Bug in CF/NSString's no-copy constructors (was Re: Use of Mac OS X 10.5 / Leopards Garbage Collection Considered Harmful)

FROM : Alastair Houghton
DATE : Wed Feb 06 16:55:56 2008

On 6 Feb 2008, at 09:39, John Engelhart wrote:

> - (NSString *)someMethod
> {
>  NSUInteger finalStringLength = 1024; // Example only
>  NSString *copySting = NULL;
>  char * __strong restrict copyBuffer = NULL;
>
>  copyBuffer = NSAllocateCollectable(finalStringLength, 0);
>  /* Since this is just an example, the part that fills contents of 
> copyBuffer with text are omitted */
>  copyString = 
> NSMakeCollectable
> ((id)CFStringCreateWithCStringNoCopy(kCFAllocatorDefault, 
> copyBuffer, kCFStringEncodingUTF8, kCFAllocatorNull));
>  /* kCFAllocatorNull = This allocator is useful as the 
> bytesDeallocator in CFData or contentsDeallocator in CFString where 
> the memory should not be freed.  So.. Don't call free() on our 
> NSAllocateCollectable buffer, which is an error. */
>  return(copyString);
> }
>
> You see where the bug is, right?


I'll just add, publicly, that I think this probably is a bug in 
CFString that John has found here.  That is, I don't see why 
CFString's pointer shouldn't be traced by the collector in this case 
(it doesn't appear to be; certainly when I try it the backing buffer 
is released).  The problem also occurs with NSString's -
initWithBytesNoCopy:length:encoding:freeWhenDone: et al.

I've asked John if he's filed a bug report (I just filed one, <rdar://5727379
>, with a working code snippet).


Kind regards,

Alastair.

--
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