[FFmpeg-devel] [PATCH v4] avformat/utils: add helper functions to retrieve index entries from an AVStream

[James Almer]

On 4/8/2021 10:09 AM, Anton Khirnov wrote:
> Quoting James Almer (2021-04-04 01:12:00)
>> Signed-off-by: James Almer <jamrial at gmail.com>
>> ---
>> Now using the avformat_ prefix as Anton requested. I forgot about it when i
>> made v3.
>>
>>   libavformat/avformat.h | 39 +++++++++++++++++++++++++++++++++++++++
>>   libavformat/utils.c    | 27 +++++++++++++++++++++++++++
>>   2 files changed, 66 insertions(+)
>>
>> diff --git a/libavformat/avformat.h b/libavformat/avformat.h
>> index 6a9b09160c..a1e87ef891 100644
>> --- a/libavformat/avformat.h
>> +++ b/libavformat/avformat.h
>> @@ -2767,6 +2767,45 @@ int av_find_default_stream_index(AVFormatContext *s);
>>    */
>>   int av_index_search_timestamp(AVStream *st, int64_t timestamp, int flags);
>>   
>> +/**
>> + * Get the index entry count for the given AVStream.
>> + *
>> + * @param st stream
>> + * @return the number of index entries in the stream
>> + */
>> +int avformat_index_get_entries_count(AVStream *st);
>> +
>> +/**
>> + * Get the AVIndexEntry corresponding to the given index.
>> + *
>> + * @param st          Stream containing the requested AVIndexEntry.
>> + * @param idx         The desired index.
>> + * @return A pointer to the requested AVIndexEntry if it exists, NULL otherwise.
>> + *
>> + * @note The pointer returned by this function is only guaranteed to be valid
>> + *       until any function that could alter the stream or the AVFormatContext
>> + *       that cointains it is called.
>> + */
>> +const AVIndexEntry *avformat_index_get_entry(AVStream *st, int idx);
>> +
>> +/**
>> + * Get the AVIndexEntry corresponding to the given timestamp.
>> + *
>> + * @param st          Stream containing the requested AVIndexEntry.
>> + * @param timestamp   Timestamp to retrieve the index entry for.
>> + * @param flags       If AVSEEK_FLAG_BACKWARD then the returned entry will correspond
>> + *                    to the timestamp which is <= the requested one, if backward
>> + *                    is 0, then it will be >=
>> + *                    if AVSEEK_FLAG_ANY seek to any frame, only keyframes otherwise.
>> + * @return A pointer to the requested AVIndexEntry if it exists, NULL otherwise.
>> + *
>> + * @note The pointer returned by this function is only guaranteed to be valid
>> + *       until any function that could alter the stream or the AVFormatContext
>> + *       that cointains it is called.
> 
> How can the user know which functions can "alter the stream or the
> AVFormatContext"?

Any function that takes a non const AVStream or AVFormatContext should 
be considered one that can potentially alter the stream.

> 
> I would prefer invalidating the pointer on ANY lavf call on that
> AVFormatContext or its children, including calls to
> avformat_index_get_entry*. That would allow us more freedom to change
> the implementation later.

avformat_index_get_entry() takes a const AVStream after i included 
Andreas suggestion when i pushed this patch the other day, so in theory 
it should not change the index array stored within the AVStream. I'd 
have to remove the const qualifier.
If that's preferred, I can send a patch to make this change to the doxy 
and signature.