av_register_all();//注册所有可解码类型
AVFormatContext *pInFmtCtx=NULL;//文件格式
AVCodecContext *pInCodecCtx=NULL;//编码格式
if (av_open_input_file(&pInFmtCtx,filename,NULL, 0, NULL)!=0)//获取文件格式
    printf("av_open_input_file error\n");
if(av_find_stream_info(pInFmtCtx) < 0)//获取文件内音视频流的信息
    printf("av_find_stream_info error\n");

unsigned int j;
// Find the first audio stream

int    audioStream = -1;
for(j=0; j<pInFmtCtx->nb_streams; j++)//找到音频对应的stream
    if(pInFmtCtx->streams[j]->codec->codec_type==CODEC_TYPE_AUDIO)
    {
        audioStream=j;
        break;
    }
    if(audioStream==-1)
    {
        printf("input file has no audio stream\n");
        return 0; // Didn't find a audio stream

    }
    printf("audio stream num: %d\n",audioStream);
    pInCodecCtx = pInFmtCtx->streams[audioStream]->codec;//音频的编码上下文
    AVCodec *pInCodec=NULL;
    /* FILE *file3 = fopen("codec_private_data_size.txt","w");
    for(int i = 0; i <200000; i++)
    {
    pInCodec = avcodec_find_decoder((CodecID)i);
    if (pInCodec!=NULL)
    {
    fprintf(file3,"%s %d\n",pInCodec->name,pInCodec->priv_data_size );
    }
    }
    fclose(file3);
    system("pause");
    */
    pInCodec = avcodec_find_decoder(pInCodecCtx->codec_id);//根据编码ID找到用于解码的结构体
    if(pInCodec==NULL)
    {
        printf("error no Codec found\n");
        return -1 ; // Codec not found
    }


    //使用test的代替pInCodecCtx也可以完成解码，可以看出只要获取以下几个重要信息就可以实现解码和重采样
    AVCodecContext *test = avcodec_alloc_context();
    test->bit_rate = pInCodecCtx->bit_rate;//重采样用
    test->sample_rate = pInCodecCtx->sample_rate;//重采样用
    test->channels = pInCodecCtx->channels;//重采样用
    test->extradata = pInCodecCtx->extradata;//若有则必有
    test->extradata_size = pInCodecCtx->extradata_size;//若有则必要
    test->codec_type = CODEC_TYPE_AUDIO;//不必要
    test->block_align = pInCodecCtx->block_align ;//必要

    
    if(avcodec_open(test, pInCodec)<0)//将两者结合以便在下面的解码函数中调用pInCodec中的对应解码函数
    {
        printf("error avcodec_open failed.\n");
        return -1; // Could not open codec

    }
    
    if(avcodec_open(pInCodecCtx, pInCodec)<0)
    {
        printf("error avcodec_open failed.\n");
        return -1; // Could not open codec

    }
    
    static AVPacket packet;
    
    printf(" bit_rate = %d \r\n", pInCodecCtx->bit_rate);
    printf(" sample_rate = %d \r\n", pInCodecCtx->sample_rate);
    printf(" channels = %d \r\n", pInCodecCtx->channels);
    printf(" code_name = %s \r\n", pInCodecCtx->codec->name);
    //printf("extra data size: %d :data%x %x %x %x\n",pInCodecCtx->extradata_size,pInCodecCtx->extradata[0]
    //     ,pInCodecCtx->extradata[1],pInCodecCtx->extradata[2],pInCodecCtx->extradata[3]);
    printf(" block_align = %d\n",pInCodecCtx->block_align);
    
    //system("pause");
    //////////////////////////////////////////////////////////////////////////

    uint8_t *pktdata;
    int pktsize;
    int out_size = AVCODEC_MAX_AUDIO_FRAME_SIZE*100;
    uint8_t * inbuf = (uint8_t *)malloc(out_size);
    FILE* pcm,*packetinfo;
    packetinfo = fopen("packetinfo.txt","w");
    pcm = fopen("result.pcm","wb");
    long start = clock();
    while(av_read_frame(pInFmtCtx, &packet)>=0)//pInFmtCtx中调用对应格式的packet获取函数
    {
        //fprintf(packetinfo," packet { pts=%d; dts =%d;%x,%x,%x,%x; size=%d; stream_index=%d; pos=%d;}\n",

        // packet.pts,packet.dts,packet.data[0],packet.data[1],packet.data[2],packet.data[3],packet.size,packet.stream_index,packet.pos);

        if(packet.stream_index==audioStream)//如果是音频
        {
            pktdata = packet.data;
            pktsize = packet.size;
            while(pktsize>0)
            {
                //fprintf(packetinfo,"packet data:%x %x %x %x %x\n",pktdata[0],pktdata[1],pktdata[2],pktdata[3],pktdata[4]);

                //fprintf(packetinfo,"packet size:%d\n\n",pktsize);

                out_size = AVCODEC_MAX_AUDIO_FRAME_SIZE*100;
                //解码
                int len = avcodec_decode_audio2(pInCodecCtx,(short*)inbuf,&out_size,pktdata,pktsize);
                if (len<0)
                {
                    printf("Error while decoding.\n");
                    break;
                }
                if(out_size>0)
                {
                    fwrite(inbuf,1,out_size,pcm);//pcm记录
                    fflush(pcm);
                }
                pktsize -= len;
                pktdata += len;
            }
        }
        av_free_packet(&packet);
    }
    long end = clock();
    printf("cost time :%f\n",double(end-start)/(double)CLOCKS_PER_SEC);
    free(inbuf);
    fclose(pcm);
    fclose(packetinfo);
    if (pInCodecCtx!=NULL)
    {
        avcodec_close(pInCodecCtx);
    }
    if (test!=NULL)
    {
        avcodec_close(test);
    }
    av_free(test);
    av_close_input_file(pInFmtCtx);
    return 0; }