c,swift,pointers,endianness , Porting C endianness & pointers black magic to Swift


Porting C endianness & pointers black magic to Swift

Question:

Tag: c,swift,pointers,endianness

I'm trying to translate this snippet :

ntohs(*(UInt16*)VALUE) / 4.0

and some other ones, looking alike, from C to Swift. Problem is, I have very few knowledge of Swift and I just can't understand what this snippet does... Here's all I know :

Thanks !


Answer:
  1. VALUE is a pointer to 32 bytes (char[32]).
  2. The pointer is cast to UInt16 pointer. That means the first two bytes of VALUE are being interpreted as UInt16 (2 bytes).
  3. * will dereference the pointer. We get the two bytes of VALUE as a 16-bit number. However it has net endianness (net byte order), so we cannot make integer operations on it.
  4. We now swap the endianness to host, we get a normal integer.
  5. We divide the integer by 4.0.

To do the same in Swift, let's just compose the byte values to an integer.

let host = (UInt(data.0) << 8) | UInt(data.1)

Note that to divide by 4.0 you will have to convert the integer to Float.


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