Comments on: Working with Bitfields

By: Anonymous

Anonymous — Thu, 01 Jan 2009 11:06:08 +0000

But i think it is not guaranteed that the compiler decides to pack them like this he might decide to align every bit to a byte adress for performance reasons.

Old versions of vc6 had to be set to explictly pack it using #pragma pack dont know about gcc though.

I think the most reliable solution is (memory << bitn) & mask.

By: uprise78

uprise78 — Mon, 22 Dec 2008 16:26:13 +0000

Rodney, Using C sockets you can actually just do a straight send/receive of the struct. Pseudo code to follow:

typedef struct
{
unsigned preLoaded: 1;
unsigned saveState: 1;
unsigned saveLoginName: 1;
} statusFlags;

// Setup send socket
send(statusFlags)

// Setup receive socket
int count = receive()
if( count > 0 )
{
statusFlags flags = ( statusFlags *)buffer;
}

By: Christian

Christian — Mon, 22 Dec 2008 16:03:51 +0000

Very nice article!
This is a very clean and compact approach to a type of data structure I seem to find myself needing time and time again. Anxious to give it a go today!

By: Rodney

Rodney — Mon, 22 Dec 2008 14:30:35 +0000

Nice article. I didn’t realize that a struct could be use like this. What I would like to know more about though, is how this could be applied to custom binary data downloaded via the network.

Imagine a custom network protocol in which a sequence of bytes where downloaded and each byte had meaning. How would one read those bytes? I’d assume you would start with NSData, convert that into a char[] and then create a pointer to move through the char array. Each char, being one byte, would then need to be masked to get the value for a particular bit in the byte.

A good example of this applied is WBXML, which is a way to encode XML into a very compact binary protocol. XML documents will, on average, will be 60% smaller when encoded as WBXML, which when gzip’d on top of that you can get some amazing compression.