Re: [FFmpeg-devel] [PATCH 2/4] avcodec/ccaption_dec: check the length of packet and return used length

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

lance.lmwang@gmail.com:
> On Thu, May 12, 2022 at 08:25:29AM +0200, Paul B Mahol wrote:
>> On Thu, May 12, 2022 at 1:39 AM <lance.lmwang@gmail.com> wrote:
>>
>>> On Wed, May 11, 2022 at 09:47:52PM +0200, Paul B Mahol wrote:
>>>> why?
>>>
>>> assuming the len is 1, the following code will access the next 3
>>> array anymore, I think it's better to check the i with len -2.
>>>
>>> for (i = 0; i < len; i += 3) {
>>> to
>>> for (i = 0; i < len - 2; i += 3) {
>>>
>>> for the return, I think it's correct to return the used length,
>>> if it's error, have return already. right?
>>
>>
>> I doubt so.
> 
> maybe I'm misunderstand it, but from the comments, the API claims that:
> libavcodec/codec_internal.h
> 175          * @return amount of bytes read from the packet on success,
> 176          *         negative error code on failure
> 177          */
> 178         int (*decode)(struct AVCodecContext *avctx, struct AVFrame *frame,
> 179                       int *got_frame_ptr, struct AVPacket *avpkt);
> 180         /**
> 181          * Decode subtitle data to an AVSubtitle.
> 182          * cb is in this state if cb_type is FF_CODEC_CB_TYPE_DECODE_SUB.
> 183          *
> 184          * Apart from that this is like the decode callback.
> 185          */
> 186         int (*decode_sub)(struct AVCodecContext *avctx, struct AVSubtitle *sub,
> 187                           int *got_frame_ptr, const struct AVPacket *avpkt);
> 

This is correct. It is not only the internal API which claims that, but
the public API, too. And this just not honoured, in particular not in
case of subtitle recoding. See
https://github.com/mkver/FFmpeg/commit/ba1564c532654888015d67b70bf73d117c2d9f75

- Andreas
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