Re: [Qemu-devel] [PATCH 02/40] elf: Add notes implementation

[Stefan Weil]

Am 01.11.2010 22:17, schrieb Alexander Graf:
> On 01.11.2010, at 16:19, Stefan Weil wrote:
>
> > Am 01.11.2010 20:51, schrieb Alexander Graf:
> > > On 01.11.2010, at 14:42, Stefan Weil wrote:
> > >
> > >
> > > > Am 01.11.2010 19:29, schrieb Blue Swirl:
> > > >
> > > > > On Mon, Nov 1, 2010 at 3:01 PM, Alexander Graf<address@hidden> wrote:
> > > > >
> > > > >
> > > > > > ---
> > > > > > hw/elf_ops.h | 61 
> > > > > > +++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
> > > > > > hw/loader.c | 7 ++++++
> > > > > > hw/loader.h | 3 ++
> > > > > > 3 files changed, 70 insertions(+), 1 deletions(-)
> > > > > >
> > > > > > diff --git a/hw/elf_ops.h b/hw/elf_ops.h
> > > > > > index 8b63dfc..645d058 100644
> > > > > > --- a/hw/elf_ops.h
> > > > > > +++ b/hw/elf_ops.h
> > > > > > @@ -189,6 +189,44 @@ static int glue(load_symbols, SZ)(struct 
> > > > > > elfhdr *ehdr, int fd, int must_swab,
> > > > > > return -1;
> > > > > > }
> > > > > >
> > > > > > +static void glue(elf_read_notes, SZ)(uint8_t *data, int data_len,
> > > > > > + ElfHandlers *handlers, int must_swab)
> > > > > > +{
> > > > > > + uint8_t *p = data;
> > > > > > +
> > > > > > + while ((ulong)&p[3]< (ulong)&data[data_len]) {
> > > > > Please use 'unsigned long'.
> > > > Why is a type cast used here? I see no reason for it.
> > > Pointers can't be compared, you have to cast them to values first.
> > >
> > >
> > > Alex
> >
> > No. Pointers of same type which are not void pointers can be compared.
> >
> > There is even a data type ptrdiff_t, so you can also compare their
> > difference with zero.
>
> Let's ask someone who definitely knows :).
>
> Michael, is code like
>
> char *x = a, *y = b;
> if (x < y) {
> ...
> }
>
> valid? Or do I first have to cast x and y to unsigned longs or uintptr_t?
>
>
> Alex


Hopefully C did not change for code like this during the last
20 years.

Then your code is always valid, but will only return useful results
if both a and b are derived from the same base pointer
(plus an individual offset):

char *base = any_value;
a = base + offset_a;
b = base + offset_b;

Then
        x - y = a - b = (ptrdiff_t)(offset_a - offset_b) / (sizeof(*x);
and
        (x < y) == (a < b) == (offset_a < offset_b);

Regards,
Stefan