Memory layout of struct having bitfields

Question

I have this C struct: (representing an IP datagram)

struct ip_dgram
{
    unsigned int ver   : 4;
    unsigned int hlen  : 4;
    unsigned int stype : 8;
            


        

Answer 1

I agree with what unwind said. Bit fields are compiler dependent.

If you need the bits to be in a specific order, pack the data into a pointer to a character array. Increment the buffer the size of the element being packed. Pack the next element.

pack( char** buffer )
{
   if ( buffer & *buffer )
   {
     //pack ver
     //assign first 4 bits to 4. 
     *((UInt4*) *buffer ) = 4;
     *buffer += sizeof(UInt4); 

     //assign next 4 bits to 5
     *((UInt4*) *buffer ) = 5;
     *buffer += sizeof(UInt4); 

     ... continue packing
   }
}