Objective-c negative Hex values - ios

Question

I have the following category method on NSData. I'm trying to extract the bit field at the given index and have it return as an NSNumber. I have it working perfectly for all positive but I need it to work with negative numbers as well.
My Implementation looks as follows:
#import <Foundation/Foundation.h>
#interface NSData (ExternalDevices)
#end
#implementation NSData (ExternalDevices)
- (NSNumber *)extractLittleEndianBitFieldAtIndex:(int)index forLength:(int)length
{
//This function has limitations on the "length" parameter that are not yet know/defined
//These limitations are due to the max size of "NSInteger intData" defined below
int first_byte = index/8; //Index of the first byte containing this bit field
int last_byte = (length+index-1)/8; //Index of the last byte containing this bit field
int byte_length = last_byte - first_byte + 1; //number of bytes containing this bit field
Byte *byteArray = (Byte*)malloc(byte_length);
memcpy(byteArray, [[self subdataWithRange:NSMakeRange(first_byte, byte_length)] bytes], byte_length);
NSInteger intData = *((NSInteger *)byteArray);
free(byteArray);
return [NSNumber numberWithInt:intData];
}
+ (NSData *)dataFromHexString:(NSString *)string
{
string = [string lowercaseString];
NSMutableData *data= [NSMutableData new];
unsigned char whole_byte;
char byte_chars[3] = {'\0','\0','\0'};
int i = 0;
NSUInteger length = string.length;
while (i < length-1) {
char c = [string characterAtIndex:i++];
if (c < '0' || (c > '9' && c < 'a') || c > 'f')
continue;
byte_chars[0] = c;
byte_chars[1] = [string characterAtIndex:i++];
whole_byte = strtol(byte_chars, NULL, 16);
[data appendBytes:&whole_byte length:1];
}
return data;
}
#end
#interface Testing:NSObject
#end
#implementation Testing
- (instancetype)init
{
self = [super init];
if (self)
{
{
NSData *data = [NSData dataFromHexString:#"e30b"];
NSLog(#"%# should be 3043", [data extractLittleEndianBitFieldAtIndex:0 forLength:16]);
}
{
NSData *data = [NSData dataFromHexString:#"46e0"];
NSLog(#"%# should be -8122", [data extractLittleEndianBitFieldAtIndex:0 forLength:16]);
}
{
NSData *data = [NSData dataFromHexString:#"f208"];
NSLog(#"%# should be 2290", [data extractLittleEndianBitFieldAtIndex:0 forLength:16]);
}
{
NSData *data = [NSData dataFromHexString:#"10e6"];
NSLog(#"%# should be -6640", [data extractLittleEndianBitFieldAtIndex:0 forLength:16]);
}
{
NSData *data = [NSData dataFromHexString:#"018900"];
NSLog(#"%# should be 137", [data extractLittleEndianBitFieldAtIndex:8 forLength:16]);
}
}
return self;
}
#end
int main(int argc, char *argv[]) {
#autoreleasepool {
[[Testing alloc] init];
}
}
The following website seems to always yield the results I want under INT16 - Little Endian (BA)
http://www.scadacore.com/field-applications/miscellaneous/online-hex-converter.html
Although it is important to note that not every number I work with will be an INT16

Answer 1

Your line:
NSInteger intData = *((NSInteger *)byteArray);
is your key problem for two reasons:
byteArray may be shorter (or less likely, longer) than an NSInteger and you'll end up reading garbage. E.g. if byteArray is 2 bytes as in your examples and NSInteger is 4 bytes - which it will be in 64-bit - you'll read two bytes of garbage.
If you are converting signed values you need to sign-extend the value - that is replicate the sign bit into the higher unused bits. E.g. if you are converting a signed 16-bit field into a 32-bit signed value then the upper 16-bits need to be a replication of the most significant bit of the 16-bit value, so 0x7000 -> 0x00007000 and 0x8000 -> 0xFFFF8000.
You need to come up with an algorithm that handles these issues. You may find it easier to do the conversion a byte at a time using masking (and'ing), or'ing and shifting.
HTH