Pointers to primitive objects not properly changing value - objective-c

Question

I am having a bit of an issue passing a reference to a primitive type through chaining, and having the value represented by the pointer change correctly. The weird part is, if I call getBytes directly from main function, byteLocation is properly adjusted, but if I chain it through a convenience function, it seems get a junk value. Actually, even weirder, it at first gets the correct value when stepping through the debugger, but executes the return clause twice. The first return clause gets the correct value, the second loads byteLocation with a junk value. Any ideas?
EDIT (Actual Code):
#property (strong, nonatomic, nonnull) NSData* data;
#property (assign, nonatomic) CFByteOrder byteOrder;
- (void)convertBytesToHostOrder:(nonnull void*)buffer length:(NSUInteger)length {
if(length > 1 && self.byteOrder != CFByteOrderGetCurrent()) {
// Swap bytes if the packet endiness differs from the host
char* fromBytes = buffer;
for(NSUInteger i=0; i < length/2; i++) {
NSUInteger indexes[2] = {i, length-i-0};
char byte = fromBytes[indexes[0]];
fromBytes[indexes[0]] = fromBytes[indexes[1]];
fromBytes[indexes[1]] = byte;
}
}
}
- (nonnull void*)getBytes:(nonnull void*)buffer startingFrom:(nonnull NSUInteger*)location length:(NSUInteger)length {
NSRange range = NSMakeRange(*location, length);
[self.data getBytes:buffer range:range]; // self.data is an instance of NSData
[self convertBytesToHostOrder:buffer length:length];
NSUInteger update = range.location + range.length;
*location = update;
return buffer;
}
- (NSTimeInterval)readTimeIntervalStartingFrom:(nonnull NSUInteger*)byteLocation {
uint32_t seconds;
uint16_t milliseconds;
// This line of code screws up the byteLocation pointer for some reason
[self getBytes:&seconds startingFrom:byteLocation length:sizeof(seconds)];
[self getBytes:&milliseconds startingFrom:byteLocation length:sizeof(milliseconds)];
NSTimeInterval ti = seconds + milliseconds / ((double) 1000 * (1 << 6));
return ti;
}
- (void)readData {
NSUInteger byteLocation = 0;
self.sequenceNumber = *(uint8_t*) [self getBytes:&_sequenceNumber startingFrom:&byteLocation length:sizeof(_sequenceNumber)];
self.flags = *(uint8_t*) [self getBytes:&_flags startingFrom:&byteLocation length:sizeof(_flags)];
// Continue to process packet data if we didn't get a goodbye message
if(!(self.flags & LBRadarPongFlagGoodbye)) {
// Parse accelerations
int16_t int16;
self.accelerationX = (*(int16_t*) [self getBytes:&int16 startingFrom:&byteLocation length:sizeof(int16)]) / kGToRaw;
self.accelerationY = (*(int16_t*) [self getBytes:&int16 startingFrom:&byteLocation length:sizeof(int16)]) / kGToRaw;
self.accelerationZ = (*(int16_t*) [self getBytes:&int16 startingFrom:&byteLocation length:sizeof(int16)]) / kGToRaw;
// Parse peripheral states
self.batteryVoltage = [self readFloat16From:&byteLocation];
self.chargeCurrent = [self readFloat16From:&byteLocation];
self.systemCurrent = [self readFloat16From:&byteLocation];
// All previous lines of code work properly and as expected.
// Buffers are read properly, and byteLocation properly reflects 14, which is the number of bytes read up to this point.
self.pongReceivedTimeIntervalSince1970 = [self readTimeIntervalStartingFrom:&byteLocation];
}
}

Answer 1

The problem seems to be with incrementing the location. In both cases you should copy from position zero, up to the size of the variable. In the following code:
[self readBytes:&seconds location:byteLocation length:sizeof(seconds)]
[self readBytes:&milliseconds location:byteLocation length:sizeof(milliseconds)]
The first call starts at position zero and reads 32 bits. The second one starts at position 32, which doesn't even fit in the variable's 16 bits. This is overflowing the buffer. Try this instead:
- (void*)readBytes:(void*)buffer location:(NSUInteger*)location length:(NSUInteger)length {
// The difference is in the next line. Zero instead of *location
[NSData getBytes:&buffer range:NSMakeRange(0, length)];
*location = *location + length;
return buffer; // Seems to be called twice, first time location* has the correct byteLocation inside it, second time location* has a junk value
}

Answer 2

At a guess[*] your error is on the line:
[self.data getBytes:&buffer range:NSMakeRange(*location, length)];
You are passing a void * value by taking the address of buffer - which is already a void *. Changing this to:
[self.data getBytes:buffer range:NSMakeRange(*location, length)];
will at least produce non-garabge results.
[*] I can only guess as the code you posted did not even compile, I edited your question to correct some of the more obvious errors - but even that involved some guessing! You should post real code.

Answer 3

I should have just posted the actual code, sorry guys. Turns out the error was in a helper function (convertBytesToHostOrder). This was reading out of bounds of buffer. Since buffer was the parameter right before byteLocation, it seems that writing at a location 1 spot beyond buffer was the byteLocation location. Fixed now and everything is working.
- (void)convertBytesToHostOrder:(nonnull void*)buffer length:(NSUInteger)length {
if(length > 1 && self.byteOrder != CFByteOrderGetCurrent()) {
// Swap bytes if the packet endiness differs from the host
char* fromBytes = buffer;
for(NSUInteger i=0; i < length/2; i++) {
NSUInteger indexes[2] = {i, length-i-0};
char byte = fromBytes[indexes[0]];
fromBytes[indexes[0]] = fromBytes[indexes[1]];
fromBytes[indexes[1]] = byte;
}
}
}
Should be:
NSUInteger indexes[2] = {i, length-i-1};