1. 程式人生 > >avformat_find_stream_info讀資料時的幾個快取資料的流向

avformat_find_stream_info讀資料時的幾個快取資料的流向

簡介

瞭解ffmpeg在使用幾個讀資料的函式時,原始資料是怎樣讀到記憶體中,在記憶體中又是從哪個結構體轉到哪個結構體,
我覺得這是很關鍵的。因為ffmpeg就是處理資料的。只知道幾個API的呼叫,不知道資料的流轉方向,對於細緻的
處理來說很難。

分析流資訊時的資料流轉

資料存放基本介紹

此圖中涉及到的快取區位置包括如下:
AVIOContext:AVFormatContext->pb,原始資料都是先進到這裡。
AVFormatInternal :AVFormatContext->internal

struct AVFormatInternal {
    /**
     * Number of streams relevant for interleaving.
     * Muxing only.
     */
int nb_interleaved_streams; /** * This buffer is only needed when packets were already buffered but * not decoded, for example to get the codec parameters in MPEG * streams. */ struct AVPacketList *packet_buffer; struct AVPacketList *packet_buffer_end; /* av_seek_frame() support */
int64_t data_offset; /**< offset of the first packet */ /** * Raw packets from the demuxer, prior to parsing and decoding. * This buffer is used for buffering packets until the codec can * be identified, as parsing cannot be done without knowing the * codec. */ struct
AVPacketList *raw_packet_buffer; struct AVPacketList *raw_packet_buffer_end; /** * Packets split by the parser get queued here. */ struct AVPacketList *parse_queue; struct AVPacketList *parse_queue_end; /** * Remaining size available for raw_packet_buffer, in bytes. */ #define RAW_PACKET_BUFFER_SIZE 2500000 int raw_packet_buffer_remaining_size; /** * Offset to remap timestamps to be non-negative. * Expressed in timebase units. * @see AVStream.mux_ts_offset */ int64_t offset; /** * Timebase for the timestamp offset. */ AVRational offset_timebase; #if FF_API_COMPUTE_PKT_FIELDS2 int missing_ts_warning; #endif int inject_global_side_data; int avoid_negative_ts_use_pts; /** * Whether or not a header has already been written */ int header_written; int write_header_ret; };

raw_packet_buffer和raw_packet_buffer_end:在ff_read_packet()中會用到。這個還沒分析清楚
parse_queue和parse_queue_end:在read_frame_internal()中由於讀到的資料可能是多包,在此過程中就不能每分析到一包就返回,所以用這個快取去來存放分析到為一包的資料。
packet_buffer和packet_buffer_end:在read_frame_internal()中會從parse_queue中讀資料,將分析到的每包都放到這個連結串列中。是為了防止資料丟失,給之後的轉碼過程提供資料。在轉碼讀資料的時候就是先檢視這個連結串列裡面有沒有資料。

這些連結串列中存放的都是沒有解碼的資料。在分析碼流的最後會嘗試解碼,並釋放解碼後的資料。

資料流轉圖

這裡寫圖片描述

幾個涉及到的函式

以下將對幾個函式進行分析。從前到後對資料的開始讀入到解碼成功做一個梳理。

av_probe_input_buffer函式中的資料流向

av_probe_input_buffer函式中的資料流向
這篇文章對在探查幀格式的過程中,資料的快取做了分析。這是對輸入流的第一步分析。
最後將探查過的資料存放到了AVIOContext *pb的buf_ptr到buf_end中。

avformat_find_stream_info

最後分析完的資料都儲存在了packet_buffer和packet_buffer_end中。提供給之後的轉碼流程。

int avformat_find_stream_info(AVFormatContext *ic, AVDictionary **options)
{
    int i, count = 0, ret = 0, j;
    int64_t read_size;
    AVStream *st;
    AVCodecContext *avctx;
    AVPacket pkt1, *pkt;
    int64_t old_offset  = avio_tell(ic->pb);
    // new streams might appear, no options for those
    int orig_nb_streams = ic->nb_streams;
    int flush_codecs;
    int64_t max_analyze_duration = ic->max_analyze_duration;
    int64_t max_stream_analyze_duration;
    int64_t max_subtitle_analyze_duration;
    int64_t probesize = ic->probesize;
    int eof_reached = 0;

    flush_codecs = probesize > 0;

    av_opt_set(ic, "skip_clear", "1", AV_OPT_SEARCH_CHILDREN);

    max_stream_analyze_duration = max_analyze_duration;
    max_subtitle_analyze_duration = max_analyze_duration;
    if (!max_analyze_duration) {
        max_stream_analyze_duration =
        max_analyze_duration        = 5*AV_TIME_BASE;
        max_subtitle_analyze_duration = 30*AV_TIME_BASE;
        if (!strcmp(ic->iformat->name, "flv"))
            max_stream_analyze_duration = 90*AV_TIME_BASE;
        if (!strcmp(ic->iformat->name, "mpeg") || !strcmp(ic->iformat->name, "mpegts"))
            max_stream_analyze_duration = 7*AV_TIME_BASE;
    }

    if (ic->pb)
        av_log(ic, AV_LOG_DEBUG, "Before avformat_find_stream_info() pos: %"PRId64" bytes read:%"PRId64" seeks:%d nb_streams:%d\n",
               avio_tell(ic->pb), ic->pb->bytes_read, ic->pb->seek_count, ic->nb_streams);

    for (i = 0; i < ic->nb_streams; i++) {
        const AVCodec *codec;
        AVDictionary *thread_opt = NULL;
        st = ic->streams[i];
        avctx = st->internal->avctx;

        if (st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO ||
            st->codecpar->codec_type == AVMEDIA_TYPE_SUBTITLE) {
/*            if (!st->time_base.num)
                st->time_base = */
            if (!avctx->time_base.num)
                avctx->time_base = st->time_base;
        }
        // only for the split stuff
        if (!st->parser && !(ic->flags & AVFMT_FLAG_NOPARSE) && st->request_probe <= 0) {
            st->parser = av_parser_init(st->codecpar->codec_id);
            if (st->parser) {
                if (st->need_parsing == AVSTREAM_PARSE_HEADERS) {
                    st->parser->flags |= PARSER_FLAG_COMPLETE_FRAMES;
                } else if (st->need_parsing == AVSTREAM_PARSE_FULL_RAW) {
                    st->parser->flags |= PARSER_FLAG_USE_CODEC_TS;
                }
            } else if (st->need_parsing) {
                av_log(ic, AV_LOG_VERBOSE, "parser not found for codec "
                       "%s, packets or times may be invalid.\n",
                       avcodec_get_name(st->codecpar->codec_id));
            }
        }

        /* check if the caller has overridden the codec id */
#if FF_API_LAVF_AVCTX
FF_DISABLE_DEPRECATION_WARNINGS
        if (st->codec->codec_id != st->internal->orig_codec_id) {
            st->codecpar->codec_id   = st->codec->codec_id;
            st->codecpar->codec_type = st->codec->codec_type;
            st->internal->orig_codec_id = st->codec->codec_id;
        }
FF_ENABLE_DEPRECATION_WARNINGS
#endif
        if (st->codecpar->codec_id != st->internal->orig_codec_id)
            st->internal->orig_codec_id = st->codecpar->codec_id;

        ret = avcodec_parameters_to_context(avctx, st->codecpar);
        if (ret < 0)
            goto find_stream_info_err;
        if (st->request_probe <= 0)
            st->internal->avctx_inited = 1;

        codec = find_decoder(ic, st, st->codecpar->codec_id);

        /* Force thread count to 1 since the H.264 decoder will not extract
         * SPS and PPS to extradata during multi-threaded decoding. */
        av_dict_set(options ? &options[i] : &thread_opt, "threads", "1", 0);

        if (ic->codec_whitelist)
            av_dict_set(options ? &options[i] : &thread_opt, "codec_whitelist", ic->codec_whitelist, 0);

        /* Ensure that subtitle_header is properly set. */
        if (st->codecpar->codec_type == AVMEDIA_TYPE_SUBTITLE
            && codec && !avctx->codec) {
            if (avcodec_open2(avctx, codec, options ? &options[i] : &thread_opt) < 0)
                av_log(ic, AV_LOG_WARNING,
                       "Failed to open codec in av_find_stream_info\n");
        }

        // Try to just open decoders, in case this is enough to get parameters.
        if (!has_codec_parameters(st, NULL) && st->request_probe <= 0) {
            if (codec && !avctx->codec)
                if (avcodec_open2(avctx, codec, options ? &options[i] : &thread_opt) < 0)
                    av_log(ic, AV_LOG_WARNING,
                           "Failed to open codec in av_find_stream_info\n");
        }
        if (!options)
            av_dict_free(&thread_opt);
    }

    for (i = 0; i < ic->nb_streams; i++) {
#if FF_API_R_FRAME_RATE
        ic->streams[i]->info->last_dts = AV_NOPTS_VALUE;
#endif
        ic->streams[i]->info->fps_first_dts = AV_NOPTS_VALUE;
        ic->streams[i]->info->fps_last_dts  = AV_NOPTS_VALUE;
    }

    read_size = 0;
    for (;;) {
        int analyzed_all_streams;
        if (ff_check_interrupt(&ic->interrupt_callback)) {
            ret = AVERROR_EXIT;
            av_log(ic, AV_LOG_DEBUG, "interrupted\n");
            break;
        }

        /* check if one codec still needs to be handled */
        for (i = 0; i < ic->nb_streams; i++) {
            int fps_analyze_framecount = 20;

            st = ic->streams[i];
            if (!has_codec_parameters(st, NULL))
                break;
            /* If the timebase is coarse (like the usual millisecond precision
             * of mkv), we need to analyze more frames to reliably arrive at
             * the correct fps. */
            if (av_q2d(st->time_base) > 0.0005)
                fps_analyze_framecount *= 2;
            if (!tb_unreliable(st->internal->avctx))
                fps_analyze_framecount = 0;
            if (ic->fps_probe_size >= 0)
                fps_analyze_framecount = ic->fps_probe_size;
            if (st->disposition & AV_DISPOSITION_ATTACHED_PIC)
                fps_analyze_framecount = 0;
            /* variable fps and no guess at the real fps */
            if (!(st->r_frame_rate.num && st->avg_frame_rate.num) &&
                st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO) {
                int count = (ic->iformat->flags & AVFMT_NOTIMESTAMPS) ?
                    st->info->codec_info_duration_fields/2 :
                    st->info->duration_count;
                if (count < fps_analyze_framecount)
                    break;
            }
            if (st->parser && st->parser->parser->split &&
                !st->codecpar->extradata)
                break;
            if (st->first_dts == AV_NOPTS_VALUE &&
                !(ic->iformat->flags & AVFMT_NOTIMESTAMPS) &&
                st->codec_info_nb_frames < ((st->disposition & AV_DISPOSITION_ATTACHED_PIC) ? 1 : ic->max_ts_probe) &&
                (st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO ||
                 st->codecpar->codec_type == AVMEDIA_TYPE_AUDIO))
                break;
        }
        analyzed_all_streams = 0;
        if (i == ic->nb_streams) {
            analyzed_all_streams = 1;
            /* NOTE: If the format has no header, then we need to read some
             * packets to get most of the streams, so we cannot stop here. */
            if (!(ic->ctx_flags & AVFMTCTX_NOHEADER)) {
                /* If we found the info for all the codecs, we can stop. */
                ret = count;
                av_log(ic, AV_LOG_DEBUG, "All info found\n");
                flush_codecs = 0;
                break;
            }
        }
        /* We did not get all the codec info, but we read too much data. */
        if (read_size >= probesize) {
            ret = count;
            av_log(ic, AV_LOG_DEBUG,
                   "Probe buffer size limit of %"PRId64" bytes reached\n", probesize);
            for (i = 0; i < ic->nb_streams; i++)
                if (!ic->streams[i]->r_frame_rate.num &&
                    ic->streams[i]->info->duration_count <= 1 &&
                    ic->streams[i]->codecpar->codec_type == AVMEDIA_TYPE_VIDEO &&
                    strcmp(ic->iformat->name, "image2"))
                    av_log(ic, AV_LOG_WARNING,
                           "Stream #%d: not enough frames to estimate rate; "
                           "consider increasing probesize\n", i);
            break;
        }

        /* NOTE: A new stream can be added there if no header in file
         * (AVFMTCTX_NOHEADER). */
        ret = read_frame_internal(ic, &pkt1);
        if (ret == AVERROR(EAGAIN))
            continue;

        if (ret < 0) {
            /* EOF or error*/
            eof_reached = 1;
            break;
        }

        pkt = &pkt1;

        if (!(ic->flags & AVFMT_FLAG_NOBUFFER)) {
            ret = add_to_pktbuf(&ic->internal->packet_buffer, pkt,
                                &ic->internal->packet_buffer_end, 0);
            if (ret < 0)
                goto find_stream_info_err;
        }

        st = ic->streams[pkt->stream_index];
        if (!(st->disposition & AV_DISPOSITION_ATTACHED_PIC))
            read_size += pkt->size;

        avctx = st->internal->avctx;
        if (!st->internal->avctx_inited) {
            ret = avcodec_parameters_to_context(avctx, st->codecpar);
            if (ret < 0)
                goto find_stream_info_err;
            st->internal->avctx_inited = 1;
        }

        if (pkt->dts != AV_NOPTS_VALUE && st->codec_info_nb_frames > 1) {
            /* check for non-increasing dts */
            if (st->info->fps_last_dts != AV_NOPTS_VALUE &&
                st->info->fps_last_dts >= pkt->dts) {
                av_log(ic, AV_LOG_DEBUG,
                       "Non-increasing DTS in stream %d: packet %d with DTS "
                       "%"PRId64", packet %d with DTS %"PRId64"\n",
                       st->index, st->info->fps_last_dts_idx,
                       st->info->fps_last_dts, st->codec_info_nb_frames,
                       pkt->dts);
                st->info->fps_first_dts =
                st->info->fps_last_dts  = AV_NOPTS_VALUE;
            }
            /* Check for a discontinuity in dts. If the difference in dts
             * is more than 1000 times the average packet duration in the
             * sequence, we treat it as a discontinuity. */
            if (st->info->fps_last_dts != AV_NOPTS_VALUE &&
                st->info->fps_last_dts_idx > st->info->fps_first_dts_idx &&
                (pkt->dts - st->info->fps_last_dts) / 1000 >
                (st->info->fps_last_dts     - st->info->fps_first_dts) /
                (st->info->fps_last_dts_idx - st->info->fps_first_dts_idx)) {
                av_log(ic, AV_LOG_WARNING,
                       "DTS discontinuity in stream %d: packet %d with DTS "
                       "%"PRId64", packet %d with DTS %"PRId64"\n",
                       st->index, st->info->fps_last_dts_idx,
                       st->info->fps_last_dts, st->codec_info_nb_frames,
                       pkt->dts);
                st->info->fps_first_dts =
                st->info->fps_last_dts  = AV_NOPTS_VALUE;
            }

            /* update stored dts values */
            if (st->info->fps_first_dts == AV_NOPTS_VALUE) {
                st->info->fps_first_dts     = pkt->dts;
                st->info->fps_first_dts_idx = st->codec_info_nb_frames;
            }
            st->info->fps_last_dts     = pkt->dts;
            st->info->fps_last_dts_idx = st->codec_info_nb_frames;
        }
        if (st->codec_info_nb_frames>1) {
            int64_t t = 0;
            int64_t limit;

            if (st->time_base.den > 0)
                t = av_rescale_q(st->info->codec_info_duration, st->time_base, AV_TIME_BASE_Q);
            if (st->avg_frame_rate.num > 0)
                t = FFMAX(t, av_rescale_q(st->codec_info_nb_frames, av_inv_q(st->avg_frame_rate), AV_TIME_BASE_Q));

            if (   t == 0
                && st->codec_info_nb_frames>30
                && st->info->fps_first_dts != AV_NOPTS_VALUE
                && st->info->fps_last_dts  != AV_NOPTS_VALUE)
                t = FFMAX(t, av_rescale_q(st->info->fps_last_dts - st->info->fps_first_dts, st->time_base, AV_TIME_BASE_Q));

            if (analyzed_all_streams)                                limit = max_analyze_duration;
            else if (avctx->codec_type == AVMEDIA_TYPE_SUBTITLE) limit = max_subtitle_analyze_duration;
            else                                                     limit = max_stream_analyze_duration;

            if (t >= limit) {
                av_log(ic, AV_LOG_VERBOSE, "max_analyze_duration %"PRId64" reached at %"PRId64" microseconds st:%d\n",
                       limit,
                       t, pkt->stream_index);
                if (ic->flags & AVFMT_FLAG_NOBUFFER)
                    av_packet_unref(pkt);
                break;
            }
            if (pkt->duration) {
                if (avctx->codec_type == AVMEDIA_TYPE_SUBTITLE && pkt->pts != AV_NOPTS_VALUE && pkt->pts >= st->start_time) {
                    st->info->codec_info_duration = FFMIN(pkt->pts - st->start_time, st->info->codec_info_duration + pkt->duration);
                } else
                    st->info->codec_info_duration += pkt->duration;
                st->info->codec_info_duration_fields += st->parser && st->need_parsing && avctx->ticks_per_frame ==2 ? st->parser->repeat_pict + 1 : 2;
            }
        }
#if FF_API_R_FRAME_RATE
        if (st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO)
            ff_rfps_add_frame(ic, st, pkt->dts);
#endif
        if (st->parser && st->parser->parser->split && !avctx->extradata) {
            int i = st->parser->parser->split(avctx, pkt->data, pkt->size);
            if (i > 0 && i < FF_MAX_EXTRADATA_SIZE) {
                avctx->extradata_size = i;
                avctx->extradata      = av_mallocz(avctx->extradata_size +
                                                   AV_INPUT_BUFFER_PADDING_SIZE);
                if (!avctx->extradata)
                    return AVERROR(ENOMEM);
                memcpy(avctx->extradata, pkt->data,
                       avctx->extradata_size);
            }
        }

        /* If still no information, we try to open the codec and to
         * decompress the frame. We try to avoid that in most cases as
         * it takes longer and uses more memory. For MPEG-4, we need to
         * decompress for QuickTime.
         *
         * If AV_CODEC_CAP_CHANNEL_CONF is set this will force decoding of at
         * least one frame of codec data, this makes sure the codec initializes
         * the channel configuration and does not only trust the values from
         * the container. */
        try_decode_frame(ic, st, pkt,
                         (options && i < orig_nb_streams) ? &options[i] : NULL);

        if (ic->flags & AVFMT_FLAG_NOBUFFER)
            av_packet_unref(pkt);

        st->codec_info_nb_frames++;
        count++;
    }

    if (eof_reached) {
        int stream_index;
        for (stream_index = 0; stream_index < ic->nb_streams; stream_index++) {
            st = ic->streams[stream_index];
            avctx = st->internal->avctx;
            if (!has_codec_parameters(st, NULL)) {
                const AVCodec *codec = find_decoder(ic, st, st->codecpar->codec_id);
                if (codec && !avctx->codec) {
                    if (avcodec_open2(avctx, codec, (options && stream_index < orig_nb_streams) ? &options[stream_index] : NULL) < 0)
                        av_log(ic, AV_LOG_WARNING,
                            "Failed to open codec in av_find_stream_info\n");
                }
            }

            // EOF already reached while reading the stream above.
            // So continue with reoordering DTS with whatever delay we have.
            if (ic->internal->packet_buffer && !has_decode_delay_been_guessed(st)) {
                update_dts_from_pts(ic, stream_index, ic->internal->packet_buffer);
            }
        }
    }

    if (flush_codecs) {
        AVPacket empty_pkt = { 0 };
        int err = 0;
        av_init_packet(&empty_pkt);

        for (i = 0; i < ic->nb_streams; i++) {

            st = ic->streams[i];

            /* flush the decoders */
            if (st->info->found_decoder == 1) {
                do {
                    err = try_decode_frame(ic, st, &empty_pkt,
                                            (options && i < orig_nb_streams)
                                            ? &options[i] : NULL);
                } while (err > 0 && !has_codec_parameters(st, NULL));

                if (err < 0) {
                    av_log(ic, AV_LOG_INFO,
                        "decoding for stream %d failed\n", st->index);
                }
            }
        }
    }

    // close codecs which were opened in try_decode_frame()
    for (i = 0; i < ic->nb_streams; i++) {
        st = ic->streams[i];
        avcodec_close(st->internal->avctx);
    }

    ff_rfps_calculate(ic);

    for (i = 0; i < ic->nb_streams; i++) {
        st = ic->streams[i];
        avctx = st->internal->avctx;
        if (avctx->codec_type == AVMEDIA_TYPE_VIDEO) {
            if (avctx->codec_id == AV_CODEC_ID_RAWVIDEO && !avctx->codec_tag && !avctx->bits_per_coded_sample) {
                uint32_t tag= avcodec_pix_fmt_to_codec_tag(avctx->pix_fmt);
                if (avpriv_find_pix_fmt(avpriv_get_raw_pix_fmt_tags(), tag) == avctx->pix_fmt)
                    avctx->codec_tag= tag;
            }

            /* estimate average framerate if not set by demuxer */
            if (st->info->codec_info_duration_fields &&
                !st->avg_frame_rate.num &&
                st->info->codec_info_duration) {
                int best_fps      = 0;
                double best_error = 0.01;

                if (st->info->codec_info_duration        >= INT64_MAX / st->time_base.num / 2||
                    st->info->codec_info_duration_fields >= INT64_MAX / st->time_base.den ||
                    st->info->codec_info_duration        < 0)
                    continue;
                av_reduce(&st->avg_frame_rate.num, &st->avg_frame_rate.den,
                          st->info->codec_info_duration_fields * (int64_t) st->time_base.den,
                          st->info->codec_info_duration * 2 * (int64_t) st->time_base.num, 60000);

                /* Round guessed framerate to a "standard" framerate if it's
                 * within 1% of the original estimate. */
                for (j = 0; j < MAX_STD_TIMEBASES; j++) {
                    AVRational std_fps = { get_std_framerate(j), 12 * 1001 };
                    double error       = fabs(av_q2d(st->avg_frame_rate) /
                                              av_q2d(std_fps) - 1);

                    if (error < best_error) {
                        best_error = error;
                        best_fps   = std_fps.num;
                    }
                }
                if (best_fps)
                    av_reduce(&st->avg_frame_rate.num, &st->avg_frame_rate.den,
                              best_fps, 12 * 1001, INT_MAX);
            }

            if (!st->r_frame_rate.num) {
                if (    avctx->time_base.den * (int64_t) st->time_base.num
                    <= avctx->time_base.num * avctx->ticks_per_frame * (int64_t) st->time_base.den) {
                    st->r_frame_rate.num = avctx->time_base.den;
                    st->r_frame_rate.den = avctx->time_base.num * avctx->ticks_per_frame;
                } else {
                    st->r_frame_rate.num = st->time_base.den;
                    st->r_frame_rate.den = st->time_base.num;
                }
            }
            if (st->display_aspect_ratio.num && st->display_aspect_ratio.den) {
                AVRational hw_ratio = { avctx->height, avctx->width };
                st->sample_aspect_ratio = av_mul_q(st->display_aspect_ratio,
                                                   hw_ratio);
            }
        } else if (avctx->codec_type == AVMEDIA_TYPE_AUDIO) {
            if (!avctx->bits_per_coded_sample)
                avctx->bits_per_coded_sample =
                    av_get_bits_per_sample(avctx->codec_id);
            // set stream disposition based on audio service type
            switch (avctx->audio_service_type) {
            case AV_AUDIO_SERVICE_TYPE_EFFECTS:
                st->disposition = AV_DISPOSITION_CLEAN_EFFECTS;
                break;
            case AV_AUDIO_SERVICE_TYPE_VISUALLY_IMPAIRED:
                st->disposition = AV_DISPOSITION_VISUAL_IMPAIRED;
                break;
            case AV_AUDIO_SERVICE_TYPE_HEARING_IMPAIRED:
                st->disposition = AV_DISPOSITION_HEARING_IMPAIRED;
                break;
            case AV_AUDIO_SERVICE_TYPE_COMMENTARY:
                st->disposition = AV_DISPOSITION_COMMENT;
                break;
            case AV_AUDIO_SERVICE_TYPE_KARAOKE:
                st->disposition = AV_DISPOSITION_KARAOKE;
                break;
            }
        }
    }

    if (probesize)
        estimate_timings(ic, old_offset);

    av_opt_set(ic, "skip_clear", "0", AV_OPT_SEARCH_CHILDREN);

    if (ret >= 0 && ic->nb_streams)
        /* We could not have all the codec parameters before EOF. */
        ret = -1;
    for (i = 0; i < ic->nb_streams; i++) {
        const char *errmsg;
        st = ic->streams[i];

        /* if no packet was ever seen, update context now for has_codec_parameters */
        if (!st->internal->avctx_inited) {
            if (st->codecpar->codec_type == AVMEDIA_TYPE_AUDIO &&
                st->codecpar->format == AV_SAMPLE_FMT_NONE)
                st->codecpar->format = st->internal->avctx->sample_fmt;
            ret = avcodec_parameters_to_context(st->internal->avctx, st->codecpar);
            if (ret < 0)
                goto find_stream_info_err;
        }
        if (!has_codec_parameters(st, &errmsg)) {
            char buf[256];
            avcodec_string(buf, sizeof(buf), st->internal->avctx, 0);
            av_log(ic, AV_LOG_WARNING,
                   "Could not find codec parameters for stream %d (%s): %s\n"
                   "Consider increasing the value for the 'analyzeduration' and 'probesize' options\n",
                   i, buf, errmsg);
        } else {
            ret = 0;
        }
    }

    compute_chapters_end(ic);

    /* update the stream parameters from the internal codec contexts */
    for (i = 0; i < ic->nb_streams; i++) {
        st = ic->streams[i];

        if (st->internal->avctx_inited) {
            int orig_w = st->codecpar->width;
            int orig_h = st->codecpar->height;
            ret = avcodec_parameters_from_context(st->codecpar, st->internal->avctx);
            if (ret < 0)
                goto find_stream_info_err;
            // The decoder might reduce the video size by the lowres factor.
            if (av_codec_get_lowres(st->internal->avctx) && orig_w) {
                st->codecpar->width = orig_w;
                st->codecpar->height = orig_h;
            }
        }

#if FF_API_LAVF_AVCTX
FF_DISABLE_DEPRECATION_WARNINGS
        ret = avcodec_parameters_to_context(st->codec, st->codecpar);
        if (ret < 0)
            goto find_stream_info_err;

        // The old API (AVStream.codec) "requires" the resolution to be adjusted
        // by the lowres factor.
        if (av_codec_get_lowres(st->internal->avctx) && st->internal->avctx->width) {
            av_codec_set_lowres(st->codec, av_codec_get_lowres(st->internal->avctx));
            st->codec->width = st->internal->avctx->width;
            st->codec->height = st->internal->avctx->height;
        }

        if (st->codec->codec_tag != MKTAG('t','m','c','d'))
            st->codec->time_base = st->internal->avctx->time_base;
        st->codec->framerate = st->avg_frame_rate;

        if (st->internal->avctx->subtitle_header) {
            st->codec->subtitle_header = av_malloc(st->internal->avctx->subtitle_header_size);
            if (!st->codec->subtitle_header)
                goto find_stream_info_err;
            st->codec->subtitle_header_size = st->internal->avctx->subtitle_header_size;
            memcpy(st->codec->subtitle_header, st->internal->avctx->subtitle_header,
                   st->codec->subtitle_header_size);
        }

        // Fields unavailable in AVCodecParameters
        st->codec->coded_width = st->internal->avctx->coded_width;
        st->codec->coded_height = st->internal->avctx->coded_height;
        st->codec->properties = st->internal->avctx->properties;
FF_ENABLE_DEPRECATION_WARNINGS
#endif

        st->internal->avctx_inited = 0;
    }

find_stream_info_err:
    for (i = 0; i < ic->nb_streams; i++) {
        st = ic->streams[i];
        if (st->info)
            av_freep(&st->info->duration_error);
        av_freep(&ic->streams[i]->info);
    }
    if (ic->pb)
        av_log(ic, AV_LOG_DEBUG, "After avformat_find_stream_info() pos: %"PRId64" bytes read:%"PRId64" seeks:%d frames:%d\n",
               avio_tell(ic->pb), ic->pb->bytes_read, ic->pb->seek_count, count);
    return ret;
}

ff_read_packet

int ff_read_packet(AVFormatContext *s, AVPacket *pkt)
{
    int ret, i, err;
    AVStream *st;

    for (;;) {
        //首先在這個連結串列中找。
        AVPacketList *pktl = s->internal->raw_packet_buffer;
        //如果連結串列中有包從連結串列中取,一開始進來時是空的。
        if (pktl) {
            *pkt = pktl->pkt;
            st   = s->streams[pkt->stream_index];
            //預設是RAW_PACKET_BUFFER_SIZE=2500000
            if (s->internal->raw_packet_buffer_remaining_size <= 0)
                if ((err = probe_codec(s, st, NULL)) < 0)
                    return err;
            if (st->request_probe <= 0) {
                s->internal->raw_packet_buffer                 = pktl->next;
                s->internal->raw_packet_buffer_remaining_size += pkt->size;
                av_free(pktl);
                return 0;
            }
        }

        pkt->data = NULL;
        pkt->size = 0;
        av_init_packet(pkt);
        //此函式如果是h264的原始流的話呼叫的是ff_raw_read_partial_packet,
        //在檔案libavformat\rawdec.c中
        ret = s->iformat->read_packet(s, pkt);
        if (ret < 0) {
            /* Some demuxers return FFERROR_REDO when they consume
               data and discard it (ignored streams, junk, extradata).
               We must re-call the demuxer to get the real packet. */
            if (ret == FFERROR_REDO)
                continue;
            if (!pktl || ret == AVERROR(EAGAIN))
                return ret;
            for (i = 0; i < s->nb_streams; i++) {
                st = s->streams[i];
                if (st->probe_packets)
                    if ((err = probe_codec(s, st, NULL)) < 0)
                        return err;
                av_assert0(st->request_probe <= 0);
            }
            continue;
        }

        if (!pkt->buf) {
            AVPacket tmp = { 0 };
            ret = av_packet_ref(&tmp, pkt);
            if (ret < 0)
                return ret;
            *pkt = tmp;
        }

        if ((s->flags & AVFMT_FLAG_DISCARD_CORRUPT) &&
            (pkt->flags & AV_PKT_FLAG_CORRUPT)) {
            av_log(s, AV_LOG_WARNING,
                   "Dropped corrupted packet (stream = %d)\n",
                   pkt->stream_index);
            av_packet_unref(pkt);
            continue;
        }

        if (pkt->stream_index >= (unsigned)s->nb_streams) {
            av_log(s, AV_LOG_ERROR, "Invalid stream index %d\n", pkt->stream_index);
            continue;
        }

        st = s->streams[pkt->stream_index];

        if (update_wrap_reference(s, st, pkt->stream_index, pkt) && st->pts_wrap_behavior == AV_PTS_WRAP_SUB_OFFSET) {
            // correct first time stamps to negative values
            if (!is_relative(st->first_dts))
                st->first_dts = wrap_timestamp(st, st->first_dts);
            if (!is_relative(st->start_time))
                st->start_time = wrap_timestamp(st, st->start_time);
            if (!is_relative(st->cur_dts))
                st->cur_dts = wrap_timestamp(st, st->cur_dts);
        }

        pkt->dts = wrap_timestamp(st, pkt->dts);
        pkt->pts = wrap_timestamp(st, pkt->pts);

        force_codec_ids(s, st);

        /* TODO: audio: time filter; video: frame reordering (pts != dts) */
        if (s->use_wallclock_as_timestamps)
            pkt->dts = pkt->pts = av_rescale_q(av_gettime(), AV_TIME_BASE_Q, st->time_base);

        if (!pktl && st->request_probe <= 0)
            return ret;

        err = add_to_pktbuf(&s->internal->raw_packet_buffer, pkt,
                            &s->internal->raw_packet_buffer_end, 0);
        if (err)
            return err;
        s->internal->raw_packet_buffer_remaining_size -= pkt->size;

        if ((err = probe_codec(s, st, pkt)) < 0)
            return err;
    }
}

ff_raw_read_partial_packet

int ff_raw_read_partial_packet(AVFormatContext *s, AVPacket *pkt)
{
    int ret, size;

    size = RAW_PACKET_SIZE;
    //為pkt分配必要的記憶體,後面有詳解
    if (av_new_packet(pkt, size) < 0)
        return AVERROR(ENOMEM);

    pkt->pos= avio_tell(s->pb);
    pkt->stream_index = 0;
    //呼叫在s->pb中註冊的讀函式將size個數據讀入pkt->data中。後面有詳解
    ret = ffio_read_partial(s->pb, pkt->data, size);
    if (ret < 0) {
        //pkt是帶引用計數的。如果失敗就拿這個函式釋放。
        av_packet_unref(pkt);
        return ret;
    }
    //把返回的ret當pkt中資料的大小進行設定,並將最後的AV_INPUT_BUFFER_PADDING_SIZE個位元組設為0.
    av_shrink_packet(pkt, ret);
    return ret;
}

av_new_packet

給AVPacket分配具體放資料的空間,

int av_new_packet(AVPacket *pkt, int size)
{
    AVBufferRef *buf = NULL;
    //這個函式做了幾個記憶體分配
    /*
    AVBufferRef *buf(malloc)
    AVBuffer    *buf->buffer(malloc)
    uint8_t *buf->buffer->data(malloc)
    buf->data = buf->buffer->data
    buf->size   = buf->data的size
    buf->buffer->refcount =1引用計數
    */
    int ret = packet_alloc(&buf, size);
    if (ret < 0)
        return ret;
    //最後將所有分配的記憶體都放到pkt中
    av_init_packet(pkt);
    pkt->buf      = buf;
    pkt->data     = buf->data;
    pkt->size     = size;

    return 0;
}

struct AVBuffer {
    uint8_t *data; /**< data described by this buffer */
    int      size; /**< size of data in bytes */

    /**
     *  number of existing AVBufferRef instances referring to this buffer
     */
    volatile int refcount;

    /**
     * a callback for freeing the data
     */
    void (*free)(void *opaque, uint8_t *data);

    /**
     * an opaque pointer, to be used by the freeing callback
     */
    void *opaque;

    /**
     * A combination of BUFFER_FLAG_*
     */
    int flags;
};
/**
 * A reference to a data buffer.
 *
 * The size of this struct is not a part of the public ABI and it is not meant
 * to be allocated directly.
 */
typedef struct AVBufferRef {
    AVBuffer *buffer;

    /**
     * The data buffer. It is considered writable if and only if
     * this is the only reference to the buffer, in which case
     * av_buffer_is_writable() returns 1.
     */
    uint8_t *data;
    /**
     * Size of data in bytes.
     */
    int      size;
} AVBufferRef;

avio_tell

/**
 * ftell() equivalent for AVIOContext.
 * @return position or AVERROR.
 */
static av_always_inline int64_t avio_tell(AVIOContext *s)
{
    return avio_seek(s, 0, SEEK_CUR);
}

ffio_read_partial

最終呼叫的還是在s中註冊的read函式。如果是記憶體讀取如下注冊。

AVIOContext * inpb = avio_alloc_context(Buf, BUF_SIZE, 0, dataFromUser, read_data, NULL, NULL);

s->read_packet(s->opaque, buf, size);呼叫的就是上面註冊的read_data函式。

int ffio_read_partial(AVIOContext *s, unsigned char *buf, int size)
{
    int len;

    if (size < 0)
        return -1;

    if (s->read_packet && s->write_flag) {
        len = s->read_packet(s->opaque, buf, size);
        if (len > 0)
            s->pos += len;
        return len;
    }
    //如果s的buff中無資料就用fill_buffer函式讀取,並放到s的buff中。具體介紹在《av_probe_input_buffer函式中的資料流向》中有介紹
    len = s->buf_end - s->buf_ptr;
    if (len == 0) {
        /* Reset the buf_end pointer to the start of the buffer, to make sure
         * the fill_buffer call tries to read as much data as fits into the
         * full buffer, instead of just what space is left after buf_end.
         * This avoids returning partial packets at the end of the buffer,
         * for packet based inputs.
         */
        s->buf_end = s->buf_ptr = s->buffer;
        fill_buffer(s);
        len = s->buf_end - s->buf_ptr;
    }
    //取資料返回。
    if (len > size)
        len = size;
    memcpy(buf, s->buf_ptr, len);
    s->buf_ptr += len;
    if (!len) {
        if (s->error)      return s->error;
        if (avio_feof(s))  return AVERROR_EOF;
    }
    return len;
}

av_shrink_packet

如果讀到的比分配的空間小的情況下,需要將pkt中的大小重新設定,並將真是資料的後AV_INPUT_BUFFER_PADDING_SIZE個
位元組設為0.防止讀越界。

void av_shrink_packet(AVPacket *pkt, int size)
{
    if (pkt->size <= size)
        return;
    pkt->size = size;
    memset(pkt->data + size, 0, AV_INPUT_BUFFER_PADDING_SIZE);
}

parse_packet

將資料放到av_parser_parse2函式去解析,如果解析成功,並且有返回的包就將包放到s->internal->parse_queue的連結串列中。在av_parser_parse2中應該有自己的快取處理之前放入的資料。

/**
 * Parse a packet, add all split parts to parse_queue.
 *
 * @param pkt Packet to parse, NULL when flushing the parser at end of stream.
 */
static int parse_packet(AVFormatContext *s, AVPacket *pkt, int stream_index)
{
    AVPacket out_pkt = { 0 }, flush_pkt = { 0 };
    AVStream *st = s->streams[stream_index];
    uint8_t *data = pkt ? pkt->data : NULL;
    int size      = pkt ? pkt->size : 0;
    int ret = 0, got_output = 0;

    if (!pkt) {
        av_init_packet(&flush_pkt);
        pkt        = &flush_pkt;
        got_output = 1;
    } else if (!size && st->parser->flags & PARSER_FLAG_COMPLETE_FRAMES) {
        // preserve 0-size sync packets
        compute_pkt_fields(s, st, st->parser, pkt, AV_NOPTS_VALUE, AV_NOPTS_VALUE);
    }

    while (size > 0 || (pkt == &flush_pkt && got_output)) {
        int len;
        int64_t next_pts = pkt->pts;
        int64_t next_dts = pkt->dts;

        av_init_packet(&out_pkt);
        len = av_parser_parse2(st->parser, st->internal->avctx,
                               &out_pkt.data, &out_pkt.size, data, size,
                               pkt->pts, pkt->dts, pkt->pos);

        pkt->pts = pkt->dts = AV_NOPTS_VALUE;
        pkt->pos = -1;
        /* increment read pointer */
        data += len;
        size -= len;

        got_output = !!out_pkt.size;

        if (!out_pkt.size)
            continue;

        if (pkt->side_data) {
            out_pkt.side_data       = pkt->side_data;
            out_pkt.side_data_elems = pkt->side_data_elems;
            pkt->side_data          = NULL;
            pkt->side_data_elems    = 0;
        }

        /* set the duration */
        out_pkt.duration = (st->parser->flags & PARSER_FLAG_COMPLETE_FRAMES) ? pkt->duration : 0;
        if (st->codecpar->codec_type == AVMEDIA_TYPE_AUDIO) {
            if (st->internal->avctx->sample_rate > 0) {
                out_pkt.duration =
                    av_rescale_q_rnd(st->parser->duration,
                                     (AVRational) { 1, st->internal->avctx->sample_rate },
                                     st->time_base,
                                     AV_ROUND_DOWN);
            }
        }

        out_pkt.stream_index = st->index;
        out_pkt.pts          = st->parser->pts;
        out_pkt.dts          = st->parser->dts;
        out_pkt.pos          = st->parser->pos;

        if (st->need_parsing == AVSTREAM_PARSE_FULL_RAW)
            out_pkt.pos = st->parser->frame_offset;

        if (st->parser->key_frame == 1 ||
            (st->parser->key_frame == -1 &&
             st->parser->pict_type == AV_PICTURE_TYPE_I))
            out_pkt.flags |= AV_PKT_FLAG_KEY;

        if (st->parser->key_frame == -1 && st->parser->pict_type ==AV_PICTURE_TYPE_NONE && (pkt->flags&AV_PKT_FLAG_KEY))
            out_pkt.flags |= AV_PKT_FLAG_KEY;

        compute_pkt_fields(s, st, st->parser, &out_pkt, next_dts, next_pts);

        ret = add_to_pktbuf(&s->internal->parse_queue, &out_pkt,
                            &s->internal->parse_queue_end, 1);
        av_packet_unref(&out_pkt);
        if (ret < 0)
            goto fail;
    }

    /* end of the stream => close and free the parser */
    if (pkt == &flush_pkt) {
        av_parser_close(st->parser);
        st->parser = NULL;
    }

fail:
    av_packet_unref(pkt);
    return ret;
}