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Date: Wed, 31 May 2006 20:13:31 +0100
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From: Rui Paulo <rpaulo@NetBSD.org>
To: Emil Kondayan <emil@ekon-bg.com>
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Cc: freebsd-net@freebsd.org
Subject: Re: Question about netinet/ip.h
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At Wed, 31 May 2006 21:57:03 -0700,
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?

u_int means unsigned int and they only fill a byte because 4 + 4 = 8
bits (a byte) (I'm not going into the "Why is a byte 8 bits?" question ;-).

This structure is composed of bitfields and it means that when you
access a bitfield element (for example, ip_hl) you are only
manipulating 4 bits instead of sizeof(unsigned int) bytes (8 bit
several computer arquitectures).

Any C book should explain bitfield structs/unions clearly.

> And my second question:do these "#define ..." directives allocate space in the 
> structure?

No, read about the C pre-processor (again, in every C book).