Running Xenomai2 binaries on Xenomai3

Philippe Gerum rpm at
Sun Dec 20 19:47:01 CET 2020

Drew via Xenomai <xenomai at> writes:

> We have a Xenomai2 based project that is being shifted to Xenomai3. It used
> the native skin. (Beaglebone black, crusty 3.8.13-xenomai kernel)
> I created a tiny file that allows me to run my unmodified
> xenomai2 binary images on xenomai3.. I also added a symlink called
> that seems to work if pointed at any used xenomai shared
> object. (those are the two shared libraries the old binaries look for.)
> I put them in /usr/lib where our old binaries look for them. My libnative
> looks for the other libs in their new location by building using --rpath.
> I just built the library to load in alchemy, cobalt, copperplate, and
> modechk when I linked my library. Our binaries find the needed methods via
> this libnative, and seem to be running fine.
> Well, they find *most* of the calls. Four of them were "static inline" in
> .h files, so I just called the inline functions, wrapped with a different
> name to avoid a redefinition at compile time. Then I used objcopy to rename
> the exported functions so they are the same as the static inline function
> names that disappeared in the new API.

As a result, I just kicked out the few inlines from libevl so that
interposing on these API calls should be made easier.

> Why would I want to do this? I have software in the field that will
> download and try to run the xenomai2 binaries. I don't want that to break..
> I do want it to work transparently. My question, I guess, is.. well.. pick
> one or more response, or write your own.
> * Good lord, that's a terrible idea because <awful thing> will happen!
> * Good lord, that's a brilliant idea! why didn't we do that in Xenomai3? Ok
> if we use it? <why sure!>
> * I'm glad it seems to work for you, but we really couldn't do that in
> general because <totally valid reason why.>
> * This is already supported in Xenomai3. you haven't read the docs, have
> you? To do this you only needed to <link to doc I didn't see>.
> * This idea isn't new. You haven't been up to date on the mailing list,
> have you? <link to someone else already doing/trying/having issues with
> this.>

It's a brilliant although terrible idea which I'm glad works for you
because this has never been meant to be supported by Xenomai 3.

This said, ABI compat is a must for some deployments, this is why there
is a general consensus that we should try hard to guarantee it.

> My main worry *was* that Xenomai2 defined structures of one size (which the
> old binaries allocate) but the new xenomai made those structures bigger and
> now memory is being trashed. (not badly enough to cause trouble
> immediately, though.)
> I took a closer look at the sizeof the structs we used, just to make sure
> the sizes aren't a problem. What I found was one (the mutex) was reduced in
> size from 2->3..(was:12 bytes. is now: 4 bytes) The other things (cond,
> pipe, rtime, task) used the same amount of memory in 2 and 3.. so, it seems
> there is no issue there.

Alternatively, in order to avoid any issue of that kind, there might be
the option to allocate a complete shadow Xenomai 3/alchemy descriptor
for each object, referencing it from the space defined by the Xenomai
2/native ABI for holding their counterparts. There are a couple of
invariants you should be able to rely on for this:

- xnhandle_t is of long type, so a pointer would fit there.

- in x3, all object creation and deletion calls are deemed secondary
  mode operations, so there cannot be any real-time constraints on these
  (e.g. you might even call malloc()/free() there). You could
  alternatively use the real-time memory allocator libcopperplate
  provides under the hood (xnmalloc(), xnfree() see below).

e.g. Somewhere in the Xenomai 3 side of the universe:

typedef union {
      xnhandle_t x2_opaque;
      struct RT_COND *x3_object;

#include <copperplate/heapobj.h>
#include <alchemy/cond.h>

int rt_cond_create_shim(RT_COND_SHIM *cond, ...)
        cond->x3_object = xnmalloc(sizeof(*RT_COND));
        return rt_cond_create(cond->x3_object, ...);

int rt_cond_delete_shim(RT_COND_SHIM *cond, ...)

> Happy holidays! Stay safe!



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