int pcm2aac()
{
    AVFormatContext* pFormatCtx;
    AVOutputFormat* fmt;
    AVStream* audio_st;//音频流
    AVCodecContext* pCodecCtx;
    AVCodec* pCodec;

    uint8_t* frame_buf;
    AVFrame* frame;
    int size;

    FILE *in_file = fopen("tdjm.pcm", "rb");    //音频PCM采样数据
    int framenum=1000;  //音频帧数
    const char* out_file = "tdjm.aac";      //输出文件路径


    av_register_all();

    //方法1.组合使用几个函数
    pFormatCtx = avformat_alloc_context();
    //猜格式
    fmt = av_guess_format(NULL, out_file, NULL);
    pFormatCtx->oformat = fmt;


    //方法2.更加自动化一些
    //avformat_alloc_output_context2(&pFormatCtx, NULL, NULL, out_file);
    //fmt = pFormatCtx->oformat;

    //注意输出路径
    if (avio_open(&pFormatCtx->pb,out_file, AVIO_FLAG_READ_WRITE) < 0)
    {
        printf("输出文件打开失败！\n");
        return -1;
    }
    //创建音频流
    audio_st = avformat_new_stream(pFormatCtx,NULL);
    //audio_st = av_new_stream(pFormatCtx, 0);

    if (audio_st==NULL){
        return -1;
    }
    pCodecCtx = audio_st->codec;
    pCodecCtx->codec_id = fmt->audio_codec;
    pCodecCtx->codec_type = AVMEDIA_TYPE_AUDIO;
    pCodecCtx->sample_fmt = AV_SAMPLE_FMT_S16;
    pCodecCtx->sample_rate= 44100;
    pCodecCtx->channel_layout=AV_CH_LAYOUT_STEREO;
    pCodecCtx->channels = av_get_channel_layout_nb_channels(pCodecCtx->channel_layout);
    pCodecCtx->bit_rate = 64000;

    //输出格式信息
    av_dump_format(pFormatCtx, 0, out_file, 1);

    pCodec = avcodec_find_encoder(pCodecCtx->codec_id);
    if (!pCodec)
    {
        printf("没有找到合适的编码器！\n");
        return -1;
    }
    if (avcodec_open2(pCodecCtx, pCodec,NULL) < 0)
    {
        printf("编码器打开失败！\n");
        return -1;
    }
    
    frame = av_frame_alloc();
    //frame = avcodec_alloc_frame();

    frame->nb_samples= pCodecCtx->frame_size;
    frame->format= pCodecCtx->sample_fmt;

    size = av_samples_get_buffer_size(NULL, pCodecCtx->channels,pCodecCtx->frame_size,pCodecCtx->sample_fmt, 1);
    frame_buf = (uint8_t *)av_malloc(size);
    avcodec_fill_audio_frame(frame, pCodecCtx->channels, pCodecCtx->sample_fmt,(const uint8_t*)frame_buf, size, 1);

    //写文件头
    avformat_write_header(pFormatCtx,NULL);

    AVPacket pkt;
    av_new_packet(&pkt,size);

    for (int i=0; i<framenum; i++){
        //读入PCM
        if (fread(frame_buf, 1, size, in_file) < 0)
        {
            printf("文件读取错误！\n");
            return -1;
        }else if(feof(in_file)){
            break;
        }
        frame->data[0] = frame_buf;  //采样信号

        frame->pts=i*100;
        int got_frame=0;
        //编码
        int ret = avcodec_encode_audio2(pCodecCtx, &pkt,frame, &got_frame);
        if(ret < 0)
        {
            printf("编码错误！\n");
            return -1;
        }
        if (got_frame==1)
        {
            printf("编码成功第%d帧！\n",i);
            pkt.stream_index = audio_st->index;
            ret = av_write_frame(pFormatCtx, &pkt);
            av_free_packet(&pkt);
        }
    }

    //写文件尾
    av_write_trailer(pFormatCtx);

    //清理
    if (audio_st)
    {
        avcodec_close(audio_st->codec);
        av_free(frame);
        av_free(frame_buf);
    }
    avio_close(pFormatCtx->pb);
    avformat_free_context(pFormatCtx);

    fclose(in_file);

    return 0;
}
//https://blog.csdn.net/leixiaohua1020/article/details/84598124