dataWithContentsOfURL returns NSCocoaErrorDomain Code=256 over cellular, but not wifi

Question

Really odd problem I'm having: dataWithContentsOfURL has begun returning an error code 256 over cellular, but not over wifi.

The operation couldn’t be completed. (Cocoa error 256.)

I do indeed have a cellular data connection, and it is functioning, so it is not a problem with my cellular connection. Plus the code works fine on wifi, so the basic code is not the issue. The code in question is:

dispatch_queue_t queue = dispatch_queue_create("com.appName.FetchImage", DISPATCH_QUEUE_SERIAL);
dispatch_async(queue, ^{
                    ...
                    NSURL *url = [NSURL URLWithString:[urlString stringByAddingPercentEscapesUsingEncoding:NSASCIIStringEncoding]];
                    NSLog(@"URL: %@", url);
                    [UIApplication sharedApplication].networkActivityIndicatorVisible = YES;
                    NSError *error = [[NSError alloc] init];
                    NSData *imgData = [NSData dataWithContentsOfURL:url options:NSDataReadingUncached error:&error];
                    if (error) NSLog(@"Error loading data: %@", error);
                    UIImage *image = [UIImage imageWithData:imgData];
                    ...
});

Any thoughts? I'm at a loss as to why this might be happening. It also occurs with the vanilla dataWithContentsOfURL (as opposed to with options).

Answer 1

NSCocoaErrorDomain Code = 256 means:

A file system or file I/O related error whose reason is unknown.

Simply, it tells us nothing.

However in general, NSData's dataWithContentsOfURL should only be use to access local file resources.

Important: Do not use this synchronous method to request network-based URLs. For network-based URLs, this method can block the current thread for tens of seconds on a slow network, resulting in a poor user experience, and in iOS, may cause your app to be terminated.

You can try improving your code and use a better way of downloading data. It might fix the issue you are experiencing. Instead of using dataWithContentsOfURL, you can use NSURLConnection's class methods like:

+ (void)sendAsynchronousRequest:(NSURLRequest *)request 
                          queue:(NSOperationQueue *)queue 
              completionHandler:(void (^)(NSURLResponse*, NSData*, NSError*))handler;

Based on:

Using NSURLConnection

NSData Class Reference

Answer 2

In my case, the issue was a bad network connection on my mobile 4G. It was not xcode-specific, it also existed in regular safari browsing. I switched to a different network and that fixed the problem. The "hint" was that the method did not return immediately - it timed out after about a minute.