netdev - Re: [PATCH v2 net-next] net: filter: export pkt_type

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Message-ID: <1409839892.26422.115.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Thu, 04 Sep 2014 07:11:32 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	Denis Kirjanov <kirjanov@...il.com>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Daniel Borkmann <dborkman@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Denis Kirjanov <kda@...ux-powerpc.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Markos Chandras <markos.chandras@...tec.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH v2 net-next] net: filter: export pkt_type_offset() helper

On Thu, 2014-09-04 at 15:40 +0200, Hannes Frederic Sowa wrote:

> The type does not matter at all. Actually I wanted to use an empty
> struct but was afraid it might not work on older compilers and didn't
> want to check that with each version.


It matters. Really.

$ cat try.c
#include <stdio.h>

struct S_int {
	char a;
	int offset[0];
	char b:1;
};

struct S_char {
	char a;
	char offset[0];
	char b:1;
};

int main()
{
	printf("sizeof(S_int) %zu\n", sizeof(struct S_int));
	printf("sizeof(S_char) %zu\n", sizeof(struct S_char));
	return 0;
}
$ gcc -o try try.c && ./try
sizeof(S_int) 8
sizeof(S_char) 2


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