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/**************************************************************************/
[bugs #6743] Latest Modifications:
Changes by:
Richard Frith-Macdonald <[EMAIL PROTECTED]>
'Date:
Sun 11/30/03 at 09:08 (GMT)
What | Removed | Added
---------------------------------------------------------------------------
Status | Open | Closed
------------------ Additional Follow-up Comments ----------------------------
Code in cvs modified to raise an exception if a cached small number is deallocated ...
more user friendly than crashing on attempting to reference a deallocated object where
user code has released a number it doesn't own.
/**************************************************************************/
[bugs #6743] Full Item Snapshot:
URL: <http://savannah.gnu.org/bugs/?func=detailitem&item_id=6743>
Project: GNUstep
Submitted by: Moses Hall
On: Tue 11/25/03 at 03:06
Category: Base/Foundation
Severity: 5 - Average
Item Group: Bug
Resolution: None
Assigned to: None
Status: Closed
Summary: Distinct NSNumbers with the same value dealloc each other
Original Submission: Distinct NSNumbers with the *same value* can dealloc each other.
The following code snippet crashes gnustep-base 1.7.3 possibly because of NSNumber
caching. If I don't call [n1 release] there is no crash. If this is due to caching
strategy, then it can make NSNumber very fragile when numbers are expected to persist
as long as retained.
#include <Foundation/Foundation.h>
int main (int argc, const char *argv[], const char *env[])
{
NSNumber* n1 = [NSNumber numberWithLong:1];
NSNumber* n2 = [NSNumber numberWithLong:1];
NSLog(@"%@(%d)n", n1, [n1 retainCount]);
NSLog(@"%@(%d)n", n2, [n2 retainCount]);
[n1 release]; //<--problem here
// The next line crashes. n2 has gone away!
NSLog(@"%@(%d)n", n2, [n2 retainCount]);
return 0;
}
Thanks for your attention.
Brian 'Moses' Hall
[EMAIL PROTECTED]
Follow-up Comments
------------------
-------------------------------------------------------
Date: Sun 11/30/03 at 09:08 By: CaS
Code in cvs modified to raise an exception if a cached small number is deallocated ...
more user friendly than crashing on attempting to reference a deallocated object where
user code has released a number it doesn't own.
-------------------------------------------------------
Date: Tue 11/25/03 at 22:36 By: moseshall
Aside from my assertion that this is counterintuitive, my face is burning with shame.
Thanks for the pointer.
-------------------------------------------------------
Date: Tue 11/25/03 at 21:52 By: alexm
[NSNumber +numberWithLong:] returns an autoreleased object, not an object you own. You
might want to read http://www.stepwise.com/Articles/Technical/HoldMe.html .
-------------------------------------------------------
Date: Tue 11/25/03 at 21:38 By: moseshall
I confess I don't understand how the test program fails to own the numbers n1 and n2,
because it allocated them. I think given "principle of least surprise" I expect these
entities to be independent of each other, just as if they were created using malloc()
or if they were some other NSObject subclass. Doing otherwise breaks retain/release
balance. I would suggest ignoring a request to dealloc a cached NSNumber if the
program must keep track of how many identical NSNumbers it has created (essentially
duplicating the cache).
(Insert newbie disclaimer here.)
I would be curious to know how Cocoa handles the test program.
-------------------------------------------------------
Date: Tue 11/25/03 at 11:09 By: ayers
Yes, that would be very good.
-------------------------------------------------------
Date: Tue 11/25/03 at 10:40 By: CaS
What I was thinking of was making the cached numbers members of some new subclass
where -dealloc had been overridden to raise an exception. So there would be no
runtime overhead on dealloc of a non-cached number.
-------------------------------------------------------
Date: Tue 11/25/03 at 10:17 By: ayers
In principal I agree, but it depends on the cost. I'm not sure how much the hit would
be, but expect numbers to be created and destroyed often in GDL2 based applications.
So if each dealloc resulted in a hash look up... well, depending on the
implementation, I guess we may need to test it.
-------------------------------------------------------
Date: Tue 11/25/03 at 08:00 By: CaS
I think raising an exception upon an attempt to deallocate a cached NSNumber would be
a nice feature.
-------------------------------------------------------
Date: Tue 11/25/03 at 06:24 By: CaS
There is a bug in the test program ... it releases an object it does not own, so a
crash does not seem unreasonable (in the general case releasing objects you don't own
is bound to cause crashes).
Is the report a suggestion that the library should be changed to crash elsewhere, or
to have code added to watch for this case and raise an exception or something similar,
or perhaps to simply ignore the extra release and keep on going?
For detailed info, follow this link:
<http://savannah.gnu.org/bugs/?func=detailitem&item_id=6743>
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