问ffmpeg函数avcodec_receive_frame总是返回EAGAIN错误

EN

Metadata:
    major_brand     : iso6
    minor_version   : 0
    compatible_brands: iso6dash
  Duration: N/A, bitrate: N/A
    Stream #0:0(und): Audio: aac (mp4a / 0x6134706D), 48000 Hz, 2 channels (default)
    Metadata:
      handler_name    : USP Sound Handler

av_register_all();
AVFormatContext *avFormatContext = avformat_alloc_context();
NSString *filePath = [[NSBundle mainBundle] pathForResource:@"bbc" ofType:@"mp4"];

int ret;
ret = avformat_open_input(&avFormatContext, [filePath UTF8String], NULL, NULL);
if (0 != ret) {
    NSLog(@"avformat_open_input failed: %d", ret);
}

ret = avformat_find_stream_info(avFormatContext, NULL);
if (0 != ret) {
    NSLog(@"avformat_find_stream_info: %d", ret);
}

// the libfdk_aac decoder
AVCodec *codec = avcodec_find_decoder_by_name("libfdk_aac");
AVCodecContext *codecContext = avcodec_alloc_context3(codec);

ret = avcodec_open2(codecContext, codec, NULL);
if (0 != ret) {
    NSLog(@"avcodec_open2 faild: %d", ret);
}

AVFrame *frame = av_frame_alloc();
AVPacket packet;
av_init_packet(&packet);

// start read data and decode data
while (true) {
    ret = av_read_frame(avFormatContext, &packet);
    if (0 != ret) {
        break;
    }
    ret = avcodec_send_packet(codecContext, &packet);
    if (ret != 0) {
        NSLog(@"send package with error: %d", ret);
        continue;
        break;
    }
    while (true) {
        // the ret below is always return -35, means AVERROR(EAGAIN)
        ret = avcodec_receive_frame(codecContext, frame);
        if (ret == AVERROR(EAGAIN)) {
            NSLog(@"avcodec_receive_frame with EAGAIN error: %d", ret);
            break;
        } else if (ret == AVERROR_EOF) {
            NSLog(@"end of file");
            break;
        }
    }
    if (ret == AVERROR(EAGAIN)) {
        continue;
    }
}