Re: [PATCH] Trivial warning fix for imap-send.c

From: H. Peter Anvin <hpa@zytor.com>
Date: Mar 13, 2006, 02:59 UTC
Message-ID: <4414E000.9030902@zytor.com>
In-Reply-To: <4414747B.7040700@gmail.com>

A Large Angry SCM wrote:

Show 18 quoted lines

> Mark Wooding wrote:
> 
>> Linus Torvalds <torvalds@osdl.org> wrote:
>>
>>> So in modern C, using NULL at the end of a varargs array as a pointer 
>>> is perfectly sane, and the extra cast is just ugly and bowing to bad 
>>> programming practices and makes no sense to anybody who never saw the 
>>> horror that is K&R.
>>
>> No!  You can still get bitten.  You're lucky that on common platforms
>> all pointers look the same, but if you find one where `char *' (and
>> hence `void *') isn't the same as `struct foo *' then, under appropriate
>> circumstances you /will/ unless you put the casts in.
> 
> Please explain how malloc() can work on such a platform. My reading of 
> the '89 ANSI C spec. finds that _ALL_ (non function) pointers _are_ 
> cast-able to/from a void * and that NULL should be #defined as (void *). 
> See 3.2.2.3 and 4.1.5 if interested.

Consider the non-hypothetical example of a word-addressed machine, which has to have extra bits in a subword pointer like char *. The C standard requires that void * has those bits as well, but it doesn't means that any void * can be cast to any arbitrary pointer -- the opposite, however, is required.

	-hpa