SMPlayer Support Forum

VLC is able to start playback of a currently downloading file and play arriving data.
SMPlayer can't do that. You have to restart playback to be able to see new file data.

Really useful feature - starting video download and being able to play right off the bat without waiting for complete download.

PS. I think it's because (S)Mplayer doesn't account the filesystem state of files it opens. On file opening the (S)Mplayer inappropriately fixate the end of file for files that are still being written at.

I think that happens only when the -idx option is used. You can disable it (uncheck "rebuild index if needed" in preferences -> advanced).

That option is off (by default). Maybe there are related mplayer command line options?

Probably the file is being downloaded by several parts in parallel. Thus, when MPlayer gets to the end of the first downloaded part, it can not proceed playing because the file has a "hole" (probably filled by zero bytes) between downloaded parts there.

But I have another question here. I usually use wget for downloading and it downloads a whole file continuously. The file can be played up to the end of its downloaded data, but no seeking forward or backward is possible. Why? Other players - such as VLC or GOM - mostly allows to seek (navigate) within already downloaded part of the file.

I think you can seek in those files with mplayer if using the lavf demuxer.

So, it appears there's no solution to the OP problem?
Will have to use VLC...

I do this all the time in Linux Mint with my HDHomerun (I'll start recording a show with my HDHR and then start viewing it while the HDHR continues to add data to the file). And I'll occasionally start watching something while Firefox (or the Downloadhelper extension) is still downloading.

Sounds to me like it's either your OS or the app that you use to download that's the issue.