Cfxtrjtrk

I'm making a custom USB HID device, and a desktop app to go with it, using HIDAPI on Lubuntu 16.04.3 LTS. We'll call it myapp for now.

Apparently, if I do $ ./myapp, libusb_open() fails with LIBUSB_ERROR_ACCESS. _{(shown in my debugger as -3; it took a while to figure that out because the enum doesn't seem to be documented very well)}

But if I do $ sudo ./myapp, it succeeds.

I really don't want myapp to have root permissions, so how can I talk to my USB device without them?

I was hoping for an answer here, but it seems to have been abandoned at exactly this point. It says something about user permissions, but I can't seem to find that on my system. I thought I might find a group called usb or libusb that I could add myself to, but no such luck.

This question on a different SE site has an answer that uses some austere text files to change the permissions either globally (probably a bad idea) or for a specific device, but:

1. It's not the "muggle's tweak" that I was looking for.


2. Most of those files have a warning against modifying directly because they're auto-generated. So how can I be sure that a random package update isn't going to wipe out my changes and thus break myapp?


3. It doesn't seem to do anything anyway. I followed those instructions and rebooted, and myapp with user permissions still can't talk to USB.

It's still not the "muggle's tweak" that I'm really looking for, but at least this works:

Apparently there are two directories for udev (I have no idea why):

• /etc/udev/rules.d


• /lib/udev/rules.d

I'd been messing with the /lib one and getting nowhere. I found the /etc one here, and it does work:

Put SUBSYSTEM=="usb", ATTRS{idVendor}=="VID", ATTRS{idProduct}=="PID", MODE="0666"

• 
  VID is the USB-IF-assigned Vendor ID of the device in question *


• 
  PID is the Vendor-assigned Product ID of the device in question *


• 
  

  0666 gives universal read/write access to whatever matches this line

  
  
  

  * _{$ lsusb to see all attached USB devices and their ID's.}

  
  

In /etc/udev/rules.d/xx-my-rule.rules (may need root/sudo permissions)

• 
  xx is any number > 50 (the defaults are in 50, and higher numbers take priority)


• 
  my-rule is whatever you want to call it


• must end in .rules
  

Then udevadm control --reload-rules (may also need root/sudo permissions), and it should "just work" for that specific VID/PID pair.

Another option, to tighten the permissions a little bit more, is to use TAG+="uaccess" in place of MODE="0666". This limits access to the currently-logged-in (physical) user instead of all users. Thanks @Lekensteyn!