There are several reasons for RTXI to not open.

1. RTXI is already running.

   If you run rtxi in the terminal and get this output, RTXI is already running:

   $ sudo rtxi
   ../src/rt_os-xenomai.cpp:123:RT::OS::createTask : failed to create task
   ../src/rt.cpp:157:RT::System::System : failed to create realtime thread

   The error happens because only one instance of RTXI can be running on the system. Check who is running it by entering:

   $ ps ax -o euser,comm | grep rtxi

   If the command shows someone else's username, that means they have RTXI open. Bug them about closing it or close it yourself by running $ sudo pkill rtxi. Note that workspaces, open files, etc. will not be saved when force-closing RTXI.

   If it's your username, RTXI may not have exited cleanly from sometime earlier. You can run $ sudo pkill rtxi for this case, too.

   Now, you should be able to run RTXI.

2. You aren't using a real-time kernel.

   Check the kernel you are using. Run this in the terminal:

   $ uname -r

   You will get one line of output that tells you what kernel you are using. If it has the word "xenomai" in it, then it's a real-time kernel. If not, then it isn't.

   If it's not a real-time kernel, you'll have to reboot your computer and pick a real-time kernel in the GRUB boot menu that pops up. If you don't see it, enter the "Advanced options..." submenu and look there.

   Also, see this page about setting a default kernel.