Re: CVS commit: src/sys/dev/usb

[Valery Ushakov <uwe%stderr.spb.ru@localhost>]

On Sat, Apr 21, 2018 at 09:44:59 +0200, Manuel Bouyer wrote:
> Date: Sat, 21 Apr 2018 09:44:59 +0200
> From: Manuel Bouyer <bouyer%antioche.eu.org@localhost>
> Subject: Re: CVS commit: src/sys/dev/usb
> To: Martin Husemann <martin%duskware.de@localhost>
> Cc: Christos Zoulas <christos%astron.com@localhost>, source-changes-d%NetBSD.org@localhost
> 
> On Sat, Apr 21, 2018 at 09:37:46AM +0200, Martin Husemann wrote:
> > On Sat, Apr 21, 2018 at 09:30:20AM +0200, Manuel Bouyer wrote:
> > > > > 
> > > > >-       axe_cmd(sc, AXE_CMD_WRITE_MCAST, 0, 0, (void *)&hashtbl);
> > > > >+       axe_cmd(sc, AXE_CMD_WRITE_MCAST, 0, 0, hashtbl);
> > > > >
> > > > >missing & ?
> > > > 
> > > >         uint8_t hashtbl[8] = { 0, 0, 0, 0, 0, 0, 0, 0 }; 
> > > 
> > > So I guess the code was wrong before; not sure how multicast could have
> > > worked.
> > 
> > No, the address is only needed as rhs of the cast. If passed directly,
> > the address will be used (due to arrays being passed as pointer to first
> > element in C).
> > 
> > Try it:
> > 
> > #include <stdio.h>
> > #include <inttypes.h>
> > 
> > int main(void)
> > {
> >         static uint8_t hashtbl[8] = { 0, 0, 0, 0, 0, 0, 0, 0 };
> > 
> >         printf("%p vs %p\n", (void *)&hashtbl, hashtbl);
> >         return 0;
> > }
> 
> I didn't know this. hashtbl and &hashtbl[0] are equivalent, and I would
> assume that &hashtbl is always a pointer to pointer. So the behavior depends
> on hashtbl being declared as pointer or as array ?
> this is confusing ...

&hashtbl is a pointer to an array of size 8.  You can see this with
pointer arithmetic:

    char hashtbl[8];
    printf("%p %p\n%p %p\n", hashtbl, hashtbl + 1, &hashtbl, &hashtbl + 1);

prints

    0x7fff54e4b720 0x7fff54e4b721
    0x7fff54e4b720 0x7fff54e4b728

-uwe