How to set bit values which are not divided on byte boundary

Question

Hi

I have a header field of 2 bytes where first four bits are packet type and other 12 bits are for length. but i don't know how to store this values and retrieve them.

I am using ms c++ for client and java for server. It is the client which has to set this values and server has to retrieve it.

Thanks

Answer 1

Storing

unsigned short get_header(unsigned char type, unsigned short length)
{
  return (static_cast<unsigned short>(type) << 12) | length;    
}

Retrieving (from unsigned short)

unsigned short header = /* get 2 header bytes */
unsigned char type = header >> 12;
unsigned short length = header & 0xFFF;

Retrieving (from unsigned char[2])

unsigned char bytes[2] = /* get 2 header bytes */
unsigned char type = bytes[0] >> 4;
unsigned short length = bytes[0] & 0xF | bytes[1];

Answer 2

You must use the numeric AND operation and right-shift to extract the values.

Your header:     TTTTLLLLLLLLLLLL
Mask for type:   1111000000000000 = 0xF000
Mask for length: 0000111111111111 = 0x0FFF

Extracting:

// Just an example, this is 0001001000110100 in binary
uint16_t exampleHeader = 0x1234;

// Mask out the length and shift type to the right to get 0001 (binary)
uint8_t type = (exampleHeader & 0xF000) >> 12;

// Same here but no shift needed, returns 001000110100 (binary)
uint16_t length = exampleHeader & 0x0FFF;

Assembling a header:

uint16_t header = (type << 12) | length;

You can replace the integer types (uintXX_t) with the corresponding type of your language, like unsigned long. The code should be the same for Java and C++, except for the type keywords.

Answer 3

on the c/c++ size atleast, you could simply use this structure: don't know if you can do this in java tho...

struct Header{
   unsigned short
         type: 4
         length: 12
         ;
};  

note, those might need to be switched around

also, you might have endian problems, endian depends from processor type to processor type (all amd and intel processors that I know of are the same, the only ones that I know of that are different are PowerPC ones I think, most noteably used in the PS3 and xbox 360)