C++ ffmpeg extract audio to mp3 (demuxing) - c++

Question

I am trying to write a C++ program that allows me to extract the audio from a video file to an mp3 file. I searched the internet and stackoverflow, but couldn't get it to work.
The library I chose is ffmpeg and I have to do it in C/C++. This is what I have so far.
// Step 1 - Register all formats and codecs
avcodec_register_all();
av_register_all();
AVFormatContext* fmtCtx = avformat_alloc_context();
// Step 2 - Open input file, and allocate format context
if(avformat_open_input(&fmtCtx, filePath.toLocal8Bit().data(), NULL, NULL) < 0)
qDebug() << "Error while opening " << filePath;
// Step 3 - Retrieve stream information
if(avformat_find_stream_info(fmtCtx, NULL) < 0)
qDebug() << "Error while finding stream info";
// Step 4 - Find the audiostream
audioStreamIdx = -1;
for(uint i=0; i<fmtCtx->nb_streams; i++) {
if(fmtCtx->streams[i]->codec->codec_type == AVMEDIA_TYPE_AUDIO) {
audioStreamIdx = i;
break;
}
}
if(audioStreamIdx != -1) {
// Step 5
AVCodec *aCodec = avcodec_find_decoder(AV_CODEC_ID_MP3);
AVCodecContext *audioDecCtx = avcodec_alloc_context3(aCodec);
avcodec_open2(audioDecCtx, aCodec, NULL);
// Step 6
AVPacket pkt;
AVFrame *frame = av_frame_alloc();
av_init_packet(&pkt);
pkt.data = NULL;
pkt.size = 0;
int got_packet = 0;
while(av_read_frame(fmtCtx, &pkt) == 0) {
int got_frame = 0;
int ret = avcodec_decode_audio4(audioDecCtx, frame, &got_frame, &pkt);
if(got_frame) {
qDebug() << "got frame";
}
}
av_free_packet(&pkt);
}
avformat_close_input(&fmtCtx);
But the error I get when executing avcodec_decode_audio4() is "[mp3 # 825fc30] Header missing".
Thanks in advance!
[EDIT]
I found out that the audio of the video was not MP3 but AAC. So I changed Step 5 to the following lines of code
// Step 5
AVCodecContext *audioDecCtx = fmtCtx->streams[audioStreamIdx]->codec;
AVCodec *aCodec = avcodec_find_decoder(audioDecCtx->codec_id);
avcodec_open2(audioDecCtx, aCodec, NULL);
Now it outputs "got frame" and the avcodec_decode_audio4() returns the number of bytes it decoded.
Now I have to write the audio to a file, preferably to an MP3 file. I found out that I have to do it with the function avcodec_encode_audio2(). But some extra help on how to use it is more then welcome!