[FFmpeg-devel] [PATCH 1/4] libavcodec/adts_header: add frame_length field and avpriv function to parse AAC ADTS header

[James Almer]

On 8/16/2021 12:08 AM, "zhilizhao(赵志立)" wrote:
> 
> 
>> On Aug 16, 2021, at 2:25 AM, Nachiket Tarate <nachiket.programmer at gmail.com> wrote:
>>
>> These will be used by HLS demuxer in case of sample decryption.
>>
>> Signed-off-by: Nachiket Tarate <nachiket.programmer at gmail.com>
>> ---
>> libavcodec/adts_header.c |  1 +
>> libavcodec/adts_header.h | 14 ++++++++++++++
>> libavcodec/adts_parser.c | 32 ++++++++++++++++++++++++++++++++
>> 3 files changed, 47 insertions(+)
>>
>> diff --git a/libavcodec/adts_header.c b/libavcodec/adts_header.c
>> index 0889820f8a..e4454529c4 100644
>> --- a/libavcodec/adts_header.c
>> +++ b/libavcodec/adts_header.c
>> @@ -66,6 +66,7 @@ int ff_adts_header_parse(GetBitContext *gbc, AACADTSHeaderInfo *hdr)
>>      hdr->sample_rate    = avpriv_mpeg4audio_sample_rates[sr];
>>      hdr->samples        = (rdb + 1) * 1024;
>>      hdr->bit_rate       = size * 8 * hdr->sample_rate / hdr->samples;
>> +    hdr->frame_length   = size;
>>
>>      return size;
>> }
>> diff --git a/libavcodec/adts_header.h b/libavcodec/adts_header.h
>> index f615f6a9f9..9ecd67fb5b 100644
>> --- a/libavcodec/adts_header.h
>> +++ b/libavcodec/adts_header.h
>> @@ -34,6 +34,7 @@ typedef struct AACADTSHeaderInfo {
>>      uint8_t  sampling_index;
>>      uint8_t  chan_config;
>>      uint8_t  num_aac_frames;
>> +    uint32_t frame_length;
>> } AACADTSHeaderInfo;
>>
> 
> The patch can be separated into two patches.
> 
>> /**
>> @@ -47,4 +48,17 @@ typedef struct AACADTSHeaderInfo {
>>   */
>> int ff_adts_header_parse(GetBitContext *gbc, AACADTSHeaderInfo *hdr);
>>
>> +/**
>> + * Parse the ADTS frame header contained in the buffer, which is
>> + * the first 54 bits.
>> + * @param[in]  buf  Pointer to buffer containing the first 54 bits of the frame.
>> + * @param[in]  size Size of buffer containing the first 54 bits of the frame.
>> + * @param[out] phdr Pointer to pointer to struct AACADTSHeaderInfo for which
>> + * memory is allocated and header info is written into it.
>> + * @return Returns 0 on success, -1 if there is a sync word mismatch,
>> + * -2 if the version element is invalid, -3 if the sample rate
>> + * element is invalid, or -4 if the bit rate element is invalid.
>> + */
>> +int avpriv_adts_header_parse(AACADTSHeaderInfo **phdr, const uint8_t *buf, size_t size);
>> +
>> #endif /* AVCODEC_ADTS_HEADER_H */
>> diff --git a/libavcodec/adts_parser.c b/libavcodec/adts_parser.c
>> index 5c9f8ff6f2..32489dadf4 100644
>> --- a/libavcodec/adts_parser.c
>> +++ b/libavcodec/adts_parser.c
>> @@ -42,3 +42,35 @@ int av_adts_header_parse(const uint8_t *buf, uint32_t *samples, uint8_t *frames)
>>      return AVERROR(ENOSYS);
>> #endif
>> }
>> +
>> +int avpriv_adts_header_parse(AACADTSHeaderInfo **phdr, const uint8_t *buf, size_t size)
>> +{
>> +#if CONFIG_ADTS_HEADER
>> +    int ret = 0;
>> +    GetBitContext gb;
>> +
>> +    if (size < AV_AAC_ADTS_HEADER_SIZE)
>> +        return AVERROR_INVALIDDATA;
>> +
>> +    if (!*phdr)
>> +        *phdr = av_mallocz(sizeof(AACADTSHeaderInfo));
>> +    if (!*phdr)
>> +        return AVERROR(ENOMEM);
> 
> I’m not keen about such design, that
> 1. you can allocate AACADTSHeaderInfo directly
> 2. if you don’t, I will allocate it for you
> 
> I’m not sure about the rule of avpriv_ part, in my opinion, the struct should
> always allocated by libavcodec and used by libavformat. libavformat shouldn’t
> allocate it on stack or heap by itself for ABI compatibility.

It's copying the behavior of avpriv_ac3_parse_header(), which may be a 
remnant from a time where it was called from within libavcodec itself.
I agree it should not be allowed to pass a pre-allocated struct by 
another library for ABI compatibility reasons, but the only user 
Nachiket is adding is letting lavc allocate it, so it's not that important.

> 
>> +
>> +    ret = init_get_bits8(&gb, buf, AV_AAC_ADTS_HEADER_SIZE);
>> +    if (ret < 0) {
>> +        av_free(*phdr);
>> +        return ret;
>> +    }
>> +
>> +    ret = ff_adts_header_parse(&gb, *phdr);
>> +    if (ret < 0) {
>> +        av_free(*phdr);
> 
> av_freep(phdr)
> 
>> +        return ret;
>> +    }
>> +
>> +    return 0;
>> +#else
>> +    return AVERROR(ENOSYS);
>> +#endif
>> +}
>> -- 
>> 2.17.1
>>
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