Hello,
On Thu, Aug 14, 2025 at 04:08:57PM +0200, Uwe Kleine-König wrote:
> On Thu, Aug 14, 2025 at 08:48:23PM +0900, Gustavo A. R. Silva wrote:
> >
> > > diff --git a/include/linux/stddef.h b/include/linux/stddef.h
> > > index dab49e2ec8c0..8ca9df87a523 100644
> > > --- a/include/linux/stddef.h
> > > +++ b/include/linux/stddef.h
> > > @@ -108,7 +108,7 @@ enum {
> > > union {
> > > \
> > > TYPE NAME;
> > > \
> > > struct {
> > > \
> > > - unsigned char __offset_to_##FAM[offsetof(TYPE, FAM)];
> > > \
> > > + unsigned char __offset_to_##FAM[sizeof(TYPE)];
> > > \
> > > MEMBERS
> > > \
> > > };
> > > \
> > > }
> > >
> > > which only leaves one usage of FAM in the name of the padding struct
> > > member. I'm sure someone is able to come up with something nice here to
> > > get rid of FAM completely or point out what I'm missing.
> >
> > Flexible structures (structs that contain a FAM) may have trailing padding.
> > Under that scenario sizeof(TYPE) causes the overlay between FAM and MEMBERS
> > to be misaligned.
>
> That sounds wrong to me; are you sure? In that case allocating space for
> such a struct using
>
> struct mystruct {
> unsigned short len;
> unsigned int array[];
> };
>
> s = malloc(sizeof(struct mystruct) + n * sizeof(unsigned int));
>
> wouldn't do the right thing.
>
> I found in the net (e.g.
> https://rgambord.github.io/c99-doc/sections/6/7/2/1/index.html):
>
> In most situations, the flexible array member is ignored. In
> particular, the size of the structure is as if the flexible
> array member were omitted except that it may have more trailing
> padding than the omission would imply.
>
> So I'd claim that sizeof does work here as intended.
>
> gcc here also behaves fine:
>
> uwe@taurus:~$ cat test.c
> #include <stdio.h>
>
> struct mystruct {
> unsigned short len;
> unsigned int array[];
> };
>
> struct mystruct2 {
> unsigned short len;
> };
>
> int main()
> {
> printf("sizeof(struct mystruct) = %zu\n", sizeof(struct
> mystruct));
> printf("sizeof(struct mystruct2) = %zu\n", sizeof(struct
> mystruct2));
> return 0;
> }
>
> uwe@taurus:~$ make test
> cc -c -o test.o test.c
> cc test.o -o test
>
> uwe@taurus:~$ ./test
> sizeof(struct mystruct) = 4
> sizeof(struct mystruct2) = 2
My claim is wrong, while sizeof() never gives a value that is too small,
it might be too big. E.g. for
struct mystruct {
unsigned short a;
unsigned char b;
unsigned char c[];
};
there is sizeof(mystruct) = 4, but c starts at offset 3.
Anyhow, I applied the original patch now to
https://git.kernel.org/pub/scm/linux/kernel/git/ukleinek/linux.git pwm/for-next
; the discussion here was somewhat orthogonal anyhow.
Best regards
Uwe
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