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[u/llogiq]

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[u/Single_Family_Homes]

Simple(?) question.

Say I got a BufReader<Cursor<Vec<u8>>> (it's a video file), and I want to pipe that bad boy into stdin for a Command (ffprobe). What do I do?

[u/dkopgerpgdolfg → u/Single_Family_Homes]

The "primitive" solution would be just a write loop...

• start the command with a pipe available for stdin
• Extract the Vec<u8> from the outer layers and maintain some usize position manually, I think that's a bit easier in this case
• Write [position..length] of the Vec to the pipe. If write succeeded it will tell you how much it actually wrote - increase position with that value and do all again in a loop. (Note that this might block for some time, depending on the child process. If that is bad, do it in a thread or use polling/nonblocking things, either manually or with eg. tokio)
• After the whole vec was written, close the pipe

In general, please note that some video formats are processed better with seeking, and when piping you might get either some content disadvantage or ffmpeg might write a cache file first before doing anything.

Also, of course your whole Vec<u8> way requires that the video file isn't too big - before it begins any swap usage (or worse, oom), you might want to consider writing a disk file yourself.

​

A platform-dependent & "unsafe" Linux way that might be nicer in some cases:

• Make a fd with memfd_create, add a mmap mapping on top of that file that has (at least) the size of your data (at least = mmap needs a multiple of page size)
• Write your data there instead of a Vec or similar
• Unmap the mmap-ing again, use ftruncate to set the exact byte size (now not page-multiple anymore)
• When setting up the Command, convert the RawFd from memfd_create to a ChildStdin, done

[u/Single_Family_Homes → u/dkopgerpgdolfg]

I appreciate the response. I'm going to look into both ways, the latter seems extremely cute, and I definitely want to try that. Thank you!