Convert NSData bytes to NSString?

How about

NSString *content = [[[NSString alloc] initWithData:myData
                                           encoding:NSUTF8StringEncoding] autorelease];

That's an important point that should be re-emphasized I think. It turns out that,

NSString *content = [NSString stringWithUTF8String:[responseData bytes]];

is not the same as,

NSString *content = [[NSString alloc]  initWithBytes:[responseData bytes]
              length:[responseData length] encoding: NSUTF8StringEncoding];

the first expects a NULL terminated byte string, the second doesn't. In the above two cases content will be NULL in the first example if the byte string isn't correctly terminated.

NSData *torrent = [BEncoding objectFromEncodedData:rawdata];

When I NSLog torrent I get the following:

{
    ⋮
}

That would be an NSDictionary, then, not an NSData.

unsigned char aBuffer[[name length]];
[name getBytes:aBuffer length:[name length]];
NSLog(@"File name: %s", aBuffer);

..which retrives the data, but seems to have additional unicode rubbish after it:

File name: ubuntu-8.10-desktop-i386.iso)

No, it retrieved the filename just fine; you simply printed it incorrectly. %s takes a C string, which is null-terminated; the bytes of a data object are not null-terminated (they are just bytes, not necessarily characters in any encoding, and 0—which is null as a character—is a perfectly valid byte). You would have to allocate one more character, and set the last one in the array to 0:

size_t length = [name length] + 1;
unsigned char aBuffer[length];
[name getBytes:aBuffer length:length];
aBuffer[length - 1] = 0;
NSLog(@"File name: %s", aBuffer);

But null-terminating the data in an NSData object is wrong (except when you really do need a C string). I'll get to the right way in a moment.

I have also tried […]..

NSString *secondtry = [NSString stringWithCharacters:[name bytes] length:[name length] / sizeof(unichar)];

..but this seems to return random Chinese characters:

扵湵畴㠭ㄮⴰ敤歳潴⵰㍩㘸椮潳

That's because your bytes are UTF-8, which encodes one character in (usually) one byte.

unichar is, and stringWithCharacters:length: accepts, UTF-16. In that encoding, one character is (usually) two bytes. (Hence the division by sizeof(unichar): it divides the number of bytes by 2 to get the number of characters.)

So you said “here's some UTF-16 data”, and it went and made characters from every two bytes; each pair of bytes was supposed to be two characters, not one, so you got garbage (which turned out to be mostly CJK ideographs).

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You answered your own question pretty well, except that stringWithUTF8String: is simpler than stringWithCString:encoding: for UTF-8-encoded strings.

However, when you have the length (as you do when you have an NSData), it is even easier—and more proper—to use initWithBytes:length:encoding:. It's easier because it does not require null-terminated data; it simply uses the length you already have. (Don't forget to release or autorelease it.)