libusb cannot open USB device, permission isse. NetBeans/Ubuntu

Question

I'm writing a C/C++ application in NetBeans based on libusb-1.0 on Ubuntu 12.04. I can get basic information from the USB device (for example, the interface description) but I am not able to open the device. The function libusb_open gives me the error:

libusb:error [op_open] libusb couldn't open USB device /dev/bus/usb/002/003: Permission denied.
libusb:error [op_open] libusb requires write access to USB device nodes.

I understand that I need to change the permissions but I don't know how (I am a very basic Linux-user). Thank you!

Answer 1

I think this might be a temporary solution for the problem while Preston's solution would work consistently.

1. Check which usb port is assigned for your device

You can figure out which usb port is assigned to your device by invoking ls command two times(first with device disconnected and second with device connected).

$ ls -l /dev/bus/usb/00*
/dev/bus/usb/001:
total 0
crw-rw-r-- 1 root root 189, 0  1월 10 12:08 001
crw-rw-r-- 1 root root 189, 1  1월 10 12:08 002

/dev/bus/usb/002:
total 0
crw-rw-r-- 1 root root 189, 128  1월 10 12:08 001
crw-rw-r-- 1 root root 189, 129  1월 10 12:08 002

/dev/bus/usb/003:
total 0
crw-rw-r-- 1 root root 189, 256  1월 10 12:08 001
crw-rw-r-- 1 root root 189, 257  1월 10 12:08 002
crw-rw-r-- 1 root root 189, 258  1월 10 12:08 003
crw-rw-r-- 1 root root 189, 259  1월 10 12:08 004
crw-rw-r-- 1 root root 189, 260  1월 10 12:08 005
crw-rw-r-- 1 root root 189, 263  1월 10 15:42 008 <-- this is your device

Let's say /dev/bus/usb/003/008 is your device.

2. Giving write permission for everyone(other)

According to the output of ls -l command, root user(group) has read/write permission on 003/008 port while other user has only read permission.

crw-rw-r-- 1 root root 189, 263  1월 10 15:42 008

You can allow every user to write on specific device using chmod command. While using chmod command, you will need sudo permission.

$ sudo chmod o+w /dev/bus/usb/003/008

or

$ sudo chmod a+w /dev/bus/usb/003/008

Now if you check the permission of usb, you have to see this output

$ ls -l /dev/bus/usb/003/008
crw-rw-rw- 1 root root 189, 263  1월 10 15:42 /dev/bus/usb/003/008

3. Everytime plugging it out or shutting down the system repeat step 1,2

If you plug the device out from usb port or shut down the system, what you did for your usb port will reset.

You have to repeat step 1,2 again.

This is why I'm saying my solution is temporary(volatile).

Further readings

I find these two blog articles would be helpful to your understanding.

• Beginners Guide to File and Directory Permissions
• Unix LS Command: 15 Practical Examples