Date: Wed, 31 May 2006 15:09:50 -0400 From: Charles Swiger <cswiger@mac.com> To: Emil Kondayan <emil@ekon-bg.com> Cc: freebsd-net@freebsd.org Subject: Re: Question about netinet/ip.h Message-ID: <6AE303E5-13FD-4328-BDF4-25BACF429806@mac.com> In-Reply-To: <200605312157.04027.emil@ekon-bg.com> References: <200605291103.k4TB38uo097477@freefall.freebsd.org> <200605312157.04027.emil@ekon-bg.com>
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On Jun 1, 2006, at 12:57 AM, Emil Kondayan wrote:
> Can someone tell me why "ip_hl" and "ip_v" are of type "u_int" when
> the
> structure is packed and they only fill a byte?
Well, that struct definition is relying on the compiler to squeeze
the bitfields into the smallest space required. Some platforms
define ip_hl and ip_v as a char called ip_vhl instead:
/*
* Structure of an internet header, naked of options.
*/
struct ip {
#ifdef _IP_VHL
u_char ip_vhl; /* version << 4 | header
length >> 2 */
#else
[ ...bitfields vary based on BYTE_ORDER... ]
> And my second question:do these "#define ..." directives allocate
> space in the
> structure?
No. They define bitmasks to access each flag bit.
--
-Chuck
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