>
> In the last episode (Oct 08), Li, Qing said:
> > The bit fields "th_x2" and "th_off" in "struct tcphdr",
> > even though defined as "u_int", actually occupies 1 byte.
>
> u_int th_x2:4, /* (unused) */
> th_off:4; /* data offset */
>
> The :4 after each variable means 4 bits long, so both fields
> together take up 8 bits = 1 byte. That's the whole purpose
> of bitfields :)
>
D'oh
I didn't ask the right question.
It seems u_int specifies the packing and alignment size
for the bit fields, is that correct ?
struct {
u_int a:4,
b:4;
}; is 4 bytes in size.
struct {
u_int a:4,
b:4;
short c;
}; is 4 bytes in size.
struct {
u_int a:4,
b:4;
short c;
u_char d;
}; is 8 bytes in size;
But
struct {
u_int a:4,
b:4;
u_char d;
short c;
}; is 4 bytes in size;
-- Qing