I tried to build this PR with:

./configure --with-realtime=uspace --enable-build-documentation=none

On the 5.4.279-rtai kernel with Debian Trixie

I get the following error, any thoughts:

Linking rtapi_app
Compiling rtapi/uspace_rtai.cc
In file included from rtapi/uspace_rtai.cc:10:
/usr/include/x86_64-linux-gnu/sys/io.h:38:12: error: declaration of ‘int iopl(int) noexcept’ has a different exception specifier
   38 | extern int iopl (int __level) __THROW;
      |            ^~~~
In file included from /usr/include/rtai/rtai_lxrt.h:524,
                 from rtapi/uspace_rtai.cc:6:
/usr/include/rtai/asm/rtai_lxrt.h:170:38: note: from previous declaration ‘int iopl(int)’
  170 | #define rtai_iopl()  do { extern int iopl(int); iopl(3); } while (0)
      |                                      ^~~~
/usr/include/rtai/rtai_lxrt.h:607:9: note: in expansion of macro ‘rtai_iopl’
  607 |         rtai_iopl();
      |         ^~~~~~~~~
make: *** [Makefile:287: objects/rtapi/uspace_rtai.o] Error 1
make: Leaving directory '/home/andypugh/linuxcnc-dev/src'

I suspect that this is a consequence of the long and complex history of this particular VM.

I should probably also create an issue for https://github.com/NTULINUX/RTAI
I think this is in a package from there. rtai-modules-5.4.279: /usr/include/rtai/asm-x86/rtai_lxrt.h

This change will fix it:

--- rtai_lxrt.h.orig	2026-03-23 21:21:53.462988753 +0100
+++ /usr/include/rtai/asm-x86/rtai_lxrt.h	2026-03-23 21:22:24.946592950 +0100
@@ -167,7 +167,7 @@
         return ret;
 }
 
-#define rtai_iopl()  do { extern int iopl(int); iopl(3); } while (0)
+#define rtai_iopl()  do { extern int iopl(int) __THROW; iopl(3); } while (0)
 
 #endif /* __KERNEL__ */

I mentioned it already here but there are many things going on: #3873 (comment)

I think you will have the same issue on master and 2.9.

andypugh@trixie:~/linuxcnc-dev/src$ halrun
halcmd: exit
Note: Using LXRT realtime

Works for me too, with the change.

I am not clear if it would work for everyone if I change my rtai includes before building a package.
I think it probably would. What do you think?

You build your packages based on https://github.com/NTULINUX/RTAI? I am on it here: NTULINUX/RTAI#20
It might well be that adding this __THROW breaks other stuff.

You build your packages based on https://github.com/NTULINUX/RTAI?

No, I typically take the current 2.9 head and merge in https://github.com/LinuxCNC/linuxcnc/tree/andypugh/2.9-rtai to enable RTAI package building.
It's kludgy and clunky but I am not familiar enough with the package build system to properly and compatibly revert 6f28560

Ah sorry, i ment the package for rtai-modules-5.4.279 containing /usr/include/rtai/asm-x86/rtai_lxrt.h, the file I had to modify. I got this debian package from the linuxcnc package repo.
It probably would make more sense to fix this package.