ObjC getting 64 bit number from stream - iphone

Question

I am successfully passing a 64 bit number from a objC client to a java client, but am unable to send to an objC client.
Java Code
/*
* Retrieve a double (64-bit) number from the stream.
*/
private double getDouble() throws IOException
{
byte[] buffer = getBytes(8);
long bits =
((long)buffer[0] & 0x0ff) |
(((long)buffer[1] & 0x0ff) << 8) |
(((long)buffer[2] & 0x0ff) << 16) |
(((long)buffer[3] & 0x0ff) << 24) |
(((long)buffer[4] & 0x0ff) << 32) |
(((long)buffer[5] & 0x0ff) << 40) |
(((long)buffer[6] & 0x0ff) << 48) |
(((long)buffer[7] & 0x0ff) << 56);
return Double.longBitsToDouble(bits);
}
objC code
/*
* Retrieve a double (64-bit) number from the stream.
*/
- (double)getDouble
{
NSRange dblRange = NSMakeRange(0, 8);
char buffer[8];
[stream getBytes:buffer length:8];
[stream replaceBytesInRange:dblRange withBytes:NULL length:0];
long long bits =
((long long)buffer[0] & 0x0ff) |
(((long long)buffer[1] & 0x0ff) << 8) |
(((long long)buffer[2] & 0x0ff) << 16) |
(((long long)buffer[3] & 0x0ff) << 24) |
(((long long)buffer[4] & 0x0ff) << 32) |
(((long long)buffer[5] & 0x0ff) << 40) |
(((long long)buffer[6] & 0x0ff) << 48) |
(((long long)buffer[7] & 0x0ff) << 56);
NSNumber *tempNum = [NSNumber numberWithLongLong:bits];
NSLog(#"\n***********\nsizeof long long %d \n tempNum: %#\nbits %lld",sizeof(long long), tempNum, bits);
return [tempNum doubleValue];
}
the result of NSLog is
sizeof long long 8
tempNum: -4616134021117358511
bits -4616134021117358511
the number should be : -1.012345
The problem is that I am trying to convert Java to objC in the getDouble func. My middleware takes into account the endian issues. The simple solution is if the target is little endian
- (double)getDouble
NSRange dblRange = NSMakeRange(0, 8);
double swapped;
[stream getBytes:&swapped length:8];
[stream replaceBytesInRange:dblRange withBytes:NULL length:0];
return swapped;
Thanks all for input - got a lot of experience and a little understanding from this exercise.

Answer 1

A double and a long long are not the same thing. A long represents an integer, which has no fractional portion, and a double represents a floating-point number, which has a fractional portion. These two types have completely different ways of representing their values in memory. That is to say, if you were to look at the bits for a long long representing the number 4000 and compare those to the bits for a double representing the number 4000, they would be different.
So as Wevah notes, the first step is for you to use the proper double type, and the correct %f formatter in your call to NSLog().
I would add, though, that you also need to be careful to get your bytes in the native order for the machine your C code is running on. For a detailed description of what I'm referring to, see http://en.wikipedia.org/wiki/Endianness The short version is that different processors may represent numbers in different ways in memory, and you need to ensure in your code that once you get a pile of bytes from the network, you are putting the bytes in the right order for your processor before you attempt to interpret it as a number.
Luckily, this is a solved issue, and is easily accounted for by using the CFConvertFloat64SwappedToHost() function from CoreFoundation:
[stream getBytes:buffer length:8];
[stream replaceBytesInRange:dblRange withBytes:NULL length:0];
double myDouble = CFConvertFloat64SwappedToHost(*((double*)buffer));
NSNumber *tempNum = [NSNumber numberWithDouble:myDouble];
NSLog(#"\n***********\nsizeof double %d \n tempNum: %#\nbits %f",sizeof(double), tempNum, myDouble);
return [tempNum doubleValue];

Answer 2

You probably want to convert it to a double (possibly/probably via a union; see Jonathan's comment) and use the %f specifier.