Opening/Reading character device

[joernheissler]

Hello,
I tried reading /dev/input/event0 with trio. This character device generates a byte stream of events.
Only function I could find is trio.open_file, but looks like its purpose is actual files.
How can I open a character device so I'll get some trio.abc.ReceiveStream?

Thanks!

[pquentin]

A shot in the dark: would FdStream help? https://trio.readthedocs.io/en/stable/reference-hazmat.html#unix-specific-api

[njsmith]

Yeah, Trio doesn't have a pre-existing wrapper for /dev/event so you need to know a bit about the lower-level Unix primitives, but here it's pretty simple. In Unix, character devices general are "selectable", meaning that they're set up to be used with the non-blocking io positives. (Unlike e.g. regular on-disk files, which is what trio.open_file is mostly targeted at.) And trio.hazmat.FdStream is already set up to handle Unix "selectable" files. So you want to do something like: def open_events(events_dev_path): return trio.hazmat.FdStream(os.open(events_dev_path, os.O_RDONLY)) (Note the use of os.open to get raw file descriptor, versus using regular open which would give us a Python file object. This is to avoid FdStream and the file object from fighting over who's in charge of closing the file descriptor.) I'm assuming here that opening a /dev/event object is effectively instantaneous. If it can block for some reason then you might want to wrap the actual open call in a trio.to_thread.run_sync.

…

On Sun, Feb 9, 2020, 12:12 Quentin Pradet ***@***.***> wrote: A shot in the dark: would FdStream help? https://trio.readthedocs.io/en/stable/reference-hazmat.html#unix-specific-api — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#1395?email_source=notifications&email_token=AAEU42C4Q6H3LXGDBIWFOCLRCBPUDA5CNFSM4KSC34GKYY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGOELGWXUY#issuecomment-583887827>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAEU42CYYFZZCC5YKEO2OTTRCBPUDANCNFSM4KSC34GA> .

[joernheissler]

Thanks, I wasn't aware of trio.hazmat.FdStream. Works like a charm: https://gist.github.com/joernheissler/b366aae6b5067a5d71b2e46a5e6d5d63

I think it would still be great to have a function which handles os.open and trio.hazmat.FdStream for the user.

[oremanj]

The combination of os.open() + wrapping in FdStream that is possible with Trio's actual current APIs would only work for some devices and named pipes, not most files on disk (nor stdin, for example). I think exposing a nice name for this combo might become a bit of an attractive nuisance wrt users expecting more from it than it's able to deliver.

Closing this in favor of #174 and #219 which discuss the more general problem of wrapping "anything you can open" in a nice API.