Re: NSImage release crash

FROM : Paul Collins
DATE : Tue Nov 02 17:54:53 2004

Followup question:

It's been suggested that my woes may stem from executing code in a
callback--the misbehaving NSImages are created in code within a
CFNetwork callback. I haven't noticed any documentation stating that
there are limitations on what code may be executed in a CFNetwork
callback.

Did I miss something? Can I create objects within a callback or not?

Thanks,

Paul Collins

On Nov 1, 2004, at 2:21 PM, Paul Collins wrote:

> My app is crashing in [NSImage dealloc]. In testing, I create about
> 3000 128 x 128 NSImages (resized from various larger images from
> disk), keep them around for browsing (a la iPhoto contact sheet view),
> then later release them while closing my document window. While
> releasing the 3000 images, one of them causes a crash (in testing,
> it's been like the 1075th image, or 2054th image (in two tests the
> same image crashed, although the image itself is not a problem when
> used in a small group of images).
>
> I can replicate the crash in the debugger with NSZombie on, and get no
> zombie exceptions, so I infer that this is not a
> double-free/double-release bug. Here's a crash log:
>
> Exception:  EXC_BAD_ACCESS (0x0001)
> Codes:      KERN_PROTECTION_FAILURE (0x0002) at 0x026eb97b
>
> Thread 0 Crashed:
> 0  com.apple.AppKit             0x92ebb5e4 AddSpace + 0x1a4
> 1  com.apple.AppKit             0x92e65b6c _NXFreeImageCache + 0x1f0
> 2  com.apple.AppKit             0x92e2fc10 -[NSCachedImageRep dealloc] +
> 0x184
> 3  com.apple.AppKit             0x92e1315c -[NSImage
> _freeRepresentation:] + 0x144
> 4  com.apple.AppKit             0x92e278a4 -[NSImage dealloc] + 0x6c
> 5  ODSlide.ob                   0x017e0704 -[ODSlide setThumbImage:] +
> 0x50 (ODSlide.m:48)
> 6  ODSlide.ob                   0x017e35d4 -[ODSlide dealloc] + 0xc4
> (ODSlide.m:823)
>
>
> Here's the code used to create the image:
>
>
>     NSImage *theImage = nil;
>     NS_DURING
>         theImage = [[NSImage alloc] initWithContentsOfFile:[aSlide
> cachePath]]; // can raise exception
>             if (theImage) {
>                 [theImage setScalesWhenResized:YES];
>                 [theImage setFlipped:YES];
>                 // calculate proportional size to fit in 128 x 128 pixels and set
> that size
>                 [theImage setSize:[(MyDocument *)myDoc bestSize:mySize
> toFit:thumbsize offset:&thumbImgDelta]];
>                 // pre-cache the image by locking focus
>                 [theImage recache];
>                 NS_DURING
>                     [theImage lockFocus];
>                     [theImage unlockFocus];
>                 NS_HANDLER
>                     ODLog(@"Exception: NSImage -lockFocus failed ");
>                 NS_ENDHANDLER
>                 // retain the image in my custom object.
>                 [slide setThumbImage: theImage];
>             
>
>                 // [aSlide makeThumbnailIfNoneForDoc:owner withImage:theImage
> size:NSMakeSize(128,128)];  // only returns image if loaded to make
> thumbnail, ignored    
>                 [theImage release];
>     NS_HANDLER
>         NSLog(@"Exception loading or using cached image (possibly unreadable
> graphic format)");
>     NS_ENDHANDLER
>
>
> This code is called within a CFNetwork callback (it's just downloaded
> the original image). Would that be a problem? I could run it from an
> NSTimer after the callback if that's better.
>
> This bug appears to have been introduced while fixing other bugs - the
> major other fix was adding several NSAutoreleasePools to ensure timely
> releases, although these are not involved this this object.
>
> Any suggestions on how to avoid or at least diagnose this bug?
>
> Thanks,
>
> Paul Collins
>
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