Selecting a correct order of API call sequences is important

In short, ffmpeg changes its API in recent versions. One of them is marking AVStream.codec as deprecated. On other side, a AVCodecParameters * AVStream.codecpar was added for codec information described in the stream. To decode data read from stream correctly, we must create a AVCodecContext through avcodec_alloc_context3(), copy codec information from AVStream.codecpar to this allocated AVCodecContext, and call avcodec_open2() to assign this AVCodecContext to AVCodec found by avcodec_finder_decoder() call.

Here is the point. We must call avcodec_parameters_to_context() to copy codec information to AVCodecContext before we call avcodec_open2() to assign AVCodecContext to AVCodec. If we incorrectly reversed this order, say, called avcodec_open2() before avcodec_parameters_to_context(), we can still decode many videos correctly, but we will encounter strange behavior when we try to play back .mkv or .mp4 files – Each call to decoding function will fail.