Use NSURLIsExcludedFromBackupKey without crashing on iOS 5.0

Question

The check for availability seems to be working fine but I can't seem to set the NSURLIsExcludedFromBackupKey key without getting this crash on launch:

dyld: Symbol not found: _NSURLIsExcludedFromBackupKey Referenced from: /Users/sam/Library/Application Support/iPhone Simulator/5.0/Applications/B0872A19-3230-481C-B5CE-D4BDE264FBDF/Transit.app/Transit Expected in: /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator5.0.sdk/System/Library/Frameworks/Foundation.framework/Foundation in /Users/sam/Library/Application Support/iPhone Simulator/5.0/Applications/B0872A19-3230-481C-B5CE-D4BDE264FBDF/Transit.app/Transit

Here's my method:

- (BOOL)addSkipBackupAttributeToItemAtURL:(NSURL *)URL {    
    if (&NSURLIsExcludedFromBackupKey == nil)
        return NO;

    NSError *error;
    [URL setResourceValue:[NSNumber numberWithBool:YES] forKey:NSURLIsExcludedFromBackupKey error:&error];
    return (error != nil);
}

Crash goes away if I comment out this line:

[URL setResourceValue:[NSNumber numberWithBool:YES] forKey:NSURLIsExcludedFromBackupKey error:&error];

Do I have to weak-link Foundation?

EDIT: not sure if it makes a difference, but this method is put inside an NSFileManager category.

Answer 1

Here's code for iOS <= 5.0.1 and >= 5.1 and includes the migration technique that @Cocoanetics mentioned.

- (BOOL)addSkipBackupAttributeToItemAtURL:(NSURL *)URL
{
    const char* filePath = [[URL path] fileSystemRepresentation];
    const char* attrName = "com.apple.MobileBackup";
    if (&NSURLIsExcludedFromBackupKey == nil) {
        // iOS 5.0.1 and lower
        u_int8_t attrValue = 1;
        int result = setxattr(filePath, attrName, &attrValue, sizeof(attrValue), 0, 0);
        return result == 0;
    } else {
        // First try and remove the extended attribute if it is present
        int result = getxattr(filePath, attrName, NULL, sizeof(u_int8_t), 0, 0);
        if (result != -1) {
            // The attribute exists, we need to remove it
            int removeResult = removexattr(filePath, attrName, 0);
            if (removeResult == 0) {
                NSLog(@"Removed extended attribute on file %@", URL);
            }
        }

        // Set the new key
        return [URL setResourceValue:[NSNumber numberWithBool:YES] forKey:NSURLIsExcludedFromBackupKey error:nil];
    }
}

Answer 2

This seems to be a bug with the iPhone 5.0 Simulator. I tried running the code on a 5.0 device and no crash. Reported this bug as rdar://11017158.

EDIT: this has been fixed in Xcode 4.5 DP2 (not sure if it is in 4.4).