How do I find out why file I/O operations fail?

Several methods I've used to write to files return a BOOL signifying their success, e.g.

NSDictionary:

- (BOOL) writeToFi开发者_开发百科le:(NSString *)path atomically:(BOOL)flag

NSKeyedArchiver:

- (BOOL) archiveRootObject:(id)rootObject toFile:(NSString *)path

Somehow, this returned NO on one of my tester's devices. That's great (well, not really), but is there anyway to find out why these operations might have failed?

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The modern way is to create NSData from the (root object|dictionary), then write that data to the desired path using writeToFile:options:error: or writeToURL:options:error:. These methods return an NSError object by reference, which contains a description of what happened, including (in at least some cases) a user-presentable error message.

To create data representing a dictionary, use NSPropertyListSerialization (if the dictionary and all of its contents, all the way down, are a valid property list) or the archiver (if not).