of the CF classes has an isa pointer, just like an Objective-C  
object.  Unlike Objective-C objects, this is set to a value below  
64KB, which is guaranteed not to be the address of a real class.  This  
number is the CFTypeID, which is a pseudo-class.

The reason that is that Apple's CoreFoundation does not depend on  
libobjc (although it does link to it).  If you use CF without libobjc,  
then you use static dispatch for all of the CF calls.  When you call  
CFSomething() the function checks that the object's isa pointer is set  
to the magic constant that it expects and if not then it calls another  
function (via some ugly macros).  This is done so CF objects can be  
used without relying on libobjc having finished loading classes.

If you use Objective-C then you get the toll-free bridging.  If you  
send a message to an object with an isa pointer that is less than  
2^16, then the runtime uses a special lookup mechanism.  If you call a  
CF function with an object with an isa pointer above 2^16, it bounces  
it to the Objective-C runtime for looking up the method (this is how,  
for example, you can use CFString functions on your own NSString  
subclass).

Unless you want to use CF for some very low-level stuff, then there is  
really no point in copying this.  You can just:

1) Define Objective-C classes that implement the CF types (many of  
these already exist in GNUstep).
2) Write wrapper functions that call the ObjC methods.
3) Return the address of the class as the isa pointer.

This will then work with all code that uses CF types unless they  
either rely on the TypeID < 2^16 thing or uses CF functions in +load  
or __attribute__((constructor)) functions.

Most of the CF equivalents of class methods take a CFAllocator.  You  
can declare this as a typedef from NSZone; they are functionally  
equivalent.  For a first pass, you can just implement most of the CF  
functions as trivial Objective-C functions, for example:

typedef NSArray *CFArrayRef;
typedef NSZone *CFAllocatorRef;
CFArrayRef CFArrayCreateCopy(CFAllocatorRef allocator, CFArrayRef  
theArray)
{
        [(NSArray*)theArray copyWithZone: allocator];
}

Once this is working, then you can start worrying about optimising  
it.  You might also consider making these in static inline functions  
in the header wrapped in #ifdef __OBJC__ so that people using CF  
functions in ObjC call the real methods.

I wrote some macros for declaring / defining these CF functions a  
while back, which I can send you if you're interested, and I'm happy  
to review any code you're working on.

David

On 23 Oct 2009, at 21:23, Stef Bidi wrote:

> Just testing the water... I've been a little bored and started  
> looking at implementing a portion of CoreFoundation on top of  
> libobjc and GNUstep-base, I'm tentatively calling it CoreBase.
>
> At this point I've spent most of my time on the runtime (CFRuntime)  
> and CFBase.  I'm not sure if it works, because in CF the runtime  
> requires CFString and CFDictionary and I'll have to at least do some  
> work there before even trying to compile.  Because I don't want to  
> have to rewrite/copy parts of what's already in GNUstep-base (like  
> reference counting) I've decided to depend on it.  Libobjc is also  
> required because I'm using that as a gateway to -base, and using  
> some of it's features (like the class number stored in class->info  
> to coinside with the CFTypeID).
>
> Before continuing with it (because, of course, time is precious) I  
> want to make sure there's at least some interest.  I'm also going to  
> need a fairly high degree of help on this project because certain  
> parts are out of my league (I still think it's an interesting  
> learning experience).
>
> Anyway, just let me know if it's a project I should keep moving  
> forward.
>
> Thanks
> Stefan
> _______________________________________________
> Gnustep-dev mailing list
> address@hidden
> http://lists.gnu.org/mailman/listinfo/gnustep-dev

-- Send from my Jacquard Loom