Re: [PATCH] drm/xe: use %z format string for ptrdiff_t

[David Laight <david.laight.linux@gmail.com>]

On Tue, 17 Mar 2026 00:07:57 +0100
"Arnd Bergmann" <arnd@arndb.de> wrote:

> On Mon, Mar 16, 2026, at 23:51, Matt Roper wrote:
> > On Mon, Mar 16, 2026 at 11:43:46PM +0100, Arnd Bergmann wrote:  
> >> From: Arnd Bergmann <arnd@arndb.de>
> >> 
> >> ptrdiff_t, size_t and long are the same size on all supported architectures,
> >> but gcc requires the use of the %zx modifier instead of %lx. On 32-bit  
> >
> > Nathan Chancellor just sent a fix here:
> >
> > https://lore.kernel.org/all/20260316-drm-xe-fix-32-bit-wformat-ptrdiff-v1-1-0108b10b2b6b@kernel.org/
> >
> > His solution uses %tx rather than %zx, which according to
> > Documentation/core-api/printk-formats.rst sounds like it's probably the
> > most accurate format string for this case?
> >  
> 
> Right, Nathan's version is correct. I wasn't aware that size_t
> and ptrdiff_t have different modifiers.

Consider:
void foo(size_t len)
{
	char *p = malloc(len);
	char *end = p + len;
	ptrdiff_t diff = p - end;
	printf("len %zu, diff %td\n", len, diff);
	free(p);
}
Clearly foo(5) output "5 -5".
But what about foo(5u << 29) on a 32bit system with a large unlimit and
a big enough hole in the address space.
More likely would be something running on an x86 in real mode.
There malloc(65000) might be reasonable, so size_t would be unsigned int
but ptrdiff_t would need to be a signed 32bit type.

Of course, in linux, they are all 'long'.

	David


> 
>       Arnd
>