• FFMPEG + SDL音频播放分析

    抽象流程：

    设置SDL的音频参数 —-> 打开声音设备，播放静音 —-> ffmpeg读取音频流中数据放入队列 —-> SDL调用用户设置的函数来获取音频数据 —-> 播放音频

    SDL内部维护了一个buffer来存放解码后的数据，这个buffer中的数据来源是我们注册的回调函数（audio_callback），audio_callback调用audio_decode_frame来做具体的音频解码工作，需要引起注意的是：从流中读取出的一个音频包(avpacket)可能含有多个音频桢(avframe)，所以需要多次调用avcodec_decode_audio4来完成整个包的解码，解码出来的数据存放在我们自己的缓冲中（audio_buf2）。SDL每一次回调都会引起数据从audio_buf2拷贝到SDL内部缓冲区，当audio_buf2中的数据大于SDL的缓冲区大小时，需要分多次拷贝。

    关键实现：

    main()函数

    1 int main(int argc, char **argv){
    2     SDL_Event event; //SDL事件变量
    3     VideoState    *is; // 纪录视频及解码器等信息的大结构体
    4     is = (VideoState*) av_mallocz(sizeof(VideoState));
    5     if(argc < 2){
    6         fprintf(stderr, "Usage: play <file> ");
    7         exit(1);
    8     }
    9     av_register_all(); //注册所有ffmpeg的解码器
    10     /* 初始化SDL，这里只实用了AUDIO，如果有视频，好需要SDL_INIT_VIDEO等等 */
    11     if(SDL_Init(SDL_INIT_AUDIO)){
    12         fprintf(stderr, "Count not initialize SDL - %s ", SDL_GetError());
    13         exit(1);
    14     }
    15     is_strlcpy(is->filename, argv[1], sizeof(is->filename));
    16     /* 创建一个SDL线程来做视频解码工作，主线程进入SDL事件循环 */
    17     is->parse_tid = SDL_CreateThread(decode_thread, is);
    18     if(!is->parse_tid){
    19         SDL_WaitEvent(&event);
    20         switch(event.type){
    21             case FF_QUIT_EVENT:
    22             case SDL_QUIT:
    23                  is->quit = 1;
    24                 SDL_Quit();
    25                 exit(0);
    26                 break;
    27             default:
    28                  break;
    29         }
    30     }
    31     return 0;
    32 }

    decode_thread()读取文件信息和音频包

    1 static int decode_thread(void *arg){
    2     VideoState *is = (VideoState*)arg;
    3     AVFormatContext *ic = NULL;
    4     AVPacket pkt1, *packet = &pkt1;
    5     int ret, i, audio_index = -1;
    6  
    7     is->audioStream = -1;
    8     global_video_state = is;
    9     /*  使用ffmpeg打开视频，解码器等 常规工作 */
    10     if(avFormat_open_input(&ic, is->filename, NULL,  NULL) != 0)  {
    11         fprintf(stderr, "open file error: %s ", is->filename);
    12         return -1;
    13     }
    14     is->ic = ic;
    15     if(avformat_find_stream_info(ic, NULL) < 0){
    16         fprintf(stderr, "find stream info error ");
    17         return -1;
    18     }
    19     av_dump_format(ic, 0, is->filename, 0);
    20     for(i  = 0; i < ic->nb_streams; i++){
    21          if(ic->streams[i])->codec->codec_type == AVMEDIA_TYPE_AUDIO && audio_index == -1){
    22             audio_index = i;
    23             break;
    24         }
    25     }
    26     if(audio_index >= 0) {
    27         /* 所有设置SDL音频流信息的步骤都在这个函数里完成 */
    28         stream_component_open(is, audio_index);
    29     }
    30     if(is->audioStream < 0){
    31         fprintf(stderr, "could not open codecs for file: %s ", is->filename);
    32         goto fail;
    33     }
    34     /* 读包的主循环， av_read_frame不停的从文件中读取数据包（这里只取音频包）*/
    35     for(;;){
    36         if(is->quit) break;
    37         /* 这里audioq.size是指队列中的所有数据包带的音频数据的总量，并不是包的数量 */
    38         if(is->audioq.size > MAX_AUDIO_SIZE){
    39             SDL_Delay(10); // 毫秒
    40             continue;
    41         }
    42          ret = av_read_frame(is->ic, packet);
    43          if(ret < 0){
    44                 if(ret == AVERROR_EOF || url_feof(is->ic->pb))    break;
    45                 if(is->ic->pb && is->ic->pb->error)    break;
    46                 contiue;                 
    47           } 
    48           if(packet->stream_index == is->audioStream){
    49                     packet_queue_put(&is->audioq, packet);
    50            } else{
    51                      av_free_packet(packet);
    52             }
    53     }
    54      while(!is->quit)    SDL_Delay(100);
    55 fail: {
    56                SDL_Event event;
    57                event.type = FF_QUIT_EVENT;
    58                event.user.data1 = is;
    59                SDL_PushEvent(&event);
    60         }
    61         return 0;
    62 }

    stream_component_open()：设置音频参数和打开设备

    1 int stream_component_open(videoState *is, int stream_index){
    2     AVFormatContext *ic = is->ic;
    3     AVCodecContext *codecCtx;
    4     AVCodec *codec;
    5     /* 在用SDL_OpenAudio()打开音频设备的时候需要这两个参数*/
    6     /* wanted_spec是我们期望设置的属性，spec是系统最终接受的参数 */
    7     /* 我们需要检查系统接受的参数是否正确 */
    8     SDL_AudioSpec wanted_spec, spec;
    9     int64_t wanted_channel_layout = 0; // 声道布局(SDL中的具体定义见“FFMPEG结构体”部分)
    10     int wanted_nb_channels; // 声道数
    11     /*  SDL支持的声道数为 1, 2, 4, 6 */
    12     /*  后面我们会使用这个数组来纠正不支持的声道数目 */
    13     const int next_nb_channels[] = { 0, 0, 1, 6,  2, 6, 4, 6 };
    14  
    15     if(stream_index < 0 || stream_index >= ic->nb_streams)    return -1;
    16     codecCtx = ic->streams[stream_index]->codec;
    17     wanted_nb_channels = codecCtx->channels;
    18     if(!wanted_channel_layout || wanted_nb_channels != av_get_channel_layout_nb_channels(wanted_channel_layout)) {
    19         wanted_channel_layout = av_get_default_channel_lauout(wanted_channel_nb_channels);
    20         wanted_channel_layout &= ~AV_CH_LAYOUT_STEREO_DOWNMIX;
    21     }
    22     wanted_spec.channels = av_get_channels_layout_nb_channels(wanted_channel_layout);
    23     wanted_spec.freq = codecCtx->sample_rate;
    24     if(wanted_spec.freq <= 0 || wanted_spec.channels <=0){
    25            fprintf(stderr, "Invaild sample rate or channel count! ");
    26             return -1;
    27     }
    28     wanted_spec.format = AUDIO_S16SYS; // 具体含义请查看“SDL宏定义”部分
    29     wanted_spec.silence = 0; // 0指示静音
    30     wanted_spec.samples = SDL_AUDIO_BUFFER_SIZE; // 自定义SDL缓冲区大小
    31     wanted_spec.callback = audio_callback; // 音频解码的关键回调函数
    32     wanted_spec.userdata = is; // 传给上面回调函数的外带数据
    33  
    34     /*  打开音频设备，这里使用一个while来循环尝试打开不同的声道数(由上面 */
    35     /*  next_nb_channels数组指定）直到成功打开，或者全部失败 */
    36     while(SDL_OpenAudio(&wanted_spec, &spec) < 0){
    37         fprintf(stderr, "SDL_OpenAudio(%d channels): %s ", wanted_spec.channels, SDL_GetError());
    38         wanted_spec.channels = next_nb_channels[FFMIN(7, wanted_spec.channels)]; // FFMIN()由ffmpeg定义的宏，返回较小的数
    39         if(!wanted_spec.channels){
    40               fprintf(stderr, "No more channel to try ");
    41               return -1;
    42         }
    43         wanted_channel_layout = av_get_default_channel_layout(wanted_spec.channels);
    44     }
    45     /* 检查实际使用的配置（保存在spec,由SDL_OpenAudio()填充） */
    46     if(spec.format != AUDIO_S16SYS){
    47         fprintf(stderr, "SDL advised audio format %d is not supported ", spec.format);
    48         return -1;
    49     }
    50     if(spec.channels != wanted_spec.channels) {
    51         wanted_channel_layout = av_get_default_channel_layout(spec.channels);
    52         if(!wanted_channel_layout){
    53                 fprintf(stderr, "SDL advised channel count %d is not support ", spec.channels);
    54                 return -1;
    55         }
    56     }
    57     /* 把设置好的参数保存到大结构中 */
    58     is->audio_src_fmt = is->audio_tgt_fmt = AV_SAMPLE_FMT_S16;
    59     is->audio_src_freq = is->audio_tgt_freq = spec.freq;
    60     is->audio_src_channel_layout = is->audio_tgt_layout = wanted_channel_layout;
    61     is->audio_src_channels = is->audio_tat_channels = spec.channels;
    62  
    63     codec = avcodec_find_decoder(codecCtx>codec_id);
    64     if(!codec || (avcodec_open2(codecCtx, codec, NULL) < 0)){
    65         fprintf(stderr, "Unsupported codec! ");
    66         return -1;
    67     }
    68     ic->streams[stream_index]->discard = AVDISCARD_DEFAULT; //具体含义请查看“FFMPEG宏定义”部分
    69     is->audioStream = stream_index;
    70     is->audio_st = ic->streams[stream_index];
    71     is->audio_buf_size = 0;
    72     is->audio_buf_index = 0;
    73     memset(&is->audio_pkt, 0, sizeof(is->audio_pkt));
    74     packet_queue_init(&is->audioq);
    75     SDL_PauseAudio(0); // 开始播放静音
    76 }

    audio_callback(): 回调函数，向SDL缓冲区填充数据

    1 void audio_callback(void *userdata, Uint8 *stream, int len){
    2     VideoState *is = (VideoState*)userdata;
    3     int len1, audio_data_size;
    4  
    5     /*   len是由SDL传入的SDL缓冲区的大小，如果这个缓冲未满，我们就一直往里填充数据 */
    6     while(len > 0){
    7         /*  audio_buf_index 和 audio_buf_size 标示我们自己用来放置解码出来的数据的缓冲区，*/
    8         /*   这些数据待copy到SDL缓冲区， 当audio_buf_index >= audio_buf_size的时候意味着我*/
    9         /*   们的缓冲为空，没有数据可供copy，这时候需要调用audio_decode_frame来解码出更
    10         /*   多的桢数据 */
    11         if(is->audio_buf_index >= is->audio_buf_size){
    12                 audio_data_size = audio_decode_frame(is);
    13                 /* audio_data_size < 0 标示没能解码出数据，我们默认播放静音 */
    14                 is(audio_data_size < 0){
    15                          is->audio_buf_size = 1024;
    16                          /* 清零，静音 */
    17                          memset(is->audio_buf, 0, is->audio_buf_size);
    18                 } else{
    19                           is->audio_buf_size = audio_data_size;
    20                  }
    21                  is->audio_buf_index = 0;
    22         }
    23         /*  查看stream可用空间，决定一次copy多少数据，剩下的下次继续copy */
    24         len1 = is->audio_buf_size - is->audio_buf_index;
    25         if(len1 > len)    len1 = len;
    26  
    27         memcpy(stream, (uint8_t*)is->audio_buf + is->audio_buf_index, len1);
    28         len -= len1;
    29         stream += len1;
    30         is->audio_buf_index += len1;
    31     }
    32 }

    audio_decode_frame()：解码音频

    1 int audio_decode_frame(VideoState *is){
    2     int len1, len2, decoded_data_size;
    3     AVPacket *pkt = &is->audio_pkt;
    4     int got_frame = 0;
    5     int64_t dec_channel_layout;
    6     int wanted_nb_samples, resampled_data_size;
    7  
    8     for(;;){
    9       while(is->audio_pkt_size > 0){
    10         if(!is->audio_frame){
    11             if(!(is->audio_frame = avacodec_alloc_frame())){
    12                 return AVERROR(ENOMEM);
    13             }
    14         } else
    15           avcodec_get_frame_defaults(is->audio_frame);
    16  
    17         len1 = avcodec_decode_audio4(is->audio_st_codec, is->audio_frame, got_frame, pkt);
    18         /* 解码错误，跳过整个包 */
    19         if(len1 < 0){
    20            is->audio_pkt_size = 0;
    21            break;
    22         }
    23         is->audio_pkt_data += len1;
    24         is->audio_pkt_size -= len1;
    25         if(!got_frame)   continue;
    26         /* 计算解码出来的桢需要的缓冲大小 */
    27         decoded_data_size = av_samples_get_buffer_size(NULL,
    28                             is->audio_frame_channels,
    29                             is->audio_frame_nb_samples,
    30                             is->audio_frame_format, 1);
    31         dec_channel_layout = (is->audio_frame->channel_layout && is->audio_frame->channels
    32                    == av_get_channel_layout_nb_channels(is->audio_frame->channel_layout))
    33                    ? is->audio_frame->channel_layout : av_get_default_channel_layout(is->audio_frame->channels);                      
    34         wanted_nb_samples =  is->audio_frame->nb_samples;
    35         if (is->audio_frame->format != is->audio_src_fmt ||
    36             dec_channel_layout != is->audio_src_channel_layout ||
    37             is->audio_frame->sample_rate != is->audio_src_freq ||
    38             (wanted_nb_samples != is->audio_frame->nb_samples && !is->swr_ctx)) {
    39                 if (is->swr_ctx) swr_free(&is->swr_ctx);
    40                 is->swr_ctx = swr_alloc_set_opts(NULL,
    41                                                  is->audio_tgt_channel_layout,
    42                                                  is->audio_tgt_fmt,
    43                                                  is->audio_tgt_freq,
    44                                                  dec_channel_layout,
    45                                                  is->audio_frame->format,
    46                                                  is->audio_frame->sample_rate,
    47                                                  0, NULL);
    48                  if (!is->swr_ctx || swr_init(is->swr_ctx) < 0) {
    49                      fprintf(stderr, "swr_init() failed ");
    50                      break;
    51                  }
    52                  is->audio_src_channel_layout = dec_channel_layout;
    53                  is->audio_src_channels = is->audio_st->codec->channels;
    54                  is->audio_src_freq = is->audio_st->codec->sample_rate;
    55                  is->audio_src_fmt = is->audio_st->codec->sample_fmt;
    56          }
    57          /* 这里我们可以对采样数进行调整，增加或者减少，一般可以用来做声画同步 */
    58          if (is->swr_ctx) {
    59              const uint8_t **in = (const uint8_t **)is->audio_frame->extended_data;
    60              uint8_t *out[] = { is->audio_buf2 };
    61              if (wanted_nb_samples != is->audio_frame->nb_samples) {
    62                 if(swr_set_compensation(is->swr_ctx,
    63                   (wanted_nb_samples - is->audio_frame->nb_samples)*is->audio_tgt_freq/is->audio_frame->sample_rate,
    64                    wanted_nb_samples * is->audio_tgt_freq/is->audio_frame->sample_rate) < 0) {
    65                         fprintf(stderr, "swr_set_compensation() failed ");
    66                         break;
    67                    }
    68              }
    69              len2 = swr_convert(is->swr_ctx, out, 
    70                   sizeof(is->audio_buf2)/is->audio_tgt_channels/av_get_bytes_per_sample(is->audio_tgt_fmt), 
    71                   in, is->audio_frame->nb_samples);
    72              if (len2 < 0) {
    73                   fprintf(stderr, "swr_convert() failed ");
    74                   break;
    75              }
    76              if(len2 == sizeof(is->audio_buf2)/is->audio_tgt_channels/av_get_bytes_per_sample(is->audio_tgt_fmt)) {
    77                  fprintf(stderr, "warning: audio buffer is probably too small ");
    78                  swr_init(is->swr_ctx);
    79              }
    80              is->audio_buf = is->audio_buf2;
    81              resampled_data_size = len2*is->audio_tgt_channels*av_get_bytes_per_sample(is->audio_tgt_fmt);
    82            } else {
    83              resampled_data_size = decoded_data_size;
    84              is->audio_buf = is->audio_frame->data[0];
    85            }
    86            /*  返回得到的数据 */
    87            return resampled_data_size;
    88        }
    89        if (pkt->data) av_free_packet(pkt);
    90        memset(pkt, 0, sizeof(*pkt));
    91        if (is->quit) return -1;
    92        if (packet_queue_get(&is->audioq, pkt, 1) < 0) return -1;
    93        is->audio_pkt_data = pkt->data;
    94        is->audio_pkt_size = pkt->size;
    95  
    96      }
    97 }

    FFMPEG结构体

    channel_layout_map

    1 static const struct {
    2 const char *name;
    3 int nb_channels;
    4 uint64_t layout;
    5 } channel_layout_map[] = {
    6 { "mono", 1, AV_CH_LAYOUT_MONO },
    7 { "stereo", 2, AV_CH_LAYOUT_STEREO },
    8 { "2.1", 3, AV_CH_LAYOUT_2POINT1 },
    9 { "3.0", 3, AV_CH_LAYOUT_SURROUND },
    10 { "3.0(back)", 3, AV_CH_LAYOUT_2_1 },
    11 { "4.0", 4, AV_CH_LAYOUT_4POINT0 },
    12 { "quad", 4, AV_CH_LAYOUT_QUAD },
    13 { "quad(side)", 4, AV_CH_LAYOUT_2_2 },
    14 { "3.1", 4, AV_CH_LAYOUT_3POINT1 },
    15 { "5.0", 5, AV_CH_LAYOUT_5POINT0_BACK },
    16 { "5.0(side)", 5, AV_CH_LAYOUT_5POINT0 },
    17 { "4.1", 5, AV_CH_LAYOUT_4POINT1 },
    18 { "5.1", 6, AV_CH_LAYOUT_5POINT1_BACK },
    19 { "5.1(side)", 6, AV_CH_LAYOUT_5POINT1 },
    20 { "6.0", 6, AV_CH_LAYOUT_6POINT0 },
    21 { "6.0(front)", 6, AV_CH_LAYOUT_6POINT0_FRONT },
    22 { "hexagonal", 6, AV_CH_LAYOUT_HEXAGONAL },
    23 { "6.1", 7, AV_CH_LAYOUT_6POINT1 },
    24 { "6.1", 7, AV_CH_LAYOUT_6POINT1_BACK },
    25 { "6.1(front)", 7, AV_CH_LAYOUT_6POINT1_FRONT },
    26 { "7.0", 7, AV_CH_LAYOUT_7POINT0 },
    27 { "7.0(front)", 7, AV_CH_LAYOUT_7POINT0_FRONT },
    28 { "7.1", 8, AV_CH_LAYOUT_7POINT1 },
    29 { "7.1(wide)", 8, AV_CH_LAYOUT_7POINT1_WIDE },
    30 { "octagonal", 8, AV_CH_LAYOUT_OCTAGONAL },
    31 { "downmix", 2, AV_CH_LAYOUT_STEREO_DOWNMIX, },
    32 };

    FFMPEG宏定义

    Audio channel convenience macros

    1 #define AV_CH_LAYOUT_MONO              (AV_CH_FRONT_CENTER)
    2  #define AV_CH_LAYOUT_STEREO            (AV_CH_FRONT_LEFT|AV_CH_FRONT_RIGHT)
    3  #define AV_CH_LAYOUT_2POINT1           (AV_CH_LAYOUT_STEREO|AV_CH_LOW_FREQUENCY)
    4  #define AV_CH_LAYOUT_2_1               (AV_CH_LAYOUT_STEREO|AV_CH_BACK_CENTER)
    5  #define AV_CH_LAYOUT_SURROUND          (AV_CH_LAYOUT_STEREO|AV_CH_FRONT_CENTER)
    6  #define AV_CH_LAYOUT_3POINT1           (AV_CH_LAYOUT_SURROUND|AV_CH_LOW_FREQUENCY)
    7  #define AV_CH_LAYOUT_4POINT0           (AV_CH_LAYOUT_SURROUND|AV_CH_BACK_CENTER)
    8  #define AV_CH_LAYOUT_4POINT1           (AV_CH_LAYOUT_4POINT0|AV_CH_LOW_FREQUENCY)
    9  #define AV_CH_LAYOUT_2_2               (AV_CH_LAYOUT_STEREO|AV_CH_SIDE_LEFT|AV_CH_SIDE_RIGHT)
    10  #define AV_CH_LAYOUT_QUAD              (AV_CH_LAYOUT_STEREO|AV_CH_BACK_LEFT|AV_CH_BACK_RIGHT)
    11  #define AV_CH_LAYOUT_5POINT0           (AV_CH_LAYOUT_SURROUND|AV_CH_SIDE_LEFT|AV_CH_SIDE_RIGHT)
    12  #define AV_CH_LAYOUT_5POINT1           (AV_CH_LAYOUT_5POINT0|AV_CH_LOW_FREQUENCY)
    13  #define AV_CH_LAYOUT_5POINT0_BACK      (AV_CH_LAYOUT_SURROUND|AV_CH_BACK_LEFT|AV_CH_BACK_RIGHT)
    14  #define AV_CH_LAYOUT_5POINT1_BACK      (AV_CH_LAYOUT_5POINT0_BACK|AV_CH_LOW_FREQUENCY)
    15  #define AV_CH_LAYOUT_6POINT0           (AV_CH_LAYOUT_5POINT0|AV_CH_BACK_CENTER)
    16  #define AV_CH_LAYOUT_6POINT0_FRONT     (AV_CH_LAYOUT_2_2|AV_CH_FRONT_LEFT_OF_CENTER|AV_CH_FRONT_RIGHT_OF_CENTER)
    17  #define AV_CH_LAYOUT_HEXAGONAL         (AV_CH_LAYOUT_5POINT0_BACK|AV_CH_BACK_CENTER)
    18  #define AV_CH_LAYOUT_6POINT1           (AV_CH_LAYOUT_5POINT1|AV_CH_BACK_CENTER)
    19  #define AV_CH_LAYOUT_6POINT1_BACK      (AV_CH_LAYOUT_5POINT1_BACK|AV_CH_BACK_CENTER)
    20  #define AV_CH_LAYOUT_6POINT1_FRONT     (AV_CH_LAYOUT_6POINT0_FRONT|AV_CH_LOW_FREQUENCY)
    21  #define AV_CH_LAYOUT_7POINT0           (AV_CH_LAYOUT_5POINT0|AV_CH_BACK_LEFT|AV_CH_BACK_RIGHT)
    22  #define AV_CH_LAYOUT_7POINT0_FRONT     (AV_CH_LAYOUT_5POINT0|AV_CH_FRONT_LEFT_OF_CENTER|AV_CH_FRONT_RIGHT_OF_CENTER)
    23  #define AV_CH_LAYOUT_7POINT1           (AV_CH_LAYOUT_5POINT1|AV_CH_BACK_LEFT|AV_CH_BACK_RIGHT)
    24 #define AV_CH_LAYOUT_7POINT1_WIDE      (AV_CH_LAYOUT_5POINT1|AV_CH_FRONT_LEFT_OF_CENTER|AV_CH_FRONT_RIGHT_OF_CENTER)
    25 #define AV_CH_LAYOUT_7POINT1_WIDE_BACK (AV_CH_LAYOUT_5POINT1_BACK|AV_CH_FRONT_LEFT_OF_CENTER|AV_CH_FRONT_RIGHT_OF_CENTER)
    26 #define AV_CH_LAYOUT_OCTAGONAL         (AV_CH_LAYOUT_5POINT0|AV_CH_BACK_LEFT|AV_CH_BACK_CENTER|AV_CH_BACK_RIGHT)
    27 #define AV_CH_LAYOUT_STEREO_DOWNMIX    (AV_CH_STEREO_LEFT|AV_CH_STEREO_RIGHT)

    SDL宏定义

    SDL_AudioSpec format

     
    1 AUDIO_U8           Unsigned 8-bit samples
    2 AUDIO_S8            Signed 8-bit samples
    3 AUDIO_U16LSB    Unsigned 16-bit samples, in little-endian byte order
    4 AUDIO_S16LSB    Signed 16-bit samples, in little-endian byte order
    5 AUDIO_U16MSB    Unsigned 16-bit samples, in big-endian byte order
    6 AUDIO_S16MSB    Signed 16-bit samples, in big-endian byte order
    7 AUDIO_U16           same as AUDIO_U16LSB (for backwards compatability probably)
    8 AUDIO_S16           same as AUDIO_S16LSB (for backwards compatability probably)
    9 AUDIO_U16SYS    Unsigned 16-bit samples, in system byte order
    10 AUDIO_S16SYS     Signed 16-bit samples, in system byte order

    git clone https://github.com/lnmcc/musicPlayer.git
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  • 原文地址：https://www.cnblogs.com/subo_peng/p/5529128.html
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