blocks|key|1077125|text|是的，网址缺少了这个方案："http://"。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1077126|“错误->+Domain=NSCocoaErrorDomain+Domain=NSCocoaErrorDomain+Code=256”|1077127|有关错误代码，请检查苹果文档|1077128|Cocoa错误域中的NSError代码。|1077129|NSFileReadUnknownError+=+256,|code-block|syntax|javascript|1077130|NSFileReadUnknownError|style|CODE|1077131|阅读错误，原因不明|1077132|并不是说错误定义是非常有用的。:-)|1077133|也不要检查error是否为nil以确定是否存在错误，请检查nil的返回值。在成功执行时，不能保证error是nil。|1077134|entityMap|0|LINK|mutability|MUTABLE|url|http://|1|https://developer.apple.com/library/mac/#documentation/Cocoa/Reference/Foundation/Miscellaneous/Foundation_Constants/Reference/reference.html^0|E|8|0|0|0|A|4|1|0|0|0|0|M|0|0|0|5|5|D|3|T|3|1C|5|1I|3|0^^$0|@$1|2|3|4|5|6|7|18|8|@]|9|@$A|19|B|1A|1|1B]]|C|$]]|$1|D|3|E|5|6|7|1C|8|@]|9|@]|C|$]]|$1|F|3|G|5|6|7|1D|8|@]|9|@$A|1E|B|1F|1|1G]]|C|$]]|$1|H|3|I|5|6|7|1H|8|@]|9|@]|C|$]]|$1|J|3|K|5|L|7|1I|8|@]|9|@]|C|$M|N]]|$1|O|3|P|5|6|7|1J|8|@$A|1K|B|1L|Q|R]]|9|@]|C|$]]|$1|S|3|T|5|6|7|1M|8|@]|9|@]|C|$]]|$1|U|3|V|5|6|7|1N|8|@]|9|@]|C|$]]|$1|W|3|X|5|6|7|1O|8|@$A|1P|B|1Q|Q|R]|$A|1R|B|1S|Q|R]|$A|1T|B|1U|Q|R]|$A|1V|B|1W|Q|R]|$A|1X|B|1Y|Q|R]]|9|@]|C|$]]|$1|Y|3|-4|5|6|7|1Z|8|@]|9|@]|C|$]]]|Z|$10|$5|11|12|13|C|$14|15]]|16|$5|11|12|13|C|$14|17]]]]

Yes, the URL is missing the scheme: "http://".

"Error -> Error Domain=NSCocoaErrorDomain Code=256"

For the error code check the <a href="https://developer.apple.com/library/mac/#documentation/Cocoa/Reference/Foundation/Miscellaneous/Foundation_Constants/Reference/reference.html" rel="noreferrer">Apple documentation</a>:

NSError codes in the Cocoa error domain.

<pre><code>NSFileReadUnknownError = 256,
</code></pre>

<code>NSFileReadUnknownError</code> 
"Read error, reason unknown"

Not that the error definition is very helpful. :-)

Also do not check if <code>error</code> is <code>nil</code> to determine if there is an error, check the return value for <code>nil</code>. <code>error</code> is not guaranteed to be <code>nil</code> on successful execution.

blocks|key|598369|text|我在访问位于我设备上的文件时也遇到了类似的问题。我跟随isFileURL总是不返回，使用NSURL+fileURLWithPath而不是NSURL+URLWithString+-这是有效的！|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|598370|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/13557191/nsurl-isfileurl-always-returns-no/13557216#13557216^0|R|E|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

I had a similar problem accessing files located on my device. I followed <a href="https://stackoverflow.com/questions/13557191/nsurl-isfileurl-always-returns-no/13557216#13557216">NSURL isFileURL always returns NO</a>
and used [NSURL fileURLWithPath] instead of [NSURL URLWithString] - this worked!

blocks|key|598293|text|当我们的ssl证书过期时，我得到了这个错误(错误Domain=NSCocoaErrorDomain+Code=256)。这可能对你没有帮助，但可以帮助别人。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|598294|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

I got this error (Error Domain=NSCocoaErrorDomain Code=256) as soon as our ssl certificate expired. that may not help you but could help someone else.

blocks|key|598326|text|沙箱|type|unstyled|depth|inlineStyleRanges|entityRanges|data|598327|如果您在应用程序中使用沙箱，您可能需要检查com.apple.security.network.client是否设置为YES.它位于Xcode+5中目标的General选项卡中|offset|length|style|BOLD|CODE|598328|网络:传出连接(客户端)|598329|还请注意，如果您在尝试访问257+url时看到了代码file:///，这可能也是因为沙箱，但这一次是文件访问部分。因为我不想向`com.apple.security.files.user-selected.read-write‘以外的任何东西打开它|598330|用户选择的文件|598331|我更喜欢在调试模式下使用一个死亡的简单文件服务器和http://localhost:3000。|598332|entityMap|0|LINK|mutability|MUTABLE|url|http://stoneship.org/software/adsf/^0|0|L|X|1N|3|25|7|L|X|1N|3|0|0|D|3|Q|8|0|0|P|L|C|C|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Z|8|@$D|10|E|11|F|G]|$D|12|E|13|F|G]|$D|14|E|15|F|G]|$D|16|E|17|F|H]|$D|18|E|19|F|H]]|9|@]|A|$]]|$1|I|3|J|5|6|7|1A|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|1B|8|@$D|1C|E|1D|F|H]|$D|1E|E|1F|F|H]]|9|@]|A|$]]|$1|M|3|N|5|6|7|1G|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|1H|8|@$D|1I|E|1J|F|H]]|9|@$D|1K|E|1L|1|1M]]|A|$]]|$1|Q|3|-4|5|6|7|1N|8|@]|9|@]|A|$]]]|R|$S|$5|T|U|V|A|$W|X]]]]

<h1>Sandboxing</h1>

If you're using sandboxing in your app, you might want to check that <code>com.apple.security.network.client</code> is set to <code>YES</code>. It's in the the General tab of your Target in Xcode 5 under

Network: Outgoing Connections (Client)

Also be aware that if you see a code <code>257</code> when trying to reach a <code>file:///</code> url, that's also probably because of sandboxing, but this time rather the File Access part. Because I didn't want to open it to anything else than `com.apple.security.files.user-selected.read-write' 

User selected files

I preferred to use <a href="http://stoneship.org/software/adsf/" rel="nofollow">A Dead Simple Fileserver</a> and use <code>http://localhost:3000</code> when in Debug mode.

blocks|key|598423|text|可能导致此特定错误的更多原因：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|598424|SSL在服务器上配置错误。|ordered-list-item|598425|服务器将http+URL重定向(301)到https+(参见#1)|598426|App传输安全还对阻塞的请求使用此代码。|offset|length|598427|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/31254725/transport-security-has-blocked-a-cleartext-http^0|0|0|0|0|7|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$]]|$1|E|3|F|5|D|7|U|8|@]|9|@]|A|$]]|$1|G|3|H|5|D|7|V|8|@]|9|@$I|W|J|X|1|Y]]|A|$]]|$1|K|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|L|$M|$5|N|O|P|A|$Q|R]]]]

More reasons that might be causing this specific error:

<ol>
<li>SSL is misconfigured on the server</li>
<li>The server redirects (301) the http URL to https (see #1)</li>
<li><a href="https://stackoverflow.com/questions/31254725/transport-security-has-blocked-a-cleartext-http">App transport security</a> also uses this code for blocked requests.</li>
</ol>

blocks|key|598412|text|我也有同样的错误。以上标记的答案是完美的。但是在我的例子中，我有“url中的http://”，但是必须在url请求中添加端口号，因为在特定端口上运行的服务实际上是响应您的请求。@"http://example.com:8084/yyy.zzz"|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|598413|entityMap^0|2G|Y|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|K|8|@]|D|@]|E|$]]]|G|$]]

I got the same error. The above marked answer is perfect. But in my case, I had the "http://" in the url but had to add the port number in the url request since there is a service running on a specific port that is actually responding to your request. 
 <code>@"http://example.com:8084/yyy.zzz"</code>

I have been stuck with this for a while and don't seem to get around this. 

I am trying to read the contents of an URL as a string from an URL, But i get a weird 

<blockquote>
 Error -> Error Domain=NSCocoaErrorDomain Code=256 "The operation couldn’t be completed. (Cocoa error 256.)" 
</blockquote>

My code :

<pre><code>fetchedString = [[NSString alloc] initWithContentsOfURL:[NSURL URLWithString:@"www.example.com/iphone"] encoding:NSUTF8StringEncoding error:&amp;error]; 
 NSLog(@"%@",fetchedString);

 // if there is something wrong with the URL 
 if (error) {
 NSLog(@"Error -&gt; %@", error);
 return ;
 }
</code></pre>

What am I doing wrong? I tried using getting as NSData as well, but I get null back.

NSCocoaErrorDomain Code=256

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我被困在这件事上已经有一段时间了，似乎没办法解决这个问题。我试图将URL的内容从URL中读取为字符串，但是我得到了一个奇怪的错误-> Domain=NSCocoaErrorDomain Code=256“操作无法完成。(可可错误256。)我的代码：fetchedString = [[NSString alloc] initWithContentsOfURL:[NSURL URLWithString

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