number of /dev/usb nodes

Chuck Robey chuckr at
Sun Jun 8 14:25:35 UTC 2008

Hash: SHA1

Bernd Walter wrote:
> On Sat, Jun 07, 2008 at 01:18:41PM -0400, Chuck Robey wrote:
>> Hash: SHA1
>> I can't seem to find how many /dev/usbN bus devices there can be.  I'm writing
>> some code that scans them all looking for anything that has my device, but I
>> while I know to start at usb0, just how high do I go?  There seem to be 128
>> device minors, is that the number?  (from dev/usb/usb.h)
> There shouldn't be a limit anymore.
> I can't see any definition of 128 in usb.h that limits the number of
> busses.
> The major/minor differenciation is long time ago.
> You must be looking at old code.

I was trying to find a good way to do scanning, whjen I create the files like
/dev/usb0, how far to go in my loop?  Does the lowest available device always
get created?  That would imply that as soon as I began to get "No such device"
errors, I could stop iterating.  If the rules for picking device filenames are
pretty loose, then I could (for instance) stop scanning, say, 4 numbers past the
first "No such device" returnee.

Any idea on this?  I didn't see this i nthe code, but I just need some sane
limit on what filenames to scan about in.  I look for item info, and if the usb
vendor and prodict look friendly, I just snag the filename involved, and use
that.  Like, a scan of the usb1 bus might yield me a uhid0 which might be my
meat, whereupon I coulld drop the usb1 open, and replace it with a uhid0 open.
There's more than 1 place that my devices could show, depending on the user's
kernel devices.  I just want to have some sane limit on how many usb buses I
open for my scanning.
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