code.c

    for (i = 0; i < fmt->nb_streams; i++) {
        in_stream = fmt->streams[i];
        out_stream = avformat_new_stream(seg->fmt_ctx, NULL);
        if (!out_stream) {
            av_log(seg->fmt_ctx, AV_LOG_WARNING, "Failed allocating output stream\n");
            continue;
        }
        ret = avcodec_parameters_copy(out_stream->codecpar, in_stream->codecpar);
        if (ret < 0) {
            av_log(seg->fmt_ctx, AV_LOG_WARNING, "Failed to copy context from input to output stream codec context\n");
            continue;
        }
        if (out_stream->codecpar->codec_type == AVMEDIA_TYPE_VIDEO) {
            if (in_stream->sample_aspect_ratio.num)
                out_stream->sample_aspect_ratio = in_stream->sample_aspect_ratio;
            out_stream->avg_frame_rate = in_stream->avg_frame_rate;
            out_stream->r_frame_rate = in_stream->r_frame_rate;
        }
        av_dict_copy(&out_stream->metadata, in_stream->metadata, 0);
    }
    av_dict_copy(&seg->fmt_ctx->metadata, fmt->metadata, 0);
    seg->fmt_ctx->pb = seg->io_ctx;
    ret = avformat_write_header(seg->fmt_ctx, NULL);
    avio_flush(seg->io_ctx);