Re: Does NSData rearrange the order of bits?
Re: Does NSData rearrange the order of bits?
- Subject: Re: Does NSData rearrange the order of bits?
- From: Bryan Henry <email@hidden>
- Date: Wed, 02 Dec 2009 05:02:23 -0500
Your log messages show that the NSData's bytes are stored completely correctly, you're just interpreting it incorrectly.
NSData's description method will list the bytes in order, so you see "510600f0". On the other hand, you used the %x format specifier to create your string, which will print the first byte last, so you get "f0000651". That's exactly the same infomation-wise, the order of bytes is just flipped.
If you want to create an NSString from the bytes of an NSData where the lower bytes are ordered first, you can use this:
NSUInteger len = [theToken length];
const unsigned char *bytes = [theToken bytes];
NSMutableString *tokenStr = [NSMutableString stringWithCapacity:len*2];
for (NSUInteger i = 0; i < len; ++i) {
[tokenStr appendFormat:@"x", bytes[i]];
}
- Bryan
(Original reply to Brad directly resubmitted to list at request)
On Nov 30, 2009, at 2:27:55 PM, Brad Gibbs wrote:
> Hi,
>
> I'm doing bit-packing via a C function. Logging the bits of the C function shows the expected result. If I create a string with a hex value format, I get the correct hex string, but, if I try to put the bytes into an NSData object with [NSData dataWithBytes: length], the order of the bits changes. All of the right elements are there, but they're in the wrong order (target data should be f0000651, as shown in the Target string is ... log).
>
> My code:
>
>
> // get the target int from the text field
> unsigned int tgtValue = [self.tgtTF intValue];
>
> // use the target int and type to pack the bits into an int
> uint32_t tgtBinary = setAnalogValueForIndex(cid, tgtValue);
> NSString *tgtString = [NSString stringWithFormat:@"%x", tgtBinary];
> NSData *tgtData = [NSData dataWithBytes: &tgtBinary length: sizeof(tgtBinary)];
> NSLog(@"Target data is %@. Target string is %@", tgtData, tgtString);
>
>
> The logs:
>
> 11110000000000000000011001010001
> 2009-11-30 11:02:26.126 CertTest[11959:a0f] Target data is <510600f0>. Target string is f0000651
> 2009-11-30 11:02:26.204 CertTest[11959:a0f] After adding target, cmdData is <510600f0>
>
> If NSData is rearranging the bits, is there some way to prevent this?
>
>
> Thanks.
>
> Brad
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