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When I use it returns an empty string.

But I would like to know the Display Aspect Ratio of the file, so I can decode it to another size than the original, compensating for the aspect ratio while decoding already. For example PAL 16:9 has 720x576 should be displayed on a square pixel screen as 720x405.

What am I doing wrong, is there another place or function where I should get this information?

Thanks

I don't think you are doing anything wrong, libvlc_video_get_aspect_ratio returns nothing when the aspect ratio has not been set.
Maybe libvlc_video_get_aspect_ratio doesn't returns the DAR defined in the input file, even if vlc handle it correctly, i don't know.