Re: Why side-effects on open(2) are evil. (was Re: [RFD

Oliver Xymoron (
Mon, 21 May 2001 12:16:18 -0500 (CDT)

On Sat, 19 May 2001, Alexander Viro wrote:

> Let's distinguish between per-fd effects (that's what name in
> open(name, flags) is for - you are asking for descriptor and telling
> what behaviour do you want for IO on it) and system-wide side effects.
> IMO encoding the former into name is perfectly fine, and no write on
> another file can be sanely used for that purpose. For the latter, though,
> we need to write commands into files and here your miscdevices (or procfs
> files, or /dev/foo/ctl - whatever) is needed.

I'm a little skeptical about the necessity of these per-fd effects in the
first place - after all, Plan 9 does without them. There's only one
floppy drive, yes? No concurrent users of serial ports? The counter that
comes to mind is sound devices supporting multiple opens, but I think
esound and friends are a better solution to that problem.

What I'd like to see:

- An interface for registering an array of related devices (almost always
two: raw and ctl) and their legacy device numbers with a single userspace
callout that does whatever /dev/ creation needs to be done. Thus, naming
and permissions live in user space. No "device node is also a directory"
weirdness which is overkill in the vast majority of cases. No kernel names
or permissions leaking into userspace.

- An unregister_devices that does the same, giving userspace a
chance to persist permissions, etc.

- A userspace program that keeps a mapping of kernel names to /dev/ names,
permissions, etc.

- An autofs hook that does the reverse mapping for running with modules
(possibly calling modprobe directly)

Possible future extension:

- Allow exporting proc as a large collection of devices. Manage /proc in
userspace on a tmpfs.

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