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

H. Peter Anvin wrote:
> A Large Angry SCM wrote:
>> 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.

ANSI X3.159-1989

3.2.2.3 Pointers
	A pointer to *void* may be converted to or from a pointer to any 
incomplete or object type. A pointer to any incomplete or object type 
may be converted to a pointer to *void* and back again; the result shall 
compare equal to the original pointer.

For any qualifier /q/, a pointer to a non-/q/-qualified type may be 
converted to a pointer to the /q/-qualified version of the type; the 
values stored in the original and converted pointers shall compare equal.

In integral constant expression with value 0, or such an expression cast 
to type <bold>void *</bold>, is called a /null pointer constant.[*33*] 
If a null pointer constant is assigned to or compared for equality to a 
pointer, the constant is converted to a pointer of that type. Such a 
pointer, called a /null pointer/, is guaranteed to compare unequal to a 
pointer to any object or function.

Two null pointers, converted through possibly different sequences of 
casts to pointer types, shall compare equal.

[*33*] the macro *NULL* is defined in <stddef.h> as a null pointer 
constant; see 4.1.5.

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