No-op remuxing of .avi file - c

Question

I want to demux and then mux .avi file without changing anything.
My program is this (redacted for brevity):
AVFormatContext *input_format_context = NULL;
avformat_open_input(
&input_format_context,
input_url,
NULL, // fmt
NULL // options
);
avformat_find_stream_info(input_format_context, NULL);
AVFormatContext *output_format_context = NULL;
avformat_alloc_output_context2(
&output_format_context,
NULL, // oformat
NULL, // format_name
output_url
);
avio_open2(
&output_format_context->pb,
output_url,
AVIO_FLAG_WRITE,
NULL, // int_cb,
NULL // options
);
for (int i = 0; i < input_format_context->nb_streams; i++) {
avformat_new_stream(output_format_context, NULL);
AVStream *input_stream = input_format_context->streams[i];
AVStream *output_stream = output_format_context->streams[i];
AVCodecParameters *params = avcodec_parameters_alloc();
avcodec_parameters_copy(params, input_stream->codecpar);
output_stream->codecpar = params;
}
avformat_write_header(output_format_context, NULL))
AVPacket *input_packet = NULL;
input_packet = av_packet_alloc();
while (!av_read_frame(
input_format_context,
input_packet
)) {
av_write_frame(output_format_context, input_packet);
av_packet_unref(input_packet);
}
av_write_trailer(output_format_context);
Problem:
Output file is created but instead of close to 10 minute video it is a 24-second slide show consisting of around 3 frames.
It seems that the problem is (perhaps not the only one) lack of PTS on the packet.
When I explicitly print it (input_packet->pts) for each packet it is -9223372036854775808. And also the following warning is printed:
[avi # 0x562868c6c000] Timestamps are unset in a packet for stream 0. This is deprecated and will stop working in the future. Fix your code to set the timestamps properly
How do I then "fix my code to set the timestamps properly"?

Answer 1

I just found a solution.
I added this:
output_stream->time_base = input_stream->time_base;
which then, I understand, allows the video player to calculate PTS on the fly.
This does not remove the warning itself, though. I understand that .avi simply does not have PTS, so it's not a bug as such. To get rid of the warning one can manually set PTS on the packets:
input_packet->pts = calculated_ts;
I would think I should be able to also just do:
output_format_context->oformat->flags |= AVFMT_NOTIMESTAMPS;
However, I cannot do that:
error: assignment of member ‘flags’ in read-only object
So, it looks like ffmpeg is requiring PTS even for .avi or there's a bug or I'm still doing something wrong.