From owner-freebsd-hackers@FreeBSD.ORG  Mon Feb  6 11:28:34 2006
Return-Path: <owner-freebsd-hackers@FreeBSD.ORG>
X-Original-To: freebsd-hackers@freebsd.org
Delivered-To: freebsd-hackers@freebsd.org
Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125])
	by hub.freebsd.org (Postfix) with ESMTP id E64EF16A420
	for <freebsd-hackers@freebsd.org>; Mon,  6 Feb 2006 11:28:34 +0000 (GMT)
	(envelope-from joseph.koshy@gmail.com)
Received: from xproxy.gmail.com (xproxy.gmail.com [66.249.82.205])
	by mx1.FreeBSD.org (Postfix) with ESMTP id 77D0843D48
	for <freebsd-hackers@freebsd.org>; Mon,  6 Feb 2006 11:28:34 +0000 (GMT)
	(envelope-from joseph.koshy@gmail.com)
Received: by xproxy.gmail.com with SMTP id s9so742470wxc
	for <freebsd-hackers@freebsd.org>; Mon, 06 Feb 2006 03:28:33 -0800 (PST)
DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=beta; d=gmail.com;
	h=received:message-id:date:from:to:subject:cc:in-reply-to:mime-version:content-type:content-transfer-encoding:content-disposition:references;
	b=Nt/RLtgwjS6o2AQt/zekFO5lPwU11iYpRrTSGJSHtTcf42GPJBHlDlWiyji/R7nH9CpCZP+i7bRlCdyhbni0fySWaJU5KbGrZhIlwbj9n7lDJ4K+QxGjAqS1VAlOGfJOpkOD1nxiOKvKdUZtPDwtlO2VS/xAO8DsTa9VytZi2g8=
Received: by 10.70.74.19 with SMTP id w19mr6066222wxa;
	Mon, 06 Feb 2006 03:28:33 -0800 (PST)
Received: by 10.70.105.2 with HTTP; Mon, 6 Feb 2006 03:28:33 -0800 (PST)
Message-ID: <84dead720602060328o39c8f964h3fcf355930262816@mail.gmail.com>
Date: Mon, 6 Feb 2006 16:58:33 +0530
From: Joseph Koshy <joseph.koshy@gmail.com>
To: Tanmay <tanmayinamdar@gmail.com>
In-Reply-To: <cdc1d1310602060234p1ade9a3dh545c57e0dfc76794@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: quoted-printable
Content-Disposition: inline
References: <cdc1d1310602060234p1ade9a3dh545c57e0dfc76794@mail.gmail.com>
Cc: freebsd-hackers@freebsd.org
Subject: Re: What is '_KERNEL' in the source ?
X-BeenThere: freebsd-hackers@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: Technical Discussions relating to FreeBSD
	<freebsd-hackers.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-hackers>, 
	<mailto:freebsd-hackers-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-hackers>
List-Post: <mailto:freebsd-hackers@freebsd.org>
List-Help: <mailto:freebsd-hackers-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-hackers>,
	<mailto:freebsd-hackers-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Mon, 06 Feb 2006 11:28:35 -0000

>           In various kernel source files,i came across '#ifdef _KERNEL'.
> What is '_KERNEL' used for ? In some files _KERNEL is #defined to nothing=
 ??
> Can anybody please explain this ?

It is used to control the visibility of types and prototypes in system head=
ers.
Kernel builds define _KERNEL, but userland compiles usually do not.  Thus a
#include <sys/foo.h> has a different meaning in userland than in the kernel=
.
See: src/sys/conf/{kern.pre,kmod}.mk

A few userland utilities (e.g., fstat) define _KERNEL before including
headers from <sys/*> because they need more knowledge of kernel
data structures than is the norm for userland.

--
FreeBSD Volunteer,     http://people.freebsd.org/~jkoshy