本文参考的博文整理：https://blog.csdn.net/leixiaohua1020。
https://blog.csdn.net/u011913612/article/details/53642355
本文简单分析FFmpeg中一个常用的函数：avformat_find_stream_info()。该函数可以读取一部分视音频数据并且获得一些相关的信息。avformat_find_stream_info()的声明位于libavformat\avformat.h，如下所示。

/**
 * 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);

从注释来看，avformat_find_stream_info主要是读一些包（packets ），然后从中提取初流的信息。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。在MPEG-2重复帧模式的情况下，该函数还计算真实的帧率。
逻辑文件位置不被此函数更改; 读出来的包会被缓存起来供以后处理。
注释很好的解释了这个函数的功能。我们可以想象一下，既然这个函数的功能是更新流的信息，那么可以猜测它的作用就是更新AVStream这个结构体中的字段了。
简单解释一下这个函数参数的含义：

ic：输入的AVFormatContext。
options：额外的选项，目前没有深入研究过。

AVStream这个结构体中的一些重要字段：

int index：该流的标示。

AVCodecContext *codec：该流的编解码器上下文。

AVRational time_base：时基。通过该值可以把PTS，DTS转化为真正的时间。FFMPEG其他结构体中也有这个字段，但是根据我的经验，只有AVStream中的######time_base是可用的。PTS*time_base=真正的时间

start_time：流中第一个pts的时间。

nb_frames：这个流中的帧的数目。

AVRational avg_frame_rate;平均帧率。

int64_t duration：该流的时间长度

AVDictionary *metadata：元数据信息

AVPacket attached_pic：附带的图片。比如说一些MP3，AAC音频文件附带的专辑封面。

untitled.png

这个函数的代码，源码在libavformat/utils.c文件中。
这里简单记录一下它的要点。该函数主要用于给每个媒体流（音频/视频）的AVStream结构体赋值。我们大致浏览一下这个函数的代码，会发现它其实已经实现了解码器的查找，解码器的打开，视音频帧的读取，视音频帧的解码等工作。换句话说，该函数实际上已经“走通”的解码的整个流程。

函数源代码分析：

part1:参数定义

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

    flush_codecs = probesize > 0;
...

这里定义了一些参数：
probesize就是探测的大小，为了获得流的信息，这个函数会尝试的读一些包出来，然后分析这些数据，probesize限制 了最大允许读出的数据的大小。
orig_nb_streams是这个文件中的流的数量。普通的电影会包含三个流：音频流，视频流和字幕流。

part 2 设置max_stream_analyze_duration 等

 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;
    }

这里定义了最大分析时长的限制。max_stream_analyze_duration 等于max_analyze_duration 等于30倍的timebase。此外，如果文件格式为flv或者Mpeg，那么他们会有各自的最大分析时长的限制。

part 3 第一次遍历流

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;
        }

        /* 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
        // 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));
            }
        }

        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_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);

        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.
        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 %s\n",__FUNCTION__);
        }
        if (!options)
            av_dict_free(&thread_opt);
    }

这个函数有很多次遍历流的循环，这里是第一次。第一次循环做了如下事情：
1.获得编码器上下文环境avctx，设置avctx的time_base,code_id,codec_type,orig_codec_id。
2.如果解析器paser为空，那么会初始化解析器。
3.把解析器中的参数对应的拷贝到编解码器上下文环境中。调用的是avcodec_parameters_to_context函数，这个函数如下：

int avcodec_parameters_to_context(AVCodecContext *codec,
                                  const AVCodecParameters *par)
{
    codec->codec_type = par->codec_type;
    codec->codec_id   = par->codec_id;
    codec->codec_tag  = par->codec_tag;

    codec->bit_rate              = par->bit_rate;
    codec->bits_per_coded_sample = par->bits_per_coded_sample;
    codec->bits_per_raw_sample   = par->bits_per_raw_sample;
    codec->profile               = par->profile;
    codec->level                 = par->level;

    switch (par->codec_type) {
    case AVMEDIA_TYPE_VIDEO:
        codec->pix_fmt                = par->format;
        codec->width                  = par->width;
        codec->height                 = par->height;
        codec->field_order            = par->field_order;
        codec->color_range            = par->color_range;
        codec->color_primaries        = par->color_primaries;
        codec->color_trc              = par->color_trc;
        codec->colorspace             = par->color_space;
        codec->chroma_sample_location = par->chroma_location;
        codec->sample_aspect_ratio    = par->sample_aspect_ratio;
        codec->has_b_frames           = par->video_delay;
        break;
    case AVMEDIA_TYPE_AUDIO:
        codec->sample_fmt       = par->format;
        codec->channel_layout   = par->channel_layout;
        codec->channels         = par->channels;
        codec->sample_rate      = par->sample_rate;
        codec->block_align      = par->block_align;
        codec->frame_size       = par->frame_size;
        codec->delay            =
        codec->initial_padding  = par->initial_padding;
        codec->trailing_padding = par->trailing_padding;
        codec->seek_preroll     = par->seek_preroll;
        break;
    case AVMEDIA_TYPE_SUBTITLE:
        codec->width  = par->width;
        codec->height = par->height;
        break;
    }

    if (par->extradata) {
        av_freep(&codec->extradata);
        codec->extradata = av_mallocz(par->extradata_size + AV_INPUT_BUFFER_PADDING_SIZE);
        if (!codec->extradata)
            return AVERROR(ENOMEM);
        memcpy(codec->extradata, par->extradata, par->extradata_size);
        codec->extradata_size = par->extradata_size;
    }

    return 0;
}

可见就是根据编码器类型做对应的参数的拷贝.
根据编码器ID查找编码器.
这里调用的是find_probe_decoder函数，该函数也定义在libavformat/utils.c中。可以看一下查找编解码器的过程：

static const AVCodec *find_probe_decoder(AVFormatContext *s, const AVStream *st, enum AVCodecID codec_id)
{
    const AVCodec *codec;

#if CONFIG_H264_DECODER
    /* Other parts of the code assume this decoder to be used for h264,
     * so force it if possible. */
    if (codec_id == AV_CODEC_ID_H264)
        return avcodec_find_decoder_by_name("h264");
#endif

    codec = find_decoder(s, st, codec_id);
    if (!codec)
        return NULL;

    if (codec->capabilities & AV_CODEC_CAP_AVOID_PROBING) {
        const AVCodec *probe_codec = NULL;
        while (probe_codec = av_codec_next(probe_codec)) {
            if (probe_codec->id == codec_id &&
                    av_codec_is_decoder(probe_codec) &&
                    !(probe_codec->capabilities & (AV_CODEC_CAP_AVOID_PROBING | AV_CODEC_CAP_EXPERIMENTAL))) {
                return probe_codec;
            }
        }
    }

    return codec;
}

调用find_decoder函数进一步查找：

static const AVCodec *find_decoder(AVFormatContext *s, const AVStream *st, enum AVCodecID codec_id)
{
#if FF_API_LAVF_AVCTX
FF_DISABLE_DEPRECATION_WARNINGS
    if (st->codec->codec)
        return st->codec->codec;
FF_ENABLE_DEPRECATION_WARNINGS
#endif

    switch (st->codecpar->codec_type) {
    case AVMEDIA_TYPE_VIDEO:
        if (s->video_codec)    return s->video_codec;
        break;
    case AVMEDIA_TYPE_AUDIO:
        if (s->audio_codec)    return s->audio_codec;
        break;
    case AVMEDIA_TYPE_SUBTITLE:
        if (s->subtitle_codec) return s->subtitle_codec;
        break;
    }

    return avcodec_find_decoder(codec_id);
}

如果编码器已经存在就根据编码器类型返回对应的编解码器，否则就根据id进行查找。其实就是遍历一个编解码器的链表，其首个结构是first_avcodec。编解码器链表是我们在av_register_all()函数中注册的。这个逐个匹配每一个编解码器的id，找到后返回对应的编解码器。
然后打开编解码器使用的是avcodec_open2函数。
此处后续再分析。。。。
所以我们可以总结下第一次遍历所有的流所做的事情：初始化time_base,codec_id等参数，初始化解析器paser,然后对于每一个流，根据codec_id找对对应的编解码器，然后打开编解码器。也就是说，第一次遍历流，使得我们对于每一个流，都有一个编解码器可用。

part 4 第二次遍历流

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;
    }

第二次遍历流是在编解码器已经打开之后，对于每一个流，设置了一些参数。last_dts 是最有的解码时间。fps_first_dts和fps_last_dts 用于帧率的计算。

part 5 死循环

接下来的这个死循环代码很长。我们回顾一下之前的工作，我们遍历了两次流，设置好了每个流需要的编解码器和计算帧率的参数，初始化了每个流的解析器paser。在介绍这个函数的功能的时候，我们说这个函数会尝试的读一些数据进来，并解析这些数据，从这些数据中进一步获得详细的流的信息。到目前为止我们并没有读任何数据进来，那么接下来，想必就是读数据进来并解码分析数据流了吧。

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;
            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->info->codec_info_duration_fields/2 :
                       st->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->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]->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 = ff_packet_list_put(&ic->internal->packet_buffer,
                                     &ic->internal->packet_buffer_end,
                                     pkt, 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     - (uint64_t)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 (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->info->frame_delay_evidence = 1;
        }
        if (!st->internal->avctx->extradata) {
            ret = extract_extradata(st, pkt);
            if (ret < 0)
                goto find_stream_info_err;
        }

        /* 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++;
    }

这个死循环做了如下事情：
1.检查用户有没有请求中断。如果有中断请求，就调用中断回调方法，并且函数返回。
2.再一次遍历流，检查是不是还有编解码器需要进一步处理。这里会检查编解码器的各个参数，如果这些参数都就绪，那么就会返回了，也就没有必要再做进一步分析了。如果还有些编解码器的信息不全，那么这里会继续向下执行。
3.读一帧数据进来。使用read_frame_internal函数。这个函数很复杂，以后在分析。
4.把读出来的数据添加到缓冲区。使用的是ff_packet_list_put函数。然后更新读到的总的数据的大小：read_size += pkt->size;
5.再次更新编解码器上下文环境的参数。调用的是avcodec_parameters_to_context函数，这个函数我们已经分析过了。
6.检查dts的连续性。 如果dts中的差异大于序列中平均分组持续时间的1000倍，我们将其视为不连续。这里不是很明白意思，后续再分析

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     - (uint64_t)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;
            }

更新存储的dts的值。代码为：

/* 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;

如果这个时候，还是没有找到足够的信息，那么就会尝试解压一些数据出来并做分析。
首先看注释：
如果仍然没有信息，我们尝试打开编解码器并且解压缩帧。 我们需要在大多数情况下尽量避免做这样的事情，因为很需要很多的时间，并使用更多的内存。 对于MPEG-4，我们需要解压QuickTime。
如果设置AV_CODEC_CAP_CHANNEL_CONF，这将强制解码至少一帧编解码器数据，这确保编解码器初始化通道配置，并且不仅信任来自容器的值
尝试解压一帧的数据使用的是try_decode_frame函数。有些参数，如vidoe的pix_fmt是需要调用h264_decode_frame才可以获取其pix_fmt的。
总结下这个死循环做的工作：其中会检测是不是所有流的信息都已经完备，如果完备就返回，如果不完备，就尝试的解压一帧的数据。并从中获取pix_fmt等必须通过解压一帧的数据才能获得的参数。

part 6 检查是否到达文件尾部

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_probe_decoder(ic, st, st->codecpar->codec_id);
                if (codec && !avctx->codec) {
                    AVDictionary *opts = NULL;
                    if (ic->codec_whitelist)
                        av_dict_set(&opts, "codec_whitelist", ic->codec_whitelist, 0);
                    if (avcodec_open2(avctx, codec, (options && stream_index < orig_nb_streams) ? &options[stream_index] : &opts) < 0)
                        av_log(ic, AV_LOG_WARNING,
                               "Failed to open codec in %s\n",__FUNCTION__);
                    av_dict_free(&opts);
                }
            }

            // 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);
            }
        }
    }

如果到了文件尾部，又会遍历一次流，检测其编解码器参数，如果其参数不完整，就会再次调用avcodec_open2来初始化编解码器的各个参数。
part 7 刷新解码器

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);
                }
            }
        }
    }

有一些帧可能在缓存区中，需要把它们flush掉。

有一个has_codec_parameters()函数，用于判断AVStream中的成员变量是否都已经设置完毕。该函数在avformat_find_stream_info()中的多个地方被使用过。下面简单看一下该函数。
has_codec_parameters()用于检查AVStream中的成员变量是否都已经设置完毕。函数的定义如下。

has_codec_parameters()

static int has_codec_parameters(AVStream *st, const char **errmsg_ptr)
{
    AVCodecContext *avctx = st->internal->avctx;

#define FAIL(errmsg) do {                                         \
        if (errmsg_ptr)                                           \
            *errmsg_ptr = errmsg;                                 \
        return 0;                                                 \
    } while (0)

    if (   avctx->codec_id == AV_CODEC_ID_NONE
        && avctx->codec_type != AVMEDIA_TYPE_DATA)
        FAIL("unknown codec");
    switch (avctx->codec_type) {
    case AVMEDIA_TYPE_AUDIO:
        if (!avctx->frame_size && determinable_frame_size(avctx))
            FAIL("unspecified frame size");
        if (st->info->found_decoder >= 0 &&
            avctx->sample_fmt == AV_SAMPLE_FMT_NONE)
            FAIL("unspecified sample format");
        if (!avctx->sample_rate)
            FAIL("unspecified sample rate");
        if (!avctx->channels)
            FAIL("unspecified number of channels");
        if (st->info->found_decoder >= 0 && !st->nb_decoded_frames && avctx->codec_id == AV_CODEC_ID_DTS)
            FAIL("no decodable DTS frames");
        break;
    case AVMEDIA_TYPE_VIDEO:
        if (!avctx->width)
            FAIL("unspecified size");
        if (st->info->found_decoder >= 0 && avctx->pix_fmt == AV_PIX_FMT_NONE)
            FAIL("unspecified pixel format");
        if (st->codecpar->codec_id == AV_CODEC_ID_RV30 || st->codecpar->codec_id == AV_CODEC_ID_RV40)
            if (!st->sample_aspect_ratio.num && !st->codecpar->sample_aspect_ratio.num && !st->codec_info_nb_frames)
                FAIL("no frame in rv30/40 and no sar");
        break;
    case AVMEDIA_TYPE_SUBTITLE:
        if (avctx->codec_id == AV_CODEC_ID_HDMV_PGS_SUBTITLE && !avctx->width)
            FAIL("unspecified size");
        break;
    case AVMEDIA_TYPE_DATA:
        if (avctx->codec_id == AV_CODEC_ID_NONE) return 1;
    }

    return 1;
}

part 8 计算rfps

ff_rfps_calculate(ic);

part9 再次遍历流

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;
                AVRational codec_frame_rate = avctx->framerate;

                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 (ic->internal->prefer_codec_framerate && codec_frame_rate.num > 0 && codec_frame_rate.den > 0) {
                        error       = fabs(av_q2d(codec_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) {
                    av_reduce(&st->r_frame_rate.num, &st->r_frame_rate.den,
                              avctx->time_base.den, (int64_t)avctx->time_base.num * avctx->ticks_per_frame, INT_MAX);
                } 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;
            }
        }
    }

这个循环用来处理音频流和视频流。
对视频流而言，首先会获得原始数据的图像格式，然后据此找到对应的编解码器的tag。之后会计算平均帧率。
对音频流而言，基于音频流的服务类型初始化disposition。disposition的作用暂时不知。

part 10 计算时间相关的参数

if (probesize)
estimate_timings(ic, old_offset);

static void estimate_timings(AVFormatContext *ic, int64_t old_offset)
{
    int64_t file_size;

    /* get the file size, if possible */
    if (ic->iformat->flags & AVFMT_NOFILE) {
        file_size = 0;
    } else {
        file_size = avio_size(ic->pb);
        file_size = FFMAX(0, file_size);
    }

    if ((!strcmp(ic->iformat->name, "mpeg") ||
         !strcmp(ic->iformat->name, "mpegts")) &&
        file_size && (ic->pb->seekable & AVIO_SEEKABLE_NORMAL)) {
        /* get accurate estimate from the PTSes */
        estimate_timings_from_pts(ic, old_offset);
        ic->duration_estimation_method = AVFMT_DURATION_FROM_PTS;
    } else if (has_duration(ic)) {
        /* at least one component has timings - we use them for all
         * the components */
        fill_all_stream_timings(ic);
        ic->duration_estimation_method = AVFMT_DURATION_FROM_STREAM;
    } else {
        /* less precise: use bitrate info */
        estimate_timings_from_bit_rate(ic);
        ic->duration_estimation_method = AVFMT_DURATION_FROM_BITRATE;
    }
    update_stream_timings(ic);

    {
        int i;
        AVStream av_unused *st;
        for (i = 0; i < ic->nb_streams; i++) {
            st = ic->streams[i];
            av_log(ic, AV_LOG_TRACE, "stream %d: start_time: %0.3f duration: %0.3f\n", i,
                   (double) st->start_time * av_q2d(st->time_base),
                   (double) st->duration   * av_q2d(st->time_base));
        }
        av_log(ic, AV_LOG_TRACE,
                "format: start_time: %0.3f duration: %0.3f bitrate=%"PRId64" kb/s\n",
                (double) ic->start_time / AV_TIME_BASE,
                (double) ic->duration   / AV_TIME_BASE,
                (int64_t)ic->bit_rate / 1000);
    }
}

从estimate_timings()的代码中可以看出，有3种估算方法：（1）通过pts（显示时间戳）。该方法调用estimate_timings_from_pts()。它的基本思想就是读取视音频流中的结束位置AVPacket的PTS和起始位置AVPacket的PTS，两者相减得到时长信息。(2）通过已知流的时长。该方法调用fill_all_stream_timings()。它的代码没有细看，但从函数的注释的意思来说，应该是当有些视音频流有时长信息的时候，直接赋值给其他视音频流。
（3）通过bitrate（码率）。该方法调用estimate_timings_from_bit_rate()。它的基本思想就是获得整个文件大小，以及整个文件的bitrate，两者相除之后得到时长信息。
在这里附上上述几种方法中最简单的函数estimate_timings_from_bit_rate()的代码。

static void estimate_timings_from_bit_rate(AVFormatContext *ic)
{
    int64_t filesize, duration;
    int i, show_warning = 0;
    AVStream *st;

    /* if bit_rate is already set, we believe it */
    if (ic->bit_rate <= 0) {
        int64_t bit_rate = 0;
        for (i = 0; i < ic->nb_streams; i++) {
            st = ic->streams[i];
            if (st->codecpar->bit_rate <= 0 && st->internal->avctx->bit_rate > 0)
                st->codecpar->bit_rate = st->internal->avctx->bit_rate;
            if (st->codecpar->bit_rate > 0) {
                if (INT64_MAX - st->codecpar->bit_rate < bit_rate) {
                    bit_rate = 0;
                    break;
                }
                bit_rate += st->codecpar->bit_rate;
            } else if (st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO && st->codec_info_nb_frames > 1) {
                // If we have a videostream with packets but without a bitrate
                // then consider the sum not known
                bit_rate = 0;
                break;
            }
        }
        ic->bit_rate = bit_rate;
    }

    /* if duration is already set, we believe it */
    if (ic->duration == AV_NOPTS_VALUE &&
        ic->bit_rate != 0) {
        filesize = ic->pb ? avio_size(ic->pb) : 0;
        if (filesize > ic->internal->data_offset) {
            filesize -= ic->internal->data_offset;
            for (i = 0; i < ic->nb_streams; i++) {
                st      = ic->streams[i];
                if (   st->time_base.num <= INT64_MAX / ic->bit_rate
                    && st->duration == AV_NOPTS_VALUE) {
                    duration = av_rescale(8 * filesize, st->time_base.den,
                                          ic->bit_rate *
                                          (int64_t) st->time_base.num);
                    st->duration = duration;
                    show_warning = 1;
                }
            }
        }
    }
    if (show_warning)
        av_log(ic, AV_LOG_WARNING,
               "Estimating duration from bitrate, this may be inaccurate\n");
}

从代码中可以看出，该函数做了两步工作：（1）如果AVFormatContext中没有bit_rate信息，就把所有AVStream的bit_rate加起来作为AVFormatContext的bit_rate信息。
（2）使用文件大小filesize除以bitrate得到时长信息。具体的方法是：

AVStream->duration=(filesize*8/bit_rate)/time_basePS：

1）filesize乘以8是因为需要把Byte转换为Bit
2）具体的实现函数是那个av_rescale()函数。x=av_rescale(a,b,c)的含义是x=a*b/c。
3）之所以要除以time_base，是因为AVStream中的duration的单位是time_base，注意这和AVFormatContext中的duration的单位（单位是AV_TIME_BASE，固定取值为1000000）是不一样的。

part 11 更新数据

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;
#if FF_API_LOWRES
            // The decoder might reduce the video size by the lowres factor.
            if (st->internal->avctx->lowres && orig_w) {
                st->codecpar->width = orig_w;
                st->codecpar->height = orig_h;
            }
#endif
        }

#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;

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

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

最后就是更新各个结构的数据了。AVStream中的AVCodecContext结构体，以及AVStream中AVStreamInternal中的AVCodecContext结构体的数据要统一起来。AVStream中的AVCodecParameters和AVStream中AVStreamInternal中的AVCodecContext结构体的数据也要统一起来。以上主要就是跟新这几个结构体中的数据。

总结

avformat_open_input函数会读文件头，对mp4文件而言，它会解析所有的box。但它只是把读到的结果保存在对应的数据结构下。这个时候，AVStream中的很多字段都是空白的。
avformat_find_stream_info则检测这些重要字段，如果是空白的，就设法填充它们。因为我们解析文件头的时候，已经掌握了大量的信息，avformat_find_stream_info就是通过这些信息来填充自己的成员，当重要的成员都填充完毕后，该函数就返回了。这中情况下，该函数效率很高。
但对于某些文件，单纯的从文件头中获取信息是不够的，比如vidoe的pix_fmt是需要调用h264_decode_frame才可以获取其pix_fmt的。因此，这个时候这个函数就会读一些数据进来，然后分析这些数据，并尝试解码这些数据，最终从中提取到所需要的信息。在所有的信息都已经获取到以后，avformat_find_stream_info函数会计算start_time，波特率等信息，其中时间相关的计算很复杂，没能看懂，以后再研究吧。计算结束后会更新相关结构体的数据成员。