Re: [PATCH] c: Split -Wcalloc-transposed-args warning from -Walloc-size, -Walloc-size fixes

[Jason Merrill <jason@redhat.com>]

On 12/19/23 03:47, Jakub Jelinek wrote:
> On Tue, Dec 19, 2023 at 08:11:11AM +0100, Martin Uecker wrote:
>> Am Montag, dem 18.12.2023 um 20:14 +0100 schrieb Jakub Jelinek:
>>> Hi!
>>>
>>> The following patch changes -Walloc-size warning to no longer warn
>>> about int *p = calloc (1, sizeof (int));, because as discussed earlier,
>>> the size is IMNSHO sufficient in that case, for alloc_size with 2
>>> arguments warns if the product of the 2 arguments is insufficiently small.
>>>
>>> Also, it warns also for explicit casts of malloc/calloc etc. calls
>>> rather than just implicit, so not just
>>>    int *p = malloc (1);
>>> but also
>>>    int *p = (int *) malloc (1);
>>>
>>> It also fixes some ICEs where the code didn't verify the alloc_size
>>> arguments properly (Walloc-size-5.c testcase ICEs with vanilla trunk).
>>>
>>> And lastly, it introduces a coding style warning, -Wcalloc-transposed-args
>>> to warn for calloc (sizeof (struct S), 1) and similar calls (regardless
>>> of what they are cast to, warning whenever first argument is sizeof and
>>> the second is not).
>>
>> I would generally see function arguments that are swapped relative
>> to the documented ABI as more than a coding style issue even in
>> cases where it can be expected to make no difference.
> 
> If you have suggestions how to reword the documentation, would that be
> sufficient for you?  I still don't see why given correct alignment one can't
> store struct S into sizeof (struct S) sized heap char array,

Seems to me one can in C++, anyway.  An unsigned char array can provide 
storage for another type, and the call to calloc can be interpreted as 
creating such an array if that gives the program defined behavior.
https://eel.is/c++draft/intro.object#def:provides_storage
https://eel.is/c++draft/intro.object#def:object,implicit_creation

Jason