Accessing a USB device using libusb-1.0 as a non-root user

Question

I am trying to connect to a USB device as a non-root user on RHEL5. The device is a GPIO interface (its documentation can be found at http://www.xdimax.com/sub20/sub20.html ), which uses libusb-1.0. The procedure for opening a device using its API:

sub_device d; d = sub_find_devices(0); sub_handle h = sub_open(d); 

When I do this, the sub_find_devices() call works, but when I call sub_open() I get the libusb -3 error, which indicates that I do not have permission to open the device for writing.

I did some research on this problem and found that I had to create a udev rule. Using udevinfo on a sysfs node device, I got:

 looking at device '/devices/pci0000:00/0000:00:1d.0/usb2/2-1/2-1.2': KERNEL=="2-1.2" SUBSYSTEM=="usb" SYSFS{configuration}=="" SYSFS{serial}=="15F2" SYSFS{product}=="SUB-20" SYSFS{manufacturer}=="XDIMAX" SYSFS{maxchild}=="0" SYSFS{version}==" 1.10" SYSFS{devnum}=="6" SYSFS{speed}=="12" SYSFS{bMaxPacketSize0}=="64" SYSFS{bNumConfigurations}=="1" SYSFS{bDeviceProtocol}=="00" SYSFS{bDeviceSubClass}=="00" SYSFS{bDeviceClass}=="ff" SYSFS{bcdDevice}=="0001" SYSFS{idProduct}=="ffc3" SYSFS{idVendor}=="04d8" SYSFS{bMaxPower}=="100mA" SYSFS{bmAttributes}=="80" SYSFS{bConfigurationValue}=="1" SYSFS{bNumInterfaces}==" 1" 

Then I created the following udev rule in /etc/udev/rules.d/991-local.rules :

 SYSFS{idVendor}=="04d8", SYSFS{idProduct}=="ffc3", NAME="sub20", GROUP="582", MODE="0660" 

582 is the GID of the group to which my regular user belongs. I also tried a rule with a group name, and that didn't work. After creating this rule, the device file /dev/sub20 is created with the correct permissions, but exists only when the device is connected, which gives me reasonable confidence in the udev rule on the correct device. However, my code still gets a -3 error.

Running strace in code showed this call:

 open("/dev/bus/usb/002/006", O_RDWR) = -1 EACCES (Permission denied) 

The permissions on /dev/bus/usb... node are still root: root, so maybe this indicates that there is a problem with my udev rule, although I don't know what it might be.

If I try to make an open("/dev/sub20", O_RDWR) call open("/dev/sub20", O_RDWR) , I get a return value of ENXIO (No such device or address) , another possible indicator of an error in the udev rule, although the /dev/sub20 clearly associated with the correct device somehow, as it only exists when the device is connected.

What else can I do to try to get this to work?

Answer 1

The udev rules that I use to access devices using libusb are as follows: SUBSYSTEMS=="usb", ATTRS{idVendor}=="04d8", ATTRS{idProduct}=="ffc3", SYMLINK+="sub20", GROUP="usb", MODE="660" . It should just add a symbolic link to the device, but permissions also work for me later (I am a member of the usb group).

Answer 2

For USB FTDI devices, I use the script:

ftdi_config.sh

 #!/bin/sh echo "This script is for modern debian systems, and does everything that should" echo "be necessary to use ftdi usb devices as a regular user with libftdi," echo "instead of the built-in dumb kernel driver." echo if [ $(id -u) != 0 ]; then echo "This script must be run as root." exit 1 else read -p "Press enter to continue, or ctrl-c to bail..." x fi echo echo "** Adding usb group" groupadd usb echo echo "** Setting udev permissions on usb devices" echo 'SUBSYSTEMS=="usb", ACTION=="add", MODE="0664", GROUP="usb"' >> /etc/udev/rules.d/99-usbftdi.rules echo echo "** Reloading udev rules" /etc/init.d/udev reload echo echo "** Blacklisting ftdi_sio driver" echo 'blacklist ftdi_sio' > /etc/modprobe.d/ftdi.conf echo echo "** Removing old ftdi_sio driver (it ok if it fails)" rmmod ftdi_sio echo echo "!! Run the following command as root, to add your user to the usb group:" echo "useradd -G usb yourusernamehere" echo echo "or" echo echo "Adding to a existing user:" echo "usermod -a -G usb yourusernamehere" echo echo "as then you must reboot the system:" echo "reboot" 

then run the application as a non-root user. He works!!!