What causes this EXC_CRASH in my iPad app? - objective-c

Question

I got this symbolicated stack trace from a crash report from my iPad app (excerpt):
Exception Type: EXC_CRASH (SIGABRT)
Exception Codes: 0x00000000, 0x00000000
Crashed Thread: 0
0 ImageIO 0x34528eb4 _CGImagePluginIdentifyPNG + 0
1 ImageIO 0x34528d90 _CGImageSourceBindToPlugin + 368
2 ImageIO 0x34528bda CGImageSourceGetCount + 26
3 UIKit 0x341b8f66 _UIImageRefAtPath + 366
4 UIKit 0x342650ce -[UIImage initWithContentsOfFile:] + 50
5 UIKit 0x342b0314 +[UIImage imageWithContentsOfFile:] + 28
6 DesignScene 0x00013a2a -[LTImageCache fetchImageforURL:] (LTImageCache.m:37)
…
Here are the contents of -[LTImageCache fetchImageforURL:]:
- (UIImage *)fetchImageforURL:(NSString *)theUrl {
NSString *key = theUrl.md5Hash;
return [UIImage imageWithContentsOfFile:[self filenameForKey:key]];
}
And the contents of -[LTImageCache filenameForKey:]:
- (NSString *) filenameForKey:(NSString *) key {
return [_cacheDir stringByAppendingPathComponent:key];
}
The _cacheDir ivar is created and retained in -init. So the question is, what caused this crash? Is the problem that:
the return value of -[LTImageCache filenameForKey:] needs to be retained (it's autoreleased)
An unhandled exception somewhere (+[UIImage imageWithContentsOfFile:] claims to return nil if the image is unrecognizable)
Something else…I'm out of guesses
I would think that an autoreleased value would be fine. An in truth, this code has been working fine for months, and this method is called 100s of times in a session. This is a rare crash under very specific circumstances (the app left loaded overnight, crash upon unlocking the iPad in the morning).
What's causing this crash?

Answer 1

I'm guessing but it looks like a bogus image file. Is this in your app bundle or do you download it?
I don't think it has anything to do with memory mgt.
To test you could try using ImageIO to open the file yourself.
CGImageSourceRef imageSource = CGImageSourceCreateWithURL((CFURLRef)self.url, NULL);
if(NULL != imageSource) {
size_t imageCount = CGImageSourceGetCount(imageSource);
if(imageCount > 0) {
NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys:
[NSNumber numberWithBool:YES], kCGImageSourceCreateThumbnailFromImageIfAbsent,
[NSNumber numberWithInteger:maxSize], kCGImageSourceThumbnailMaxPixelSize, nil];
CGImageRef thumbImage = CGImageSourceCreateThumbnailAtIndex(imageSource, 0, (CFDictionaryRef)options);
self.image = [UIImage imageWithCGImage:thumbImage scale:scale orientation:imageOrientation];
CGImageRelease(thumbImage);
CFRelease(imageSource);
[pool drain];
}
} else {
NSLog(#"Unable to open image %#", self.url);
}
and then try to find the image count.
Using maxSize and getting a thumbnail will ensure you don't load a 5 mega-pixel image to put into a 100x100 tile on your UI.
scale is the window's scale (will be 2 for iPhone 4 and 1 for everything else).
To find the orientation you need to use the CGImageSourceCopyPropertiesAtIndex function and then the kCGImagePropertyOrientation key to get the orientation of the particular image.