[FFmpeg-devel] [PATCH] avcodec/get_bits: consider bit_size of 0 an error

[Ronald S. Bultje]

Hi,

On Mon, Oct 28, 2013 at 2:17 PM, Paul B Mahol <onemda at gmail.com> wrote:

> On 10/28/13, Ronald S. Bultje <rsbultje at gmail.com> wrote:
> > Hi,
> >
> > On Mon, Oct 28, 2013 at 1:35 PM, Paul B Mahol <onemda at gmail.com> wrote:
> >
> >> On 10/28/13, Reimar Doeffinger <Reimar.Doeffinger at gmx.de> wrote:
> >> > On Mon, Oct 28, 2013 at 05:04:38PM +0000, Paul B Mahol wrote:
> >> >> On 10/28/13, Reimar Doeffinger <Reimar.Doeffinger at gmx.de> wrote:
> >> >> > On Mon, Oct 28, 2013 at 04:52:27PM +0000, Paul B Mahol wrote:
> >> >> >> Signed-off-by: Paul B Mahol <onemda at gmail.com>
> >> >> >
> >> >> > Could you add the reason for this change to the commit message?
> >> >> > In theory being able to use 0 might work as an optimization in
> >> >> > some cases, so I don't think supporting it is as nonsense as it
> >> >> > might seem.
> >> >>
> >> >> What kind of optimization?
> >> >
> >> > Purely theoretical: You have a value A, depending on that following
> >> > value B is either not encoded, encoded with 2, 5 or 8 bits.
> >> > If get_bits supports 0 bits then you can just do something like
> >> > get_bits(... (int){0, 2, 5, 8}[A]);
> >> > if 0 is not allowed, you have to explicitly skip the get_bits for
> >> > A == 0.
> >> >
> >> >> If return value is never checked and bit_size is negative number next
> >> >> get_bits will happily crash.
> >> >
> >> > But negative number is already checked, your patch is about checking
> >> > 0?!
> >>
> >> Yes, because here is what happens for 0 case:
> >>
> >> buffer is not set to NULL (for negative case this causes crash)
> >>
> >> get_bits may return 1 even if bit count is 0, and it will happily
> >> return it forever (until  whatever buffer points to is changed to 0)
> >>
> >> so something like
> >>
> >> while(get_bits1(gb)) {
> >> }
> >>
> >> loops forever.
> >>
> >
> > That is a bug in the caller.
>
> Are you saying I should use get_bits_left ?


In this particular case, no; If you buffer is padded with
FF_INPUT_BUFFER_PADDING_SIZE zero bytes, at EOF, get_bits should always
return 0.

More generally, yes, because I don't see why we wouldn't support inverted
VLC schemes (while(!get_bits(gb)) { .. }), and to support that, you need
some explicit exit clause (think e.g. golomb codes).

Ronald