从头用脚分析FFmpeg源码 --- avformat_find_stream_info

本文所使用的是FFmpeg n4.4的源码，所有分析均来自博主瞎猜，如果有误，欢迎批评指正。

这个函数太难分析了，太复杂了，慢慢补全吧~

avformat_find_stream_info 作用

先看一下FFmpeg是怎么定义avformat_find_stream_info函数的

/**
 * Read packets of a media file to get stream information. This
 * is useful for file formats with no headers such as MPEG. This
 * function also computes the real framerate in case of MPEG-2 repeat
 * frame mode.
 * The logical file position is not changed by this function;
 * examined packets may be buffered for later processing.
 *
 * @param ic media file handle
 * @param options  If non-NULL, an ic.nb_streams long array of pointers to
 *                 dictionaries, where i-th member contains options for
 *                 codec corresponding to i-th stream.
 *                 On return each dictionary will be filled with options that were not found.
 * @return >=0 if OK, AVERROR_xxx on error
 *
 * @note this function isn't guaranteed to open all the codecs, so
 *       options being non-empty at return is a perfectly normal behavior.
 *
 * @todo Let the user decide somehow what information is needed so that
 *       we do not waste time getting stuff the user does not need.
 */
int avformat_find_stream_info(AVFormatContext *ic, AVDictionary **options);

通过从媒体文件中读取部分packet，来获取流的信息。有一些文件格式没有头，比如说MPEG格式的，这个时候，这个函数就很有用，因为它可以从读取到的包中获得到流的信息。可以获得MPEG-2 的repeat mode下的真实帧率。 (MPEG-2 支持对于大量静止的画面设置 repeat mode，重复的帧不用编码和存储，可以减少体积）。并且读取的这些包都会被缓存起来，等待下次使用。

在用avformat_open_input初始化AVFormatContext的时候，只是通过解析封装体结构实现的，比如MP4类型，avformat_open_input会解析所有的box，但是如果box缺失或者通过单纯解析box无法了解的信息，比如pix_fmt（像素格式）等，就需要读取甚至解码部分的AVPacket来完成。

总而言之，这个函数的作用就如函数名所讲avformat_find_stream_info，找到流相关的信息。

avformat_find_stream_info 源码

avformat_find_stream_info的源码比较长，有好几百行，下面分段来介绍这些功能。

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 = ic->internal->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;
    int *missing_streams = av_opt_ptr(ic->iformat->priv_class, ic->priv_data, "missing_streams");

    flush_codecs = probesize > 0;

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

	// 开始的一段，就是设置本次探测的最大时长
	// 首帧秒开等需要立马显示的可以减少这里的时间限制。
    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; //默认是5s
        max_subtitle_analyze_duration = 30*AV_TIME_BASE;
        if (!strcmp(ic->iformat->name, "flv")) // flv格式默认是90s
            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);

这个一段最主要就是设置探测时长。

第一遍循环：
遍历一遍所有的AVStream，然后根据格式设置设置AVCodecContext，并打开对应的AVCodec

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 = */
            //根据从容器中获得的timebase，先设置一下AVCodecContext的timbase
            if (!avctx->time_base.num)
                avctx->time_base = st->time_base;
        }

        /* 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
		// 初始化AVCodecParserContext， need_parsing参数是在read_header的时候设置的
		// AVCodecParserContext是类似于AVCodecContext的存在，只不过AVCodecParserContext是专门来解析packet，然后把从packet中得到的info设置到AVCodecContext上，而AVCodecContext是利用存在的信息，解码packet
		// AVCodecContext中编码相关的信息，基本都是从AVCodecParserContext得到的
        // only for the split stuff
        if (!st->parser && !(ic->flags & AVFMT_FLAG_NOPARSE) && st->internal->request_probe <= 0) {
        	//parse也是根据编码类型设置的，h264有自己专门的parse，类似解码器
            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));
            }
        }

        if (st->codecpar->codec_id != st->internal->orig_codec_id)
            st->internal->orig_codec_id = st->codecpar->codec_id;

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

		// 根据容器中的记录的流格式，找到对应的解码器
        codec = find_probe_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);
        /* Force lowres to 0. The decoder might reduce the video size by the
         * lowres factor, and we don't want that propagated to the stream's
         * codecpar */
        av_dict_set(options ? &options[i] : &thread_opt, "lowres", "0", 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 %s\n",__FUNCTION__);
        }

        // Try to just open decoders, in case this is enough to get parameters.
        // 上面找到codec之后，这里就open 这个codec
        if (!has_codec_parameters(st, NULL) && st->internal->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 %s\n",__FUNCTION__);
        }
        if (!options)
            av_dict_free(&thread_opt);
    }

这一段最主要就是根据read_header时获得的编码参数设置AVCodecContext，然后打开对应的解码器AVCodec 。

下面还是一段遍历， 这一段就简单多了，初始化这些值，这些会在接下来的计算中使用。

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

第二遍循环：
经过上面的第一遍循环，已经打开的解码器，但是还没有从实际的媒体Packet中获取宽高 ，fps等信息，所以下来需要读取部分packet，根据需要解码，然后从读的packet和解码得到的frame中获取更多的参数信息，然后填充到AVCodecContext。

    for (;;) {
        const AVPacket *pkt;
        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;
            int count;

            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 */
            count = (ic->iformat->flags & AVFMT_NOTIMESTAMPS) ?
                       st->internal->info->codec_info_duration_fields/2 :
                       st->internal->info->duration_count;
            if (!(st->r_frame_rate.num && st->avg_frame_rate.num) &&
                st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO) {
                if (count < fps_analyze_framecount)
                    break;
            }
            // Look at the first 3 frames if there is evidence of frame delay
            // but the decoder delay is not set.
            if (st->internal->info->frame_delay_evidence && count < 2 && st->internal->avctx->has_b_frames == 0)
                break;
            if (!st->internal->avctx->extradata &&
                (!st->internal->extract_extradata.inited ||
                 st->internal->extract_extradata.bsf) &&
                extract_extradata_check(st))
                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 (!missing_streams || !*missing_streams)
            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]->internal->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). */
        //读取一个packet， 然后调用上面创建的AVCodecParserContext，解析packet中的内容，比如在h264流中就会根据当前块是否为sps解析宽高，根据其余帧解析时长等。
        ret = read_frame_internal(ic, pkt1);
        if (ret == AVERROR(EAGAIN))
            continue;

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

        if (!(ic->flags & AVFMT_FLAG_NOBUFFER)) {
            ret = avpriv_packet_list_put(&ic->internal->packet_buffer,
                                     &ic->internal->packet_buffer_end,
                                     pkt1, NULL, 0);
            if (ret < 0)
                goto unref_then_goto_end;

            pkt = &ic->internal->packet_buffer_end->pkt;
        } else {
            pkt = pkt1;
        }

        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 unref_then_goto_end;
            st->internal->avctx_inited = 1;
        }

        if (pkt->dts != AV_NOPTS_VALUE && st->codec_info_nb_frames > 1) {
            /* check for non-increasing dts */
            if (st->internal->info->fps_last_dts != AV_NOPTS_VALUE &&
                st->internal->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->internal->info->fps_last_dts_idx,
                       st->internal->info->fps_last_dts, st->codec_info_nb_frames,
                       pkt->dts);
                st->internal->info->fps_first_dts =
                st->internal->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->internal->info->fps_last_dts != AV_NOPTS_VALUE &&
                st->internal->info->fps_last_dts_idx > st->internal->info->fps_first_dts_idx &&
                (pkt->dts - (uint64_t)st->internal->info->fps_last_dts) / 1000 >
                (st->internal->info->fps_last_dts     - (uint64_t)st->internal->info->fps_first_dts) /
                (st->internal->info->fps_last_dts_idx - st->internal->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->internal->info->fps_last_dts_idx,
                       st->internal->info->fps_last_dts, st->codec_info_nb_frames,
                       pkt->dts);
                st->internal->info->fps_first_dts =
                st->internal->info->fps_last_dts  = AV_NOPTS_VALUE;
            }

            /* update stored dts values */
            if (st->internal->info->fps_first_dts == AV_NOPTS_VALUE) {
                st->internal->info->fps_first_dts     = pkt->dts;
                st->internal->info->fps_first_dts_idx = st->codec_info_nb_frames;
            }
            st->internal->info->fps_last_dts     = pkt->dts;
            st->internal->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->internal->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->internal->info->fps_first_dts != AV_NOPTS_VALUE
                && st->internal->info->fps_last_dts  != AV_NOPTS_VALUE) {
                int64_t dur = av_sat_sub64(st->internal->info->fps_last_dts, st->internal->info->fps_first_dts);
                t = FFMAX(t, av_rescale_q(dur, 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(pkt1);
                break;
            }
            if (pkt->duration) {
                if (avctx->codec_type == AVMEDIA_TYPE_SUBTITLE && pkt->pts != AV_NOPTS_VALUE && st->start_time != AV_NOPTS_VALUE && pkt->pts >= st->start_time) {
                    st->internal->info->codec_info_duration = FFMIN(pkt->pts - st->start_time, st->internal->info->codec_info_duration + pkt->duration);
                } else
                    st->internal->info->codec_info_duration += pkt->duration;
                st->internal->info->codec_info_duration_fields += st->parser && st->need_parsing && avctx->ticks_per_frame ==2 ? st->parser->repeat_pict + 1 : 2;
            }
        }
        if (st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO) {
#if FF_API_R_FRAME_RATE
            ff_rfps_add_frame(ic, st, pkt->dts);
#endif
            if (pkt->dts != pkt->pts && pkt->dts != AV_NOPTS_VALUE && pkt->pts != AV_NOPTS_VALUE)
                st->internal->info->frame_delay_evidence = 1;
        }
        if (!st->internal->avctx->extradata) {
            ret = extract_extradata(st, pkt);
            if (ret < 0)
                goto unref_then_goto_end;
        }

        /* 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(pkt1);

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

//上面还没分析完