FFmepg 多线程解码历程 - 4:avcodec_open2

//在初始化codec后，接下来就是打开解码器
int attribute_align_arg avcodec_open2(AVCodecContext *avctx, const AVCodec *codec, AVDictionary **options)
{
    int ret = 0;
    AVDictionary *tmp = NULL;

    if (avcodec_is_open(avctx))  //返回avctx-》internal，见注释1
        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;
//判断额外数据，在硬解吗h264的时候必须人为设置sps pps给extradata
    if (avctx->extradata_size < 0 || avctx->extradata_size >= FF_MAX_EXTRADATA_SIZE)
        return AVERROR(EINVAL);

    if (options)   //一般options设置NULL
        av_dict_copy(&tmp, *options, 0);

    ret = ff_lock_avcodec(avctx);
    if (ret < 0)
        return ret;

    avctx->internal = av_mallocz(sizeof(AVCodecInternal));
    if (!avctx->internal) {
        ret = AVERROR(ENOMEM);
        goto end;
    }

    avctx->internal->pool = av_mallocz(sizeof(*avctx->internal->pool));
    if (!avctx->internal->pool) {
        ret = AVERROR(ENOMEM);
        goto free_and_end;
    }

    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 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, &tmp)) < 0)
            goto free_and_end;
    } else {
        avctx->priv_data = NULL;
    } //上面主要是分配内存
    if ((ret = av_opt_set_dict(avctx, &tmp)) < 0)
        goto free_and_end;

    // only call avcodec_set_dimensions() for non H.264/VP6F 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))) {
    if (avctx->coded_width && avctx->coded_height)
        avcodec_set_dimensions(avctx, avctx->coded_width, avctx->coded_height);
    else if (avctx->width && avctx->height)
        avcodec_set_dimensions(avctx, avctx->width, avctx->height);
    }

    if ((avctx->coded_width || avctx->coded_height || avctx->width || avctx->height)
        && (  av_image_check_size(avctx->coded_width, avctx->coded_height, 0, avctx) < 0
           || av_image_check_size(avctx->width,       avctx->height,       0, avctx) < 0)) {
        av_log(avctx, AV_LOG_WARNING, "Ignoring invalid width/height values\n");
        avcodec_set_dimensions(avctx, 0, 0);
    }

     /* if the decoder init function was already called previously,如果解码器的init函数已经调用过
     * free the already allocated subtitle_header before overwriting it 释放已经分配subtitle_header ，在它覆盖前
*/
    if (av_codec_is_decoder(codec))
        av_freep(&avctx->subtitle_header);

    if (avctx->channels > FF_SANE_NB_CHANNELS) {
        ret = AVERROR(EINVAL);
        goto free_and_end;
    }

    avctx->codec = 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");
        ret = AVERROR(EINVAL);
        goto free_and_end;
    }
    avctx->frame_number = 0;
    avctx->codec_descriptor = avcodec_descriptor_get(avctx->codec_id);

    if (avctx->codec->capabilities & CODEC_CAP_EXPERIMENTAL &&
        avctx->strict_std_compliance > FF_COMPLIANCE_EXPERIMENTAL) {
        const char *codec_string = av_codec_is_encoder(codec) ? "encoder" : "decoder";
        AVCodec *codec2;
        av_log(NULL, 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 & CODEC_CAP_EXPERIMENTAL))
            av_log(NULL, 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 (!HAVE_THREADS)
        av_log(avctx, AV_LOG_WARNING, "Warning: not compiled with thread support, using thread emulation\n");

    if (CONFIG_FRAME_THREAD_ENCODER) {
        ff_unlock_avcodec(); //we will instanciate a few encoders thus kick the counter to prevent false detection of a problem
        ret = ff_frame_thread_encoder_init(avctx, options ? *options : NULL);
        ff_lock_avcodec(avctx);
        if (ret < 0)
            goto free_and_end;
    }
//帧线程编码
    if (HAVE_THREADS && !avctx->thread_opaque
        && !(avctx->internal->frame_thread_encoder && (avctx->active_thread_type&FF_THREAD_FRAME))) {
        ret = ff_thread_init(avctx);//会根据用户设置不同的变量来判断是片，帧，来初始化线程
        if (ret < 0) {
            goto free_and_end;
        }
    }
    if (!HAVE_THREADS && !(codec->capabilities & CODEC_CAP_AUTO_THREADS))
        avctx->thread_count = 1;

    if (avctx->codec->max_lowres < avctx->lowres || avctx->lowres < 0) {
        av_log(avctx, AV_LOG_ERROR, "The maximum value for lowres supported by the decoder is %d\n",
               avctx->codec->max_lowres);
        ret = AVERROR(EINVAL);
        goto free_and_end;

    }

//下面就会根据是编码器还是解码器来初始化

    if (av_codec_is_encoder(avctx->codec)) {
        int i;
        if (avctx->codec->sample_fmts) {
            for (i = 0; avctx->codec->sample_fmts[i] != AV_SAMPLE_FMT_NONE; i++) {
                if (avctx->sample_fmt == avctx->codec->sample_fmts[i])
                    break;
                if (avctx->channels == 1 &&
                    av_get_planar_sample_fmt(avctx->sample_fmt) ==
                    av_get_planar_sample_fmt(avctx->codec->sample_fmts[i])) {
                    avctx->sample_fmt = avctx->codec->sample_fmts[i];
                    break;
                }
            }
            if (avctx->codec->sample_fmts[i] == AV_SAMPLE_FMT_NONE) {
                char buf[128];
                snprintf(buf, sizeof(buf), "%d", avctx->sample_fmt);
                av_log(avctx, AV_LOG_ERROR, "Specified sample format %s is invalid or not supported\n",
                       (char *)av_x_if_null(av_get_sample_fmt_name(avctx->sample_fmt), buf));
                ret = AVERROR(EINVAL);
                goto free_and_end;
            }
        }
        if (avctx->codec->pix_fmts) {
            for (i = 0; avctx->codec->pix_fmts[i] != AV_PIX_FMT_NONE; i++)
                if (avctx->pix_fmt == avctx->codec->pix_fmts[i])
                    break;
            if (avctx->codec->pix_fmts[i] == AV_PIX_FMT_NONE
                && !((avctx->codec_id == AV_CODEC_ID_MJPEG || avctx->codec_id == AV_CODEC_ID_LJPEG)
                     && avctx->strict_std_compliance <= FF_COMPLIANCE_UNOFFICIAL)) {
                char buf[128];
                snprintf(buf, sizeof(buf), "%d", avctx->pix_fmt);
                av_log(avctx, AV_LOG_ERROR, "Specified pixel format %s is invalid or not supported\n",
                       (char *)av_x_if_null(av_get_pix_fmt_name(avctx->pix_fmt), buf));
                ret = AVERROR(EINVAL);
                goto free_and_end;
            }
        }
        if (avctx->codec->supported_samplerates) {
            for (i = 0; avctx->codec->supported_samplerates[i] != 0; i++)
                if (avctx->sample_rate == avctx->codec->supported_samplerates[i])
                    break;
            if (avctx->codec->supported_samplerates[i] == 0) {
                av_log(avctx, AV_LOG_ERROR, "Specified sample rate %d is not supported\n",
                       avctx->sample_rate);
                ret = AVERROR(EINVAL);
                goto free_and_end;
            }
        }
        if (avctx->codec->channel_layouts) {
            if (!avctx->channel_layout) {
                av_log(avctx, AV_LOG_WARNING, "Channel layout not specified\n");
            } else {
                for (i = 0; avctx->codec->channel_layouts[i] != 0; i++)
                    if (avctx->channel_layout == avctx->codec->channel_layouts[i])
                        break;
                if (avctx->codec->channel_layouts[i] == 0) {
                    char buf[512];
                    av_get_channel_layout_string(buf, sizeof(buf), -1, avctx->channel_layout);
                    av_log(avctx, AV_LOG_ERROR, "Specified channel layout '%s' is not supported\n", buf);
                    ret = AVERROR(EINVAL);
                    goto free_and_end;
                }
            }
        }
        if (avctx->channel_layout && avctx->channels) {
            int channels = av_get_channel_layout_nb_channels(avctx->channel_layout);
            if (channels != avctx->channels) {
                char buf[512];
                av_get_channel_layout_string(buf, sizeof(buf), -1, avctx->channel_layout);
                av_log(avctx, AV_LOG_ERROR,
                       "Channel layout '%s' with %d channels does not match number of specified channels %d\n",
                       buf, channels, avctx->channels);
                ret = AVERROR(EINVAL);
                goto free_and_end;
            }
        } else if (avctx->channel_layout) {
            avctx->channels = av_get_channel_layout_nb_channels(avctx->channel_layout);
        }
        if(avctx->codec_type == AVMEDIA_TYPE_VIDEO &&
           avctx->codec_id != AV_CODEC_ID_PNG // For mplayer
        ) {
            if (avctx->width <= 0 || avctx->height <= 0) {
                av_log(avctx, AV_LOG_ERROR, "dimensions not set\n");
                ret = AVERROR(EINVAL);
                goto free_and_end;
            }
        }
        if (   (avctx->codec_type == AVMEDIA_TYPE_VIDEO || avctx->codec_type == AVMEDIA_TYPE_AUDIO)
            && avctx->bit_rate>0 && avctx->bit_rate<1000) {
            av_log(avctx, AV_LOG_WARNING, "Bitrate %d is extremely low, maybe you mean %dk\n", avctx->bit_rate, avctx->bit_rate);
        }

        if (!avctx->rc_initial_buffer_occupancy)
            avctx->rc_initial_buffer_occupancy = avctx->rc_buffer_size * 3 / 4;
    }

    avctx->pts_correction_num_faulty_pts =
    avctx->pts_correction_num_faulty_dts = 0;
    avctx->pts_correction_last_pts =
    avctx->pts_correction_last_dts = INT64_MIN;

    if (   avctx->codec->init && (!(avctx->active_thread_type&FF_THREAD_FRAME)
        || avctx->internal->frame_thread_encoder)) {
        ret = avctx->codec->init(avctx);
        if (ret < 0) {
            goto free_and_end;
        }
    }

    ret=0;

//上面部分是判断codec是否是编码器，然后如果是编码器则进行选择编码的方式，这里我们只讨论解码，下面的解码器也是一样，根据不同的条件，进行编码初始化

    if (av_codec_is_decoder(avctx->codec)) {
        if (!avctx->bit_rate)
            avctx->bit_rate = get_bit_rate(avctx);//如果没有设置bit_rate则会更加avctx类型将avctx->bit_rate复制给
        /* 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->sub_charenc) {
            if (avctx->codec_type != AVMEDIA_TYPE_SUBTITLE) {
                av_log(avctx, AV_LOG_ERROR, "Character encoding is only "
                       "supported with subtitles codecs\n");
                ret = AVERROR(EINVAL);
                goto free_and_end;
            } else if (avctx->codec_descriptor->props & AV_CODEC_PROP_BITMAP_SUB) {
                av_log(avctx, AV_LOG_WARNING, "Codec '%s' is bitmap-based, "
                       "subtitles character encoding will be ignored\n",
                       avctx->codec_descriptor->name);
                avctx->sub_charenc_mode = FF_SUB_CHARENC_MODE_DO_NOTHING;
            } else {
                /* input character encoding is set for a text based subtitle
                 * codec at this point */
                if (avctx->sub_charenc_mode == FF_SUB_CHARENC_MODE_AUTOMATIC)
                    avctx->sub_charenc_mode = FF_SUB_CHARENC_MODE_PRE_DECODER;

                if (avctx->sub_charenc_mode == FF_SUB_CHARENC_MODE_PRE_DECODER) {
#if CONFIG_ICONV
                    iconv_t cd = iconv_open("UTF-8", avctx->sub_charenc);
                    if (cd == (iconv_t)-1) {
                        av_log(avctx, AV_LOG_ERROR, "Unable to open iconv context "
                               "with input character encoding \"%s\"\n", avctx->sub_charenc);
                        ret = AVERROR(errno);
                        goto free_and_end;
                    }
                    iconv_close(cd);
#else
                    av_log(avctx, AV_LOG_ERROR, "Character encoding subtitles "
                           "conversion needs a libavcodec built with iconv support "
                           "for this codec\n");
                    ret = AVERROR(ENOSYS);
                    goto free_and_end;
#endif
                }
            }
        }
    }
end:
    ff_unlock_avcodec();
    if (options) {
        av_dict_free(options);
        *options = tmp;
    }

    return ret;
free_and_end:
    av_dict_free(&tmp);
    av_freep(&avctx->priv_data);
    if (avctx->internal)
        av_freep(&avctx->internal->pool);
    av_freep(&avctx->internal);
    avctx->codec = NULL;
    goto end;
}

注释：

1. /**
     * Private context used for internal data.
     *私有的上下文内容用于内部数据
     * Unlike priv_data, this is not codec-specific. It is used in general
     * libavcodec functions.

*不像priv_data ，这是不特定的编解码器。这是用在一般libavcodec的功能
     */
    struct AVCodecInternal *internal;