可以参考：https://blog.csdn.net/Martin_chen2/article/details/100103655中的错误案例。
我的frame采样数是2048，直接编码aac格式会失败，采用重采样的方式，把一个frame拆分成两个采样数1024的frame，分两次调用swr_convert接口返回采样。

#define AAC_ENCODE_SIZE 1024

// 定义重采样
SwrContext *swr = NULL;
swr = swr_alloc();
av_opt_set_int(swr, "in_channel_layout",  AV_CH_LAYOUT_MONO, 0);
av_opt_set_int(swr, "out_channel_layout", AV_CH_LAYOUT_MONO,  0);
av_opt_set_int(swr, "in_sample_rate",     44100, 0);
av_opt_set_int(swr, "out_sample_rate",    44100, 0);
av_opt_set_sample_fmt(swr, "in_sample_fmt",  AV_SAMPLE_FMT_S16, 0);
av_opt_set_sample_fmt(swr, "out_sample_fmt", AV_SAMPLE_FMT_S16,  0);
ret = swr_init(swr);
if (ret < 0){
    LOGD("swr_init error ret = %d, %s", ret, av_err2str(ret));
    return -1;
}

***
// 定义输出
uint8_t *outs[2];
outs[0]=(uint8_t *)malloc(AAC_ENCODE_SIZE);
outs[1]=(uint8_t *)malloc(AAC_ENCODE_SIZE);
memset(outs[0], 0, AAC_ENCODE_SIZE);
memset(outs[1], 0, AAC_ENCODE_SIZE);

***

// nb_samples为2014 重采样为1024
int count = swr_convert(swr, outs, AAC_ENCODE_SIZE, (const uint8_t **)pFrameOut->extended_data, pFrameOut->nb_samples);
if (count < 0){
   LOGD("swr_convert error 1 ret = %d, %s", count, av_err2str(count));
} else {
   LOGD("count = %d, outs[0] = %s, outs[1] = %s", count, NULL, NULL/*outs[0], outs[1]*/);
   pFrameOut->nb_samples = count;//aac 为1024
   pFrameOut->data[0] = (uint8_t*)outs[0];
   pFrameOut->data[1] = (uint8_t*)outs[1];
}

// 进行编码
encode:
      ret = avcodec_encode_audio2(pOutFormatCtx->streams[audioIndex]->codec, &enc_pkt, pFrameOut, &enc_got_frame);

***

//重采样剩余的1024个采样，不需要输入数据
count = swr_convert(swr, outs, AAC_ENCODE_SIZE, NULL, 0);
LOGD("cache count = %d, outs[0] = %s, outs[1] = %s", count, NULL, NULL/*outs[0], outs[1]*/);
if (count > 0){
   pFrameOut->nb_samples = count;
   pFrameOut->data[0] = (uint8_t*)outs[0];
   pFrameOut->data[1] = (uint8_t*)outs[1];
}

注意第二次重采样不需要输入，而是直接从缓存区把剩余的AAC_ENCODE_SIZE个数据拿出来。

AAC对应的采样数（nb_samples）和frame大小（frame_size）都是1024.
AAC对应的采样数（nb_samples）和frame大小（frame_size）都是1152.