G. Wade Johnson > Device-USB-0.35 > Device::USB::FAQ


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Device::USB::FAQ - Frequently Asked Questions for Device::USB


perldoc Device::USB::FAQ


This is an attempt to answer some of the frequently asked questions about the Device::USB module


Which platforms does Device::USB support?

Device:USB supports any platform that libusb supports. This list currently includes Linux, FreeBSD, NetBSD, OpenBSD, Darwin, and MacOS X.

There is a port of the libusb library to the Windows environment called LibUsb-Win32. Because I don't have a development environment for testing this library, Device::USB does not yet support this library.

Do I have to use Device::USB as root?

By default, access to the USB devices on a Unix-based system appear to be limited to the root account. This usually causes access to most of the libusb features to fail with a permission error.

Using the Device::USB module as root avoids this feature, but is not very satisfying from a security standpoint. (See the next question for more options.)

How do I enable use of Device::USB as a non-root user?

Some of the attributes of USB devices are available to non-root users, but accessing many of the more interesting features require special privileges. According to the libusb source, the open() function requires either device nodes to be present or the usbfs file system to be mounted in specific locations. Those places in order are:


/dev/bus/usb - pre-2.6.11: via devfs / post-2.6.11: via udev


/proc/bus/usb - usbfs

Look in both locations on your system for which of these two methods your libusb will use.

No matter which method your system uses, you will probably want to create a separate group to control access. Run this command to add a system group:

  addgroup --system usb


  groupadd --system usb

You can then add users to that group to allow access to your usb devices.




If you use Debian/Ubuntu, look in the /etc/udev/permissions.rules file. If you want to allow global access to all usb devices, make this change:

Change this: SUBSYSTEM=="usb_device", MODE="0664"

To this: SUBSYSTEM=="usb_device", MODE="0664", GROUP="usb"

After you reboot, all usb devices will inherit the mode and group specified.

If you want to only change permissions for certain devices, you can add this on one line and adjust the product and vendor IDs:

  SUBSYSTEM=="usb_device", GROUP="usb", \
    SYSFS{idVendor}=="1234", SYSFS{idProduct}=="1234"


The usbfs defaults to root as the user and group. This can be changed in the /etc/fstab by adding the following on one line:

  none /proc/bus/usb usbfs noauto,\
      0 0

The value 118 in the above should be replaced with the group id of your usb group (created above). The list* values are to allow listing devices, the bus* is to control access to the bus directories and the dev* values control access to the device files. This approach does not allow the kind of granular permission that the udev approach gives, so it is all or nothing unless permissions are changed programmatically.

If your /etc/fstab file already has a line for /proc/bus/usb, add the options above to the line that is already there rather than adding the new line. For example, you would change

  usbfs   /proc/bus/usb   usbfs   noauto  0 0


  usbfs   /proc/bus/usb   usbfs  noauto,\
      0 0

Once again, this needs to be all on one line with the \ characters removed.


Device::USB and the libusb library site at http://libusb.sourceforge.net/.


G. Wade Johnson (wade at anomaly dot org) Paul Archer (paul at paularcher dot org)

Houston Perl Mongers Group


Thanks go to various users who submitted questions and answers for the list. In particular, Anthony L. Awtrey who contributed the first FAQ answer.


Copyright 2006 Houston Perl Mongers

This document is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

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