ffmpeg学习七：avformat_find_stream_info函数源码分析

前面两篇文章分析avformat_open_input和avcodec_open2两个函数，我们所做的函数分析工作都是为了能够很好的理解前面一篇博客：ffmpeg学习四：写第一个程序-视频解码中所给的视频解码的程序。avformat_find_stream_info函数也是视频解码程序中必须要有的函数，因此这篇文章主要来分析这个函数。

一、功能简介

先看看avformat_find_stream_info函数的注释：

/*** 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。我们先浏览下这个结构体：

二、AVStream结构体解析

/*** Stream structure.* New fields can be added to the end with minor version bumps.* Removal, reordering and changes to existing fields require a major* version bump.* sizeof(AVStream) must not be used outside libav*.*/
typedef struct AVStream {int index;    /**< stream index in AVFormatContext *//*** Format-specific stream ID.* decoding: set by libavformat* encoding: set by the user, replaced by libavformat if left unset*/int id;
#if FF_API_LAVF_AVCTX/*** @deprecated use the codecpar struct instead*/attribute_deprecatedAVCodecContext *codec;
#endifvoid *priv_data;#if FF_API_LAVF_FRAC/*** @deprecated this field is unused*/attribute_deprecatedstruct AVFrac pts;
#endif/*** This is the fundamental unit of time (in seconds) in terms* of which frame timestamps are represented.** decoding: set by libavformat* encoding: May be set by the caller before avformat_write_header() to*           provide a hint to the muxer about the desired timebase. In*           avformat_write_header(), the muxer will overwrite this field*           with the timebase that will actually be used for the timestamps*           written into the file (which may or may not be related to the*           user-provided one, depending on the format).*/AVRational time_base;/*** Decoding: pts of the first frame of the stream in presentation order, in stream time base.* Only set this if you are absolutely 100% sure that the value you set* it to really is the pts of the first frame.* This may be undefined (AV_NOPTS_VALUE).* @note The ASF header does NOT contain a correct start_time the ASF* demuxer must NOT set this.*/int64_t start_time;/*** Decoding: duration of the stream, in stream time base.* If a source file does not specify a duration, but does specify* a bitrate, this value will be estimated from bitrate and file size.*/int64_t duration;int64_t nb_frames;                 ///< number of frames in this stream if known or 0int disposition; /**< AV_DISPOSITION_* bit field */enum AVDiscard discard; ///< Selects which packets can be discarded at will and do not need to be demuxed./*** sample aspect ratio (0 if unknown)* - encoding: Set by user.* - decoding: Set by libavformat.*/AVRational sample_aspect_ratio;AVDictionary *metadata;/*** Average framerate** - demuxing: May be set by libavformat when creating the stream or in*             avformat_find_stream_info().* - muxing: May be set by the caller before avformat_write_header().*/AVRational avg_frame_rate;/*** For streams with AV_DISPOSITION_ATTACHED_PIC disposition, this packet* will contain the attached picture.** decoding: set by libavformat, must not be modified by the caller.* encoding: unused*/AVPacket attached_pic;/*** An array of side data that applies to the whole stream (i.e. the* container does not allow it to change between packets).** There may be no overlap between the side data in this array and side data* in the packets. I.e. a given side data is either exported by the muxer* (demuxing) / set by the caller (muxing) in this array, then it never* appears in the packets, or the side data is exported / sent through* the packets (always in the first packet where the value becomes known or* changes), then it does not appear in this array.** - demuxing: Set by libavformat when the stream is created.* - muxing: May be set by the caller before avformat_write_header().** Freed by libavformat in avformat_free_context().** @see av_format_inject_global_side_data()*/AVPacketSideData *side_data;/*** The number of elements in the AVStream.side_data array.*/int            nb_side_data;/*** Flags for the user to detect events happening on the stream. Flags must* be cleared by the user once the event has been handled.* A combination of AVSTREAM_EVENT_FLAG_*.*/int event_flags;
#define AVSTREAM_EVENT_FLAG_METADATA_UPDATED 0x0001 ///< The call resulted in updated metadata./****************************************************************** All fields below this line are not part of the public API. They* may not be used outside of libavformat and can be changed and* removed at will.* New public fields should be added right above.******************************************************************//*** Stream information used internally by av_find_stream_info()*/
#define MAX_STD_TIMEBASES (30*12+30+3+6)struct {int64_t last_dts;int64_t duration_gcd;int duration_count;int64_t rfps_duration_sum;double (*duration_error)[2][MAX_STD_TIMEBASES];int64_t codec_info_duration;int64_t codec_info_duration_fields;/*** 0  -> decoder has not been searched for yet.* >0 -> decoder found* <0 -> decoder with codec_id == -found_decoder has not been found*/int found_decoder;int64_t last_duration;/*** Those are used for average framerate estimation.*/int64_t fps_first_dts;int     fps_first_dts_idx;int64_t fps_last_dts;int     fps_last_dts_idx;} *info;int pts_wrap_bits; /**< number of bits in pts (used for wrapping control) */// Timestamp generation support:/*** Timestamp corresponding to the last dts sync point.** Initialized when AVCodecParserContext.dts_sync_point >= 0 and* a DTS is received from the underlying container. Otherwise set to* AV_NOPTS_VALUE by default.*/int64_t first_dts;int64_t cur_dts;int64_t last_IP_pts;int last_IP_duration;/*** Number of packets to buffer for codec probing*/int probe_packets;/*** Number of frames that have been demuxed during av_find_stream_info()*/int codec_info_nb_frames;/* av_read_frame() support */enum AVStreamParseType need_parsing;struct AVCodecParserContext *parser;/*** last packet in packet_buffer for this stream when muxing.*/struct AVPacketList *last_in_packet_buffer;AVProbeData probe_data;
#define MAX_REORDER_DELAY 16int64_t pts_buffer[MAX_REORDER_DELAY+1];AVIndexEntry *index_entries; /**< Only used if the format does notsupport seeking natively. */int nb_index_entries;unsigned int index_entries_allocated_size;/*** Real base framerate of the stream.* This is the lowest framerate with which all timestamps can be* represented accurately (it is the least common multiple of all* framerates in the stream). Note, this value is just a guess!* For example, if the time base is 1/90000 and all frames have either* approximately 3600 or 1800 timer ticks, then r_frame_rate will be 50/1.** Code outside avformat should access this field using:* av_stream_get/set_r_frame_rate(stream)*/AVRational r_frame_rate;/*** Stream Identifier* This is the MPEG-TS stream identifier +1* 0 means unknown*/int stream_identifier;int64_t interleaver_chunk_size;int64_t interleaver_chunk_duration;/*** stream probing state* -1   -> probing finished*  0   -> no probing requested* rest -> perform probing with request_probe being the minimum score to accept.* NOT PART OF PUBLIC API*/int request_probe;/*** Indicates that everything up to the next keyframe* should be discarded.*/int skip_to_keyframe;/*** Number of samples to skip at the start of the frame decoded from the next packet.*/int skip_samples;/*** If not 0, the number of samples that should be skipped from the start of* the stream (the samples are removed from packets with pts==0, which also* assumes negative timestamps do not happen).* Intended for use with formats such as mp3 with ad-hoc gapless audio* support.*/int64_t start_skip_samples;/*** If not 0, the first audio sample that should be discarded from the stream.* This is broken by design (needs global sample count), but can't be* avoided for broken by design formats such as mp3 with ad-hoc gapless* audio support.*/int64_t first_discard_sample;/*** The sample after last sample that is intended to be discarded after* first_discard_sample. Works on frame boundaries only. Used to prevent* early EOF if the gapless info is broken (considered concatenated mp3s).*/int64_t last_discard_sample;/*** Number of internally decoded frames, used internally in libavformat, do not access* its lifetime differs from info which is why it is not in that structure.*/int nb_decoded_frames;/*** Timestamp offset added to timestamps before muxing* NOT PART OF PUBLIC API*/int64_t mux_ts_offset;/*** Internal data to check for wrapping of the time stamp*/int64_t pts_wrap_reference;/*** Options for behavior, when a wrap is detected.** Defined by AV_PTS_WRAP_ values.** If correction is enabled, there are two possibilities:* If the first time stamp is near the wrap point, the wrap offset* will be subtracted, which will create negative time stamps.* Otherwise the offset will be added.*/int pts_wrap_behavior;/*** Internal data to prevent doing update_initial_durations() twice*/int update_initial_durations_done;/*** Internal data to generate dts from pts*/int64_t pts_reorder_error[MAX_REORDER_DELAY+1];uint8_t pts_reorder_error_count[MAX_REORDER_DELAY+1];/*** Internal data to analyze DTS and detect faulty mpeg streams*/int64_t last_dts_for_order_check;uint8_t dts_ordered;uint8_t dts_misordered;/*** Internal data to inject global side data*/int inject_global_side_data;/*** String containing paris of key and values describing recommended encoder configuration.* Paris are separated by ','.* Keys are separated from values by '='.*/char *recommended_encoder_configuration;/*** display aspect ratio (0 if unknown)* - encoding: unused* - decoding: Set by libavformat to calculate sample_aspect_ratio internally*/AVRational display_aspect_ratio;struct FFFrac *priv_pts;/*** An opaque field for libavformat internal usage.* Must not be accessed in any way by callers.*/AVStreamInternal *internal;/** Codec parameters associated with this stream. Allocated and freed by* libavformat in avformat_new_stream() and avformat_free_context()* respectively.** - demuxing: filled by libavformat on stream creation or in*             avformat_find_stream_info()* - muxing: filled by the caller before avformat_write_header()*/AVCodecParameters *codecpar;
} AVStream;

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。

重要的数据结构框图如下：

下面来看这个函数的代码，源码在libavformat/utils.c文件中。
这个函数非常的长，我们分开来看.

三、函数源码分析

part 1 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 thoseint 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;...

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。

接着往下看。

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。
继续往下看：

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_WARNINGSif (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 stuffif (!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 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);}

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;}

可见就是根据编码器类型做对应的参数的拷贝。
4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");
#endifcodec = 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_WARNINGSif (st->codec->codec)return st->codec->codec;
FF_ENABLE_DEPRECATION_WARNINGS
#endifswitch (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。avcodec_find_decoder函数还是在utils.c文件中：

AVCodec *avcodec_find_decoder(enum AVCodecID id)
{return find_encdec(id, 0);
}

find_encdec函数也是在utils.c文件中：

static AVCodec *find_encdec(enum AVCodecID id, int encoder)
{AVCodec *p, *experimental = NULL;p = first_avcodec;id= remap_deprecated_codec_id(id);while (p) {if ((encoder ? av_codec_is_encoder(p) : av_codec_is_decoder(p)) &&p->id == id) {if (p->capabilities & AV_CODEC_CAP_EXPERIMENTAL && !experimental) {experimental = p;} elsereturn p;}p = p->next;}return experimental;
}

这个函数的功能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。
5.打开编解码器
打开使用的是avcodec_open2函数。这个函数我们在前一篇博客ffmpeg学习六：avcodec_open2函数源码分析一文中已经分析过了。

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_RATEic->streams[i]->info->last_dts = AV_NOPTS_VALUE;
#endific->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;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->internal->avctx->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);} elsest->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_RATEif (st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO)ff_rfps_add_frame(ic, st, pkt->dts);
#endifif (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++;}

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add_to_pktbuf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。
代码为：

            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;}

7.更新存储的dts的值。代码为：

            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;}

8.调用解析器的aplit方法。加入我们的解析器是h.264,那么这个解析器会像下面这样：

AVCodecParser ff_h264_parser = {.codec_ids      = { AV_CODEC_ID_H264 },.priv_data_size = sizeof(H264ParseContext),.parser_init    = init,.parser_parse   = h264_parse,.parser_close   = h264_close,.split          = h264_split,
};

这个split方法暂时看不懂，以后再来解析。
9.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) {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);}}}

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。

part 8 关闭可能可打开的编解码器

    for (i = 0; i < ic->nb_streams; i++) {st = ic->streams[i];avcodec_close(st->internal->avctx);}

再次遍历流，逐个调用avcodec_close方法来关闭流。

part 9 计算rfps

ff_rfps_calculate(ic);

part10 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;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 typeswitch (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 11 计算时间相关的参数

    if (probesize)estimate_timings(ic, old_offset);

estimate_timings函数如下：

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) {/* 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);}
}

主要是调用update_stream_timings函数更新流的时间相关的参数，之后的代码块是打印日志。
update_stream_timings函数定义在libavfomat/utils.c中。

/*** Estimate the stream timings from the one of each components.** Also computes the global bitrate if possible.*/
static void update_stream_timings(AVFormatContext *ic)
{int64_t start_time, start_time1, start_time_text, end_time, end_time1, end_time_text;int64_t duration, duration1, filesize;int i;AVStream *st;AVProgram *p;start_time = INT64_MAX;start_time_text = INT64_MAX;end_time   = INT64_MIN;end_time_text   = INT64_MIN;duration   = INT64_MIN;for (i = 0; i < ic->nb_streams; i++) {st = ic->streams[i];if (st->start_time != AV_NOPTS_VALUE && st->time_base.den) {start_time1 = av_rescale_q(st->start_time, st->time_base,AV_TIME_BASE_Q);if (st->codecpar->codec_type == AVMEDIA_TYPE_SUBTITLE || st->codecpar->codec_type == AVMEDIA_TYPE_DATA) {if (start_time1 < start_time_text)start_time_text = start_time1;} elsestart_time = FFMIN(start_time, start_time1);end_time1 = av_rescale_q_rnd(st->duration, st->time_base,AV_TIME_BASE_Q,AV_ROUND_NEAR_INF|AV_ROUND_PASS_MINMAX);if (end_time1 != AV_NOPTS_VALUE && (end_time1 > 0 ? start_time1 <= INT64_MAX - end_time1 : start_time1 >= INT64_MIN - end_time1)) {end_time1 += start_time1;if (st->codecpar->codec_type == AVMEDIA_TYPE_SUBTITLE || st->codecpar->codec_type == AVMEDIA_TYPE_DATA)end_time_text = FFMAX(end_time_text, end_time1);elseend_time = FFMAX(end_time, end_time1);}for (p = NULL; (p = av_find_program_from_stream(ic, p, i)); ) {if (p->start_time == AV_NOPTS_VALUE || p->start_time > start_time1)p->start_time = start_time1;if (p->end_time < end_time1)p->end_time = end_time1;}}if (st->duration != AV_NOPTS_VALUE) {duration1 = av_rescale_q(st->duration, st->time_base,AV_TIME_BASE_Q);duration  = FFMAX(duration, duration1);}}if (start_time == INT64_MAX || (start_time > start_time_text && start_time - start_time_text < AV_TIME_BASE))start_time = start_time_text;else if (start_time > start_time_text)av_log(ic, AV_LOG_VERBOSE, "Ignoring outlier non primary stream starttime %f\n", start_time_text / (float)AV_TIME_BASE);if (end_time == INT64_MIN || (end_time < end_time_text && end_time_text - end_time < AV_TIME_BASE)) {end_time = end_time_text;} else if (end_time < end_time_text) {av_log(ic, AV_LOG_VERBOSE, "Ignoring outlier non primary stream endtime %f\n", end_time_text / (float)AV_TIME_BASE);}if (start_time != INT64_MAX) {ic->start_time = start_time;if (end_time != INT64_MIN) {if (ic->nb_programs > 1) {for (i = 0; i < ic->nb_programs; i++) {p = ic->programs[i];if (p->start_time != AV_NOPTS_VALUE && p->end_time > p->start_time)duration = FFMAX(duration, p->end_time - p->start_time);}} elseduration = FFMAX(duration, end_time - start_time);}}if (duration != INT64_MIN && duration > 0 && ic->duration == AV_NOPTS_VALUE) {ic->duration = duration;}if (ic->pb && (filesize = avio_size(ic->pb)) > 0 && ic->duration > 0) {/* compute the bitrate */double bitrate = (double) filesize * 8.0 * AV_TIME_BASE /(double) ic->duration;if (bitrate >= 0 && bitrate <= INT64_MAX)ic->bit_rate = bitrate;}
}

时间相关的计算非常复杂，是在看不懂。这里还计算了波特率： bitrate = (double) filesize * 8.0 * AV_TIME_BASE / (double) ic->duration;
这里很好理解。filesize*8是这个文件的所有bit数。一字节=8bit。
然后比特率=bit数/时间。
这里时间为ic->duration/AV_TIME_BASE 。

part 12 更新数据

   /* 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_WARNINGSret = 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->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 AVCodecParametersst->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
#endifst->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。

四、总结

最后总结一下吧：
avformat_open_input函数会读文件头，对mp4文件而言，它会解析所有的box。但它知识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。
虽然由于能力限制，没能够彻底搞懂这个函数的方方面面，但是，通过这次分析，更好的理解了这个函数的功能，对这个函数做的事情有了一定的了解。这也是这次分析的主要收获了，至于那些搞不懂的，只能留着以后慢慢研究了。