[FFmpeg-user] Is there a video or audio file can't be seekable or can't get it's duration?

[littlebat]

Hi,

Three quesiton:
1, Is there a video or audio file can't be seekable and can't get it's
duration?
2, Is there a video or audio file can be seekable but we can't get it's
duration ? 
3, Is there a video or audio file we can get it's duration but can't be
seekable?

It's could be better if you can tell me more detail about the questions.

Thanks.