Adding metadata informations with ffmpeg - c++

Question

I am trying to read and most important to write metadata into file using ffmpeg. But I am getting sigseg.
int main(int argc, char **argv) {
av_register_all();
AVFormatContext* ctx;
std::string path("/home/stefan/test_track.mp3");
ctx = avformat_alloc_context();
if (avformat_open_input(&ctx, path.c_str(), 0, 0) < 0)
std::cout << "error1" << std::endl;
if (avformat_find_stream_info(ctx, 0) < 0)
std::cout << "error2" << std::endl;
AVDictionaryEntry *tag = nullptr;
tag = av_dict_get(ctx->metadata, "artist", tag, AV_DICT_IGNORE_SUFFIX);
std::cout << tag->key << " : " << tag->value << std::endl;
av_dict_set(&ctx->metadata, "TPFL", "testtest", 0);
std::cout << "test!" << std::endl;
int status = avformat_write_header(ctx, &ctx->metadata);
if(status == 0)
std::cout << "test1" << std::endl;
return 0;
}
I also tried to make a entire copy of AVDictionary and than saving it with new addition of field I want to save. But still sigseg.
What am I missing?

Answer 1

You need to use another AVFormatContext to write your output.
In your example, just add some meta data and copy the codec, so the muxing steps for ffmpeg library is
create the desired output format context, avformat_alloc_output_context2
add streams to the output format context, avformat_new_stream
add some custom meta data and write header
use av_write_frame to write the encoded data
write trailer
Below is a workable example with small modification of your code.
#include <iostream>
#ifdef __cplusplus
extern "C" {
#endif
#include <libavformat/avformat.h>
#ifdef __cplusplus
}
#endif
int main(int argc, char **argv) {
av_register_all();
avcodec_register_all();
AVFormatContext* ctx;
std::string path("./input.mp3");
ctx = avformat_alloc_context();
if (avformat_open_input(&ctx, path.c_str(), 0, 0) < 0)
std::cout << "error1" << std::endl;
if (avformat_find_stream_info(ctx, 0) < 0)
std::cout << "error2" << std::endl;
AVDictionaryEntry *tag = nullptr;
tag = av_dict_get(ctx->metadata, "artist", tag, AV_DICT_IGNORE_SUFFIX);
std::cout << tag->key << " : " << tag->value << std::endl;
av_dict_set(&ctx->metadata, "TPFL", "testtest", 0);
std::cout << "test!" << std::endl;
int status;
AVFormatContext* ofmt_ctx;
AVOutputFormat* ofmt = av_guess_format("mp3", "./out.mp3", NULL);
status = avformat_alloc_output_context2(&ofmt_ctx, ofmt, "mp3", "./out.mp3");
if (status < 0) {
std::cerr << "could not allocate output format" << std::endl;
return 0;
}
int audio_stream_index = 0;
for (unsigned i = 0; i < ctx->nb_streams; i++) {
if (ctx->streams[i]->codecpar->codec_type == AVMEDIA_TYPE_AUDIO) {
audio_stream_index = i;
const AVCodec *c = avcodec_find_encoder(ctx->streams[i]->codecpar->codec_id);
if (c) {
AVStream *ostream = avformat_new_stream(ofmt_ctx, c);
avcodec_parameters_copy(ostream->codecpar, ctx->streams[i]->codecpar);
ostream->codecpar->codec_tag = 0;
}
break;
}
}
av_dict_set(&ofmt_ctx->metadata, "TPFL", "testtest", 0);
av_dump_format(ofmt_ctx, 0, "./out.mp3", 1);
if (!(ofmt_ctx->oformat->flags & AVFMT_NOFILE)) {
avio_open(&ofmt_ctx->pb, "./out.mp3", AVIO_FLAG_WRITE);
}
if (avformat_init_output(ofmt_ctx, NULL) == AVSTREAM_INIT_IN_WRITE_HEADER) {
status = avformat_write_header(ofmt_ctx, NULL);
}
AVPacket *pkt = av_packet_alloc();
av_init_packet(pkt);
pkt->data = NULL;
pkt->size = 0;
while (av_read_frame(ctx, pkt) == 0) {
if (pkt->stream_index == audio_stream_index) {
// this is optional, we are copying the stream
av_packet_rescale_ts(pkt, ctx->streams[audio_stream_index]->time_base,
ofmt_ctx->streams[audio_stream_index]->time_base);
av_write_frame(ofmt_ctx, pkt);
}
}
av_packet_free(&pkt);
av_write_trailer(ofmt_ctx);
avformat_close_input(&ctx);
avformat_free_context(ofmt_ctx);
avformat_free_context(ctx);
if(status == 0)
std::cout << "test1" << std::endl;
return 0;
}
After done, you can use ffprobe output.mp3 to check the written meta data.
Input #0, mp3, from 'out.mp3':
Metadata:
TPFL : testtest
encoder : Lavf57.75.100
Duration: 00:00:08.75, start: 0.011995, bitrate: 128 kb/s
Stream #0:0: Audio: mp3, 44100 Hz, stereo, s16p, 128 kb/s
Metadata:
encoder : Lavf