[FFmpeg-devel] [PATCH 18/18] lavf: document some AVStream fields as private

[James Almer]

On 10/9/2020 6:57 PM, James Almer wrote:
> On 10/9/2020 6:55 PM, Michael Niedermayer wrote:
>> On Fri, Oct 09, 2020 at 03:04:30PM +0200, Anton Khirnov wrote:
>>> Specifically: pts_wrap_bits, first_dts, cur_dts.
>>> They are supposed to be private and are located in the private section
>>> of AVStream, but ffmpeg.c currently accesses them regardless. They
>>> should be moved to AVStreamInternal once that bug is fixed.
>>>
>>> Remove the marker for the private AVStream section, as there are no
>>> other accessible fields left there. It has proven highly confusing and
>>> people kept adding supposedly-public fields into the private section.
>>> New private per-stream fields should be added to AVStreamInternal.
>>> ---
>>>  libavformat/avformat.h | 20 +++++++++-----------
>>>  1 file changed, 9 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/libavformat/avformat.h b/libavformat/avformat.h
>>> index a01912d654..612791a2eb 100644
>>> --- a/libavformat/avformat.h
>>> +++ b/libavformat/avformat.h
>>> @@ -1013,22 +1013,16 @@ typedef struct AVStream {
>>>       */
>>>      AVCodecParameters *codecpar;
>>>  
>>> -    /*****************************************************************
>>> -     * All fields below this line are not part of the public API. They
>>> -     * may not be used outside of libavformat and can be changed and
>>> -     * removed at will.
>>> -     * Internal note: be aware that physically removing these fields
>>> -     * will break ABI. Replace removed fields with dummy fields, and
>>> -     * add new fields to AVStreamInternal.
>>> -     *****************************************************************
>>> -     */
>>> -
>>>  #if LIBAVFORMAT_VERSION_MAJOR < 59
>>>      // kept for ABI compatibility only, do not access in any way
>>>      void *unused;
>>>  #endif
>>>  
>>
>>> -    int pts_wrap_bits; /**< number of bits in pts (used for wrapping control) */
>>> +    /**
>>> +     * number of bits in pts (used for wrapping control)
>>> +     * private, do not access from outside libavformat.
>>> +     */
>>> +    int pts_wrap_bits;
>>
>> This is maybe a really bad way to export "pts_wrap_bits" but i think
>> User applications could quite reasonably need to know at which point pts wrap.
>> Or whats the max duration for a timebase where pts are still unique or valid.
>>
>> Based on this a user app might warn the user at the begin of transcoding that
>> timestamps will wrap and that with non streaming output, wrap might equal fail.
>>
>> so maybe this should not be declared private without replacement.
> 
> It already is private. This commit doesn't change that.

I'm not saying that introducing a proper way to export this same
information is a bad idea (It would in fact simply "fixing" the relevant
code in ffmpeg.c), just that this commit is merely moving a notification
from one part of the struct to another. The field itself remains
unaffected after this change.

> 
>>
>> thx
>>
>> [...]
>>
>>
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>