Re: [C PATCH, v2] Add Walloc-size to warn about insufficient size in allocations [PR71219]

[Jakub Jelinek <jakub@redhat.com>]

On Wed, Dec 06, 2023 at 08:31:12PM +0800, Xi Ruoyao wrote:
> On Wed, 2023-12-06 at 13:24 +0100, Jakub Jelinek wrote:
> > I wonder if this part isn't too pedantic or more of a code style.
> > Some packages fail to build with this with -Werror because they do
> >   struct S *p = calloc (sizeof (struct S), 1);
> > or similar.  It is true that calloc arguments are documented to be
> > nmemb, size, but given sufficient alignment (which is not really different
> > between either order of arguments) isn't it completely valid to allocate
> > char array with sizeof (struct S) elements and then store a struct S object
> > into it?
> 
> In PR112364 Martin Uecker has pointed out the alignment may be different
> with the different order of arguments, per C23 (N2293).  With earlier
> versions of the standard some people believe the alignment should not be
> different, while the other people disagree (as the text is not very
> clear).

I can understand implementations which use smaller alignment based on
allocation size, but are there any which consider for that just the second
calloc argument rather than the product of both arguments?
I think they'd quickly break a lot of real-world code.
Further I think
"size less than or equal to the size requested"
is quite ambiguous in the calloc case, isn't the size requested in the
calloc case actually nmemb * size rather than just size?

	Jakub