[Libav-user] Flushing Audio Frame Crash

Paul B Mahol onemda at gmail.com
Fri May 10 19:23:53 CEST 2013

On 5/10/13, Brad O'Hearne <brado at bighillsoftware.com> wrote:
> On May 10, 2013, at 3:44 AM, mohM <phuze9 at gmail.com> wrote:
>> I saw another thread on here similar to this, but it didn't really solve
>> my
>> problem. Basically, I'm still working on getting my audio encoding to
>> work,
>> and I'm now having trouble getting a basic audio encode to work...What
>> I'm
>> trying to do is get a single frame of audio to encode. The first call to
>> avcodec_encode_audio2() is successfull, and if I leave out the flush
>> loop,
>> the console output tells me:
>> As far as I can understand from the documentation and my experience with
>> video encoding, this is pretty normal. When I try to flush the frame by
>> passing NULL into avcodec_encode_audio2(), however, the program crashes
>> and
>> says "Integer division by zero" in mlock.c. Am I doing something wrong
>> here?
>> Any ideas?
> Hey there...I assume the "another thread" you are referring to is mine that
> was posted a few days ago on the same topic. I can identify where you are
> coming from. This issue, its proper handling, and surrounding implications
> are not outright documented as a whole in a single place, and where it is
> referred to in documentation (several places), there seems to be details
> that have to be inferred. Disclaimer -- I am no expert on this, and don't
> know if what I put here will help solve your problem, but it's the best of
> what I understand about it, for better or worse. Hopefully it will help.
> Here's where you can find some references to discussion of this:
>  - avcodec.h, line 4014, frame parameter. Here it mentions using NULL to
> flush. It doesn't mention exactly whether flushing is a bulk or repeated
> operation, nor the implications for pts or dts for flushed frames here. But
> it does reference CODEC_CAP_DELAY, and if you follow to to that parameter's
> doc, there's much more helpful info.
> - avcodec.h, line 737, CODEC_CAP_DELAY define. Explains that encoding needs
> to end with flushing, and says in so many words that you need to repeat the
> process of passing a NULL parameter until the encoder has no more frames.
> Make sure to read the NOTE, as it gives some important information about pts
> and dts.
> - decoding_encoding.h, line 447, flush loop -- it says "get the delayed
> frames" --  that's essentially flushing the encoder. It doesn't do anything
> with pts and dts (nor did it for anywhere in this example, but see the
> CODEC_CAP_DELAY doc for that detail).
> As I understand it, the general idea is this: when you execute
> avcodec_encode_audio2, you are giving the encoder an audio buffer of samples
> to encode. The encoder may optionally populate and output a packet for you
> to do further processing (presumably stream or write to drive). That part
> you probably know and have working. In the case where the encoder does not
> populate and output a packet in that call, it may have decided for some
> encoding reason to hold the data. What that means is that after you send the
> last of your sample data to the encoder in your last normal
> avcodec_encode_audio2 call, the encoder may still be holding on to data that
> you previously sent it.
> You need to flush this data out of the encoder so that you can process it
> like the rest of the encoded data by repeating calls to
> avcodec_encode_audio2 with NULL as the frame parameter to tell the codec to
> release a single buffered frame. That's important -- this isn't a single
> call, it may be several. Repeat the process until got_packet_ptr returns a 0
> value, indicating that no packet has been returned. Note that the
> decoding_encoding.c example references a use case where all of the data is
> known at the time of encoding -- in a streaming scenario, it will usually be
> the input's attempt to close the stream which should trigger the flushing of
> the codec.
> Note there are a number of unknowns still for me:
> - I do not know what codecs support CODEC_CAP_DELAY, or if there's something
> significant which determines if a codec supports it, or if it is entirely
> arbitrary.

Look at encoder AVCodec .capabilities that holds flags.

> - I do not know whether you can or what determines if you can change the
> CODEC_CAP_DELAY value for an encoder from what it is set at after you create
> the encoder.

No. that flag means encoder is capable of that. you can not change it.

> - I do not know if a codec without CODEC_CAP_DELAY set can still buffer
> frames, and if so, how it would be flushed.

It should not buffer frames.

> That's the best I've pieced together. I hope it helps. If not, I wish you
> the best of success solving the problem. If I encounter anything which might
> seem relevant, I'll post it.
> Cheers,
> Brad
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