[FFmpeg-devel] [PATCH] avformat/avienc: Remove unneeded seekable tests

[Ronald S. Bultje]

Hi,

On Sun, Mar 6, 2016 at 7:37 AM, Mats Peterson <
matsp888-at-yahoo.com at ffmpeg.org> wrote:

> On 03/06/2016 01:29 PM, Ronald S. Bultje wrote:
>
>> Hi Mats,
>>
>> On Sun, Mar 6, 2016 at 6:51 AM, Mats Peterson <
>> matsp888-at-yahoo.com at ffmpeg.org> wrote:
>>
>> On 03/06/2016 12:09 PM, Michael Niedermayer wrote:
>>>
>>> Once again, why are these tests unneeded?
>>>
>>
>>
>> You shouldn't send the same message 5x within 2 seconds. Please be
>> patient.
>>
>> Why are they unneeded? Imagine this situation:
>>
>> ----
>> int a;
>>
>> if (condition)
>>      a = create_a();
>>
>> [..]
>>
>> if (condition)
>>      do_something_with_a(a);
>> ----
>>
>> versus:
>>
>> ----
>> int a = create_a();
>>
>> [..]
>>
>> if (condition)
>>      do_something_with_a(a);
>> ----
>>
>> Oddly, both codes do exactly the same thing, assuming they don't change
>> global state. In this case, avio_tell() shouldn't change global state,
>> that
>> is, we're assuming that it will not cause bitstream corruption simply
>> because it's unseekable, which is a fair assumption.
>>
>> On the positive side, with that assumption, code fragment #2 is much
>> simpler than #1, which is why the patch is net positive.
>>
>> Ronald
>> _______________________________________________
>> ffmpeg-devel mailing list
>>
>
>
> Well, that part I understand. It's for simplification purposes.
>
> By the way, I just noticed that avio_tell() (and avio_seek() will work on
> stdout as well. That's the part I *don't* understand: How can they work on
> an "officially" unseekable stream like stdout, when ftell() and lseek()
> won't? Is it due to the buffering in FFmpeg? And where's the limit where a
> normally "unseekable stream" like stdout is no longer seekable with
> avio_seek()? Hope you understand what I mean.


See the implementation [1]. If not seekable, and the seek is forward, it
fills buffer until it reaches that position. If not seekable and seek is
backward but fits in buffer, it works as expected by changing buffer index.
Etc. See also avio_tell [2]. Line 230 of avio_seek tells you the shortcut
that makes this all work without calls to fseek/ftell.

Ronald

[1] https://ffmpeg.org/doxygen/trunk/aviobuf_8c_source.html#l00209
[2] https://ffmpeg.org/doxygen/trunk/avio_8h_source.html#l00447