Hello, I have an application that decodes an input stream into AVFrames and stores these in a ring buffer. On a trigger, I can create an output file containing video from the past N seconds until I stop. I am seeing a problem with the output video that it sometimes contains duplicate frames and sometimes contains frames from "the future" (i.e. the wrong point in the ringbuffer). I have added debug and it appears that avcodec_receive_frame() sometimes returns the same data buffer for multiple decoded frames, even though I have not unref'ed the previous AVFrame. Specifically I am checking that the returned av_frame->data[0] is not being used by any other AVFrames and this is failing.
This is on Ubuntu bionic (18.04) using the bionic backports of v4.1.3 and v4.2.0 from https://launchpad.net/~jonathonf/+archive/ubuntu/ffmpeg-4. The source of the video stream is a v4l2 device. Cut-down example of the frame retrieve function: void *reader_run() { AVFrame *ff = NULL; while (is_running) { AVPacket; av_init_packet(&pkt); ff = av_alloc_frame(); /* Get packet from input */ ret = av_read_frame(reader_fctx, &pkt); /* error checking ... */ ret = avcodec_send_packet(decoder_cctx, &pkt); /* error checking ... */ ret = avcodec_receive_frame(decoder_cctx, ff); /* error checking ... */ /* Debug to check buffers in other frames */ for (int i = 0; i< ringbuffer->size; i++) { if ringbuffer->items[i]->data[0] == ff->data[0] { ERROR("The buffer at %u is a duplicate when inserting %u %p", i, ringbuffer->next, ff->data[0]); } } /* End of debug */ av_packet_unref(&pkt); rb_push(ringbuffer, ff); // Push onto head of ringbuffer and free oldest frame } /* Exiting. Flush decoder ... */ } The buffer at 0 is a duplicate when inserting 1 0x7f1eb881e000 The buffer at 0 is a duplicate when inserting 2 0x7f1eb881e000 The buffer at 1 is a duplicate when inserting 2 0x7f1eb881e000 The buffer at 3 is a duplicate when inserting 301 0x7f1eb8a55000 The AVFrame's are not duplicated (new one alloc'ed each time), but they are being populated with the same "data[0]" as other AVFrames. Is there anything I am misunderstanding about these buffers? This usually happens 2-3 times as soon as the decoder starts, and then again when the buffer is full and these frames are being freed. It is not the same buffer pointers each time. This code is being run in a thread, but it is the only thread using this decoder. It happens even when not passing the frames to an encoder. Can anyone see something that would cause this? I am going to build my own copy of the libraries, but if this exhibits the same issue what can I check further to find the source of this problem (e.g. any data that would be useful in a bug report) Many thanks, Hugh
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