linux-kernel - Re: [PATCH 1/2] net: tcp_header_len_th and tcp_option_len

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Date:	Wed, 13 Jan 2010 11:00:16 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	William Allen Simpson <william.allen.simpson@...il.com>
CC:	Linux Kernel Developers <linux-kernel@...r.kernel.org>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	Michael Chan <mchan@...adcom.com>
Subject: Re: [PATCH 1/2] net: tcp_header_len_th and tcp_option_len_th

Le 13/01/2010 09:53, William Allen Simpson a écrit :
> Eric Dumazet wrote:
> 
>> About cast games, maybe following way is the cleanest one.
>>
>> int tcp_options_len_th(struct tcphdr *th)
>> {
>>     return tcp_header_len_th(th) - sizeof(*th);
>> }
>>
> If you'd have been one of my C students, you'd have failed the exam
> question.  That's unsigned int tcp_header_len_th() -- subtracting an
> untyped constant could be a negative number (stored in an unsigned).
> Then demotion to int (which many compilers truncate to a very large
> positive number).

Thats simply not true, you are very confused.

Once again you type too much text and ignore my comments.
I really would hate being your student, thanks God it wont happen in this life.

1) You wrote tcp_header_len_th(), you should know that it
returns an unsigned int.

2) You also should know that sizeof() is *strongly* typed (size_t),
not an "untyped constant".

unsigned int arg = some_expression;
size_t sz = sizeof(something)
int res = arg - sz;
return res;

is *perfectly* legal and very well defined by C standards.

It *will* return a negative value is arg < sz



> 
> It's one of the reasons that folks used to do all this with macros, so
> that the types and truncation were handled well by the compiler.
> 
> Of course, this is an inline function, which is more like macros.  I've
> not studied how gcc works internally since egcs.
> 
> Let's keep (int)(th->doff * 4) - sizeof(*th) -- self documenting, and
> should work with a wide variety of compilers.

So you wrote tcp_header_len_th(), but you keep (th->doff * 4) thing all over
the code...

The (int) cast it not only _not_ needed, its also confusing.

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