[FFmpeg-devel] [PATCH] flvenc: flush after write header

[Martin Storsjö]

On Thu, 28 Oct 2021, Steven Liu wrote:

>
>
>> 2021年10月28日 下午2:32，Lingjiang Fang <vacingfang at foxmail.com> 写道：
>> 
>> keep align with movenc, or it seems a little weired when debug
>> ---
>> libavformat/flvenc.c | 2 ++
>> 1 file changed, 2 insertions(+)
>> 
>> diff --git a/libavformat/flvenc.c b/libavformat/flvenc.c
>> index 3f24c7e192..f4798dc9a6 100644
>> --- a/libavformat/flvenc.c
>> +++ b/libavformat/flvenc.c
>> @@ -767,6 +767,8 @@ static int flv_write_header(AVFormatContext *s)
>>         flv_write_codec_header(s, s->streams[i]->codecpar, 0);
>>     }
>> 
>> +    avio_flush(pb);
>> +
>>     flv->datastart_offset = avio_tell(pb);
>>     return 0;
>> }
>> -- 
>> 2.29.2
>> 
>
> LGTM

No, please don't apply this.

We intentionally move in the other direction; every explicit avio_flush() 
is a potential performance bottleneck, and we've been working on reducing 
the number of extra avio_flush() calls.

When streaming, where latency is relevant, the libavformat generic code 
already flushes automatically after the header and after each packet. When 
latency is not relevant, we explicitly want to avoid extra avio_flush() 
because each of them cause extra writes (that otherwise could be 
buffered). You can also set the option "-flush_packets 1" (or "-fflags 
flush_packets") to make the generic code flush when relevant. See e.g. the 
flush_if_needed() function in libavformat/mux.c.

// Martin