[Development] WSL Tips

Running CNCjs under WSL

WSL (Wndows Subsystem for Linux) lets you run Linux programs under Windows 10 with somewhat less hassle than virtualization alternatives like VirtualBox, Docker, etc. You still have to install a Linux distro, but afterwards WSL starts faster than a VM and is somewhat better integrated with the Windows filesystem. It's not perfect, but many things work rather well, and it is getting better, as Microsoft appears to be committed to improving it. Microsoft responded almost instantly to a bug report that I filed, and fixed it quickly.

CNCjs can be made to run under WSL. Compilation is identical to CNCjs compilation under Linux. Just clone the git repo and say npm install, and the build process succeeds.

The difficulty is accessing serial ports. WSL exposes Windows COM ports under the standard Linux names; for example WSL presents COM3 as /dev/ttyS3. But there are two problems:

1. In WSL on Windows builds prior to 1809, the tty ports are owned by root,root with permissions crw-r-----. You can change that ownership, but when you restart WSL they revert back to the original ownership and permissions. The problem is fixed in build 1809 and later (https://github.com/Microsoft/WSL/issues/3042). The workaround, if you are on an earlier build, is to run cncjs as root, e.g. sudo bin/cncjs

If you are on build 1809 or later, the device files /dev/ttyS* are owned by root,dialout crw-rw----, so you just have to add yourself to the dialout group. To do that, sudo usermod -a dialout `whoami` , exit from WSL and restart. Then you should be a member of the dialout group (check with groups) and cncjs can access serial ports when started from your login.

2. The node.js serialport package doesn't list the serial ports under WSL. serialport, when running under WSL, thinks it is on an ordinary Linux and uses Linux's udev facility to discover serial ports. WSL's rudimentary implementation of udev does not include serial ports. To work around this, I modified serialport as follows. In node_modules/@serialport/bindings/lib/ I created a new file wsl-list.js with the following contents:

function listLinux() {
    const ports = [
        { comName: "/dev/ttyS3"},
        { comName: "/dev/ttyS4"}
    ]
    return Promise.resolve(ports);
}
module.exports = listLinux

For simplicity, I hardcoded only the serial ports that connect to my test controllers.

In node_modules/@serialport/bindings/lib/linux.js, I changed the line

const linuxList = require('./linux-list')

to

const linuxList = require('./wsl-list')

You can, of course, compile CNCjs for Windows without WSL, but, apart from the serialport thing, WSL is a bit less trouble if you already have it set up. There is no need to rebuild cncjs. All you have to do is kill the cncjs process and restart it.

Alternative to modifying cncjs

An alternative that does not require modifying cncjs mods is to create an executable shell script named udevadm to override the system version. That script would contain, for example:

#! /bin/sh
echo P: dummy
echo N: ttyS3
echo E: DEVNAME=/dev/ttyS3
echo P: dummy
echo N: ttyS4
echo E: DEVNAME=/dev/ttyS4

If you run cncjs as root (per problem 1. above), the script must be in the super user search path before the real version of udevadm. In my WSL setup, the super user search path is /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin and the real udevadm is found in /sbin, so you could put your udevadm in, for example, /usr/local/sbin. The downside of this approach is that any other command that uses udevadm will probably fail. I think that apt-get upgrade could be a problem area, per this WSL FAQ . If you run into problems, you could temporarily rename your spoofed version of udevadm . Or make a fancier script that bounces to the system udevadm unless it is called just the way that cncjs calls it (udevadm info -e). Or just use the wsl-list.js hack instead.