CODEWITHSUNDEEP
objective-chex
Le Mot Juiced
Le Mot Juiced

Reputation: 3851

How can I turn NSData's hex string into a normal hex string?

So, take an unsigned int, say 4286578687.

From this site: http://www.mathsisfun.com/binary-decimal-hexadecimal-converter.html

I get the hex value to be: FF7FFFFF

However, if I put that int into NSData like so:

//The unsigned int is unsignedInt and its length is unsignedLength 

NSData *thisData = [NSData dataWithBytes:&unsignedInt length:unsignedLength];

And then use the description method, which supposedly returns the data's hex value as a string:

NSLog(@"data as hex: %@", [thisData description]);

The output is:

data as hex: <ffff7fff>

Which on the same website evaluates to 4294934527.

So it seems like NSData is using some non-standard hex format. Can anyone tell me how to get back to the real hex format?

Upvotes: 0

Views: 352

Answers (2)

rmaddy
rmaddy

Reputation: 318774

You are seeing a difference between storing the bytes of the unsigned int in big-endian versus little-endian.

If you want to guarantee that the output of the NSData is in big-endian format then you should do the following:

unsigned int x = 4286578687;
unsigned int big = NSSwapHostIntToBig(x);
NSData *thisData = [NSData dataWithBytes:&big length:sizeof(big)];
NSLog(@"data as hex: %@", thisData);

This logs the expected result of data as hex: <ff7fffff>.

This code will work on any processor type and always give you the result in big-endian format.

When going back the other way you would need to use the NSSwapBigIntToHost function to ensure the big-endian data is properly converted to the local format.

Upvotes: 2

Jordan Montel
Jordan Montel

Reputation: 8247

You can try something like this :

NSInteger i = 4286578687;
NSLog(@"hex : %@", [NSString stringWithFormat:@"%X", i]);

// Result
>>> hex : FF7FFFFF

Upvotes: 0

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