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Re: Garbage Collection

From: Thomas Davie
Subject: Re: Garbage Collection
Date: Wed, 25 May 2011 15:42:29 +0100

On 25 May 2011, at 13:04, David Chisnall wrote:

> On 25 May 2011, at 12:47, David Chisnall wrote:
>> {lots of stuff, and then forgot to say half of what he actually meant say}
> Apple's GC APIs are designed to be trivial to use, there are just a few 
> things you need to do:
> If you are allocating memory that contains pointers, use 
> NSAllocateCollectable(size, NSScannedOption), not malloc().
> All memory allocated by NSAllocateCollectable() will be GC'd automatically, 
> so it does not require an explicit call to any function like free().  If you 
> add NSCollectorDisabledOption, then the GC will be temporarily disabled for 
> that allocation.  You can reenable it by calling NSMakeCollectable() on the 
> pointer.  This is only intended for interoperability with CoreFoundation, so 
> generally GNUstep code can ignore this.
> Implement -finalize to do cleanup, not -dealloc (well, you can implement 
> both, but -dealloc won't be called in non-GC mode).  This should free memory 
> allocated with malloc() and close file descriptors, but little else.  For 
> example, you don't need to unregister with notification centres here - they 
> use __weak pointers.
> Use __strong {type}* on pointers that will store GC'd references.  This is 
> not actually required currently, but it may be in the future (it is on OS X, 
> because they are a bit more aggressive about reducing the number of paths 
> that the GC has to follow than I am).  
> Use __weak id for zeroing weak references.  These will not prevent the 
> referenced object from being finalized, and will become 0 when the object is 
> finalized.
> You can skip -retain/-release/-autorelease messages, but the compile will 
> ignore them if you compile in -fobjc-gc-only mode, so you can leave them in 
> if you want to write code that works with GC or non-GC mode.
> If you want to pass an object to some C code that is not GC-aware, you can 
> use CFRetain() and CFRelease().  These increment and decrement a reference 
> count for the object, so must be paired.  When an object's reference count is 
> nonzero, it will not be eligible for collection.  Objects do not have a 
> reference count associated with them by default.  
> If you write code that only works in GC mode, then add this to prevent people 
> compiling it in non-GC mode and wondering why it's broken:
> #ifndef __OBJC_GC__
> #error This code requires garbage collection
> #endif

I take it that along with compiling your runtime with gc on, we need a brand 
new svn head of clang?


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