/**
 * Initialize the AVCodecContext to use the given AVCodec. Prior to using this
 * function the context has to be allocated with avcodec_alloc_context3().
 *
 * The functions avcodec_find_decoder_by_name(), avcodec_find_encoder_by_name(),
 * avcodec_find_decoder() and avcodec_find_encoder() provide an easy way for
 * retrieving a codec.
 *
 * @warning This function is not thread safe!
 *
 * @note Always call this function before using decoding routines (such as
 * @ref avcodec_receive_frame()).
 *
 * @code
 * av_dict_set(&opts, "b", "2.5M", 0);
 * codec = avcodec_find_decoder(AV_CODEC_ID_H264);
 * if (!codec)
 *     exit(1);
 *
 * context = avcodec_alloc_context3(codec);
 *
 * if (avcodec_open2(context, codec, opts) < 0)
 *     exit(1);
 * @endcode
 *
 * @param avctx The context to initialize.
 * @param codec The codec to open this context for. If a non-NULL codec has been
 *              previously passed to avcodec_alloc_context3() or
 *              for this context, then this parameter MUST be either NULL or
 *              equal to the previously passed codec.
 * @param options A dictionary filled with AVCodecContext and codec-private options.
 *                On return this object will be filled with options that were not found.
 *
 * @return zero on success, a negative value on error
 * @see avcodec_alloc_context3(), avcodec_find_decoder(), avcodec_find_encoder(),
 *      av_dict_set(), av_opt_find().
 */
int avcodec_open2(AVCodecContext *avctx, const AVCodec *codec, AVDictionary **options);

int attribute_align_arg avcodec_open2(AVCodecContext *avctx, const AVCodec *codec, AVDictionary **options)
{
    int ret = 0;
    AVCodecInternal *avci;

    // 编解码器是否打开，如果已经打开，直接返回
    if (avcodec_is_open(avctx))
        return 0;

    if (!codec && !avctx->codec) {
        av_log(avctx, AV_LOG_ERROR, "No codec provided to avcodec_open2()\n");
        return AVERROR(EINVAL);
    }
    if (codec && avctx->codec && codec != avctx->codec) {
        av_log(avctx, AV_LOG_ERROR, "This AVCodecContext was allocated for %s, "
                                    "but %s passed to avcodec_open2()\n", avctx->codec->name, codec->name);
        return AVERROR(EINVAL);
    }
    if (!codec)
        codec = avctx->codec;

    if ((avctx->codec_type == AVMEDIA_TYPE_UNKNOWN || avctx->codec_type == codec->type) &&
        avctx->codec_id == AV_CODEC_ID_NONE) {
        avctx->codec_type = codec->type;
        avctx->codec_id   = codec->id;
    }
    if (avctx->codec_id != codec->id || (avctx->codec_type != codec->type &&
                                         avctx->codec_type != AVMEDIA_TYPE_ATTACHMENT)) {
        av_log(avctx, AV_LOG_ERROR, "Codec type or id mismatches\n");
        return AVERROR(EINVAL);
    }
    avctx->codec = codec;

    if (avctx->extradata_size < 0 || avctx->extradata_size >= FF_MAX_EXTRADATA_SIZE)
        return AVERROR(EINVAL);

    lock_avcodec(codec);

    //各种alloc
    avci = av_mallocz(sizeof(*avci));
    if (!avci) {
        ret = AVERROR(ENOMEM);
        goto end;
    }
    avctx->internal = avci;

    avci->buffer_frame = av_frame_alloc();
    avci->buffer_pkt = av_packet_alloc();
    avci->es.in_frame = av_frame_alloc();
    avci->ds.in_pkt = av_packet_alloc();
    avci->last_pkt_props = av_packet_alloc();
    avci->pkt_props = av_fifo_alloc(sizeof(*avci->last_pkt_props));
    if (!avci->buffer_frame || !avci->buffer_pkt          ||
        !avci->es.in_frame  || !avci->ds.in_pkt           ||
        !avci->last_pkt_props || !avci->pkt_props) {
        ret = AVERROR(ENOMEM);
        goto free_and_end;
    }

    avci->skip_samples_multiplier = 1;

    if (codec->priv_data_size > 0) {
        if (!avctx->priv_data) {
            avctx->priv_data = av_mallocz(codec->priv_data_size);
            if (!avctx->priv_data) {
                ret = AVERROR(ENOMEM);
                goto free_and_end;
            }
            if (codec->priv_class) {
                *(const AVClass **)avctx->priv_data = codec->priv_class;
                av_opt_set_defaults(avctx->priv_data);
            }
        }
        if (codec->priv_class && (ret = av_opt_set_dict(avctx->priv_data, options)) < 0)
            goto free_and_end;
    } else {
        avctx->priv_data = NULL;
    }
    //将输入的AVDictionary形式的选项设置到AVCodecContext
    if ((ret = av_opt_set_dict(avctx, options)) < 0)
        goto free_and_end;

    if (avctx->codec_whitelist && av_match_list(codec->name, avctx->codec_whitelist, ',') <= 0) {
        av_log(avctx, AV_LOG_ERROR, "Codec (%s) not on whitelist \'%s\'\n", codec->name, avctx->codec_whitelist);
        ret = AVERROR(EINVAL);
        goto free_and_end;
    }

    // only call ff_set_dimensions() for non H.264/VP6F/DXV codecs so as not to overwrite previously setup dimensions
    if (!(avctx->coded_width && avctx->coded_height && avctx->width && avctx->height &&
          (avctx->codec_id == AV_CODEC_ID_H264 || avctx->codec_id == AV_CODEC_ID_VP6F || avctx->codec_id == AV_CODEC_ID_DXV))) {
        if (avctx->coded_width && avctx->coded_height)
            ret = ff_set_dimensions(avctx, avctx->coded_width, avctx->coded_height);
        else if (avctx->width && avctx->height)
            ret = ff_set_dimensions(avctx, avctx->width, avctx->height);
        if (ret < 0)
            goto free_and_end;
    }

    //检测宽和高
    if ((avctx->coded_width || avctx->coded_height || avctx->width || avctx->height)
        && (  av_image_check_size2(avctx->coded_width, avctx->coded_height, avctx->max_pixels, AV_PIX_FMT_NONE, 0, avctx) < 0
           || av_image_check_size2(avctx->width,       avctx->height,       avctx->max_pixels, AV_PIX_FMT_NONE, 0, avctx) < 0)) {
        av_log(avctx, AV_LOG_WARNING, "Ignoring invalid width/height values\n");
        ff_set_dimensions(avctx, 0, 0);
    }
    //检测宽高比
    if (avctx->width > 0 && avctx->height > 0) {
        if (av_image_check_sar(avctx->width, avctx->height,
                               avctx->sample_aspect_ratio) < 0) {
            av_log(avctx, AV_LOG_WARNING, "ignoring invalid SAR: %u/%u\n",
                   avctx->sample_aspect_ratio.num,
                   avctx->sample_aspect_ratio.den);
            avctx->sample_aspect_ratio = (AVRational){ 0, 1 };
        }
    }

    if (avctx->channels > FF_SANE_NB_CHANNELS || avctx->channels < 0) {
        av_log(avctx, AV_LOG_ERROR, "Too many or invalid channels: %d\n", avctx->channels);
        ret = AVERROR(EINVAL);
        goto free_and_end;
    }

    if (avctx->sample_rate < 0) {
        av_log(avctx, AV_LOG_ERROR, "Invalid sample rate: %d\n", avctx->sample_rate);
        ret = AVERROR(EINVAL);
        goto free_and_end;
    }
    if (avctx->block_align < 0) {
        av_log(avctx, AV_LOG_ERROR, "Invalid block align: %d\n", avctx->block_align);
        ret = AVERROR(EINVAL);
        goto free_and_end;
    }

    avctx->frame_number = 0;
    avctx->codec_descriptor = avcodec_descriptor_get(avctx->codec_id);

    //检查编码器是否出于“实验”阶段
    if ((avctx->codec->capabilities & AV_CODEC_CAP_EXPERIMENTAL) &&
        avctx->strict_std_compliance > FF_COMPLIANCE_EXPERIMENTAL) {
        const char *codec_string = av_codec_is_encoder(codec) ? "encoder" : "decoder";
        const AVCodec *codec2;
        av_log(avctx, AV_LOG_ERROR,
               "The %s '%s' is experimental but experimental codecs are not enabled, "
               "add '-strict %d' if you want to use it.\n",
               codec_string, codec->name, FF_COMPLIANCE_EXPERIMENTAL);
        codec2 = av_codec_is_encoder(codec) ? avcodec_find_encoder(codec->id) : avcodec_find_decoder(codec->id);
        if (!(codec2->capabilities & AV_CODEC_CAP_EXPERIMENTAL))
            av_log(avctx, AV_LOG_ERROR, "Alternatively use the non experimental %s '%s'.\n",
                codec_string, codec2->name);
        ret = AVERROR_EXPERIMENTAL;
        goto free_and_end;
    }

    if (avctx->codec_type == AVMEDIA_TYPE_AUDIO &&
        (!avctx->time_base.num || !avctx->time_base.den)) {
        avctx->time_base.num = 1;
        avctx->time_base.den = avctx->sample_rate;
    }

    if (av_codec_is_encoder(avctx->codec))
        //执行编码器初始化和验证。
        ret = ff_encode_preinit(avctx);
    else
        //执行解码器初始化和验证
        ret = ff_decode_preinit(avctx);
    if (ret < 0)
        goto free_and_end;

    if (!HAVE_THREADS)
        av_log(avctx, AV_LOG_WARNING, "Warning: not compiled with thread support, using thread emulation\n");

    if (CONFIG_FRAME_THREAD_ENCODER && av_codec_is_encoder(avctx->codec)) {
        unlock_avcodec(codec); //we will instantiate a few encoders thus kick the counter to prevent false detection of a problem
        ret = ff_frame_thread_encoder_init(avctx);
        lock_avcodec(codec);
        if (ret < 0)
            goto free_and_end;
    }

    if (HAVE_THREADS
        && !(avci->frame_thread_encoder && (avctx->active_thread_type&FF_THREAD_FRAME))) {
        ret = ff_thread_init(avctx);
        if (ret < 0) {
            goto free_and_end;
        }
    }
    if (!HAVE_THREADS && !(codec->caps_internal & FF_CODEC_CAP_AUTO_THREADS))
        avctx->thread_count = 1;

    if (!(avctx->active_thread_type & FF_THREAD_FRAME) ||
        avci->frame_thread_encoder) {
        if (avctx->codec->init) {
            ret = avctx->codec->init(avctx);
            if (ret < 0) {
                avci->needs_close = avctx->codec->caps_internal & FF_CODEC_CAP_INIT_CLEANUP;
                goto free_and_end;
            }
        }
        avci->needs_close = 1;
    }

    ret=0;

    if (av_codec_is_decoder(avctx->codec)) {
        if (!avctx->bit_rate)
            avctx->bit_rate = get_bit_rate(avctx);
        /* validate channel layout from the decoder */
        if (avctx->channel_layout) {
            int channels = av_get_channel_layout_nb_channels(avctx->channel_layout);
            if (!avctx->channels)
                avctx->channels = channels;
            else if (channels != avctx->channels) {
                char buf[512];
                av_get_channel_layout_string(buf, sizeof(buf), -1, avctx->channel_layout);
                av_log(avctx, AV_LOG_WARNING,
                       "Channel layout '%s' with %d channels does not match specified number of channels %d: "
                       "ignoring specified channel layout\n",
                       buf, channels, avctx->channels);
                avctx->channel_layout = 0;
            }
        }
        if (avctx->channels && avctx->channels < 0 ||
            avctx->channels > FF_SANE_NB_CHANNELS) {
            ret = AVERROR(EINVAL);
            goto free_and_end;
        }
        if (avctx->bits_per_coded_sample < 0) {
            ret = AVERROR(EINVAL);
            goto free_and_end;
        }

#if FF_API_AVCTX_TIMEBASE
        if (avctx->framerate.num > 0 && avctx->framerate.den > 0)
            avctx->time_base = av_inv_q(av_mul_q(avctx->framerate, (AVRational){avctx->ticks_per_frame, 1}));
#endif
    }
    if (codec->priv_data_size > 0 && avctx->priv_data && codec->priv_class) {
        av_assert0(*(const AVClass **)avctx->priv_data == codec->priv_class);
    }

end:
    unlock_avcodec(codec);

    return ret;
free_and_end:
    avcodec_close(avctx);
    goto end;
}