From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ozgur AKAN Subject: Re: size of a rule Date: Mon, 26 Apr 2004 17:08:30 +0300 Sender: netfilter-devel-admin@lists.netfilter.org Message-ID: <408D17DE.3050900@aiqa.com> References: <408D1482.5030009@aiqa.com> Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="------------040509080701040606020802" Cc: netfilter-devel@lists.netfilter.org Return-path: In-Reply-To: <408D1482.5030009@aiqa.com> Errors-To: netfilter-devel-admin@lists.netfilter.org List-Help: List-Post: List-Subscribe: , List-Unsubscribe: , List-Archive: List-Id: netfilter-devel.vger.kernel.org This is a multi-part message in MIME format. --------------040509080701040606020802 Content-Type: text/plain; charset=ISO-8859-9; format=flowed Content-Transfer-Encoding: 7bit Actually when I print the address of ipt_entry and subtract it from the next ipt_entry I can get the size of the rule but I am not sure if I am doing something wrong or right. I still can not understand ((type *)0)->member. Ozgur AKAN wrote: > Hi, > I am trying to find the size of a rule in memory. > > const struct ipt_entry *e; > > sizeof (*e) is equal to 112 always. But the size of rule should be > different because we can give different parameters to each rule. (I > guess that this is the size of the header-like place) > > So I looked at the source of libiptc and found this in linux_list.h > while searching for the size value. > > #define list_entry(ptr, type, member) \ > ((type *) ( (char *)(ptr)-(unsigned long)(&((type > *)0)->member) ) ) > > What is the meaning of ((type *)0)->member ? I can not understand the > part with 0. > >thanks, >-- >Ozgur Akan > > -- Ozgur Akan --------------040509080701040606020802 Content-Type: text/html; charset=ISO-8859-9 Content-Transfer-Encoding: 8bit Actually when I print the address of ipt_entry and subtract it from the next ipt_entry I can get the size of the rule but I am not sure if I am doing something wrong or right.

I still can not understand ((type *)0)->member.

Ozgur AKAN wrote:
Hi,
I am trying to find the size of a rule in memory.

const struct ipt_entry *e;

sizeof (*e) is equal to 112 always. But the size of rule should be different because we can give different parameters to each rule. (I guess that this is the size of the header-like place)

So I looked at the source of libiptc and found this in linux_list.h while searching for the size value.

#define list_entry(ptr, type, member) \
        ((type *) ( (char *)(ptr)-(unsigned long)(&((type *)0)->member) ) )

What is the meaning of ((type *)0)->member ? I can not understand the part with 0.   
thanks,
-- 
Ozgur Akan
  


-- 
Ozgur Akan
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