Packed bit fields in c structures - GCC

Question

I am working with structs in c on linux. I started using bit fields and the \"packed\" attribute and I came across a wierd behavior:

struct t1
{
    int a:12;
           


        

Answer 1

struct t1 // 6 bytes
{
    int a:12; // 0:11
    int b:32; // 12:43
    int c:4;  // 44:47
}__attribute__((packed));

struct t1 // 7 bytes
{
    int a:12; // 0:11
    int b;    // 16:47
    int c:4;  // 48:51
}__attribute__((packed));

The regular int b must be aligned to a byte boundary. So there is padding before it. If you put c right next to a this padding will no longer be necessary. You should probably do this, as accessing non-byte-aligned integers like int b:32 is slow.