access errno in Xeno 3.1

[C Smith via Xenomai]

I can compile a rtdm kernel module OK with Xenomai 3.1 but I can't
seem to get my module to see errno.

In several Xenomai example sources I see :
#include <errno.h>
but when I put that in my rtdm code I must also add to my Makefile:
-I/usr/src/linux/. Is that the right include path?

Even once I do all that, when I compile I get:
error: ‘errno’ undeclared (first use in this function)
          rtdm_printk("warning, munmap returned errno: %d\n", errno);

This is what I get when I do xeno-config:
/usr/xenomai/bin/xeno-config --skin=rtdm --cflags --ldflags
-I/usr/xenomai/include/cobalt -I/usr/xenomai/include -m32
-D_FILE_OFFSET_BITS=64 -D_GNU_SOURCE -D_REENTRANT
-fasynchronous-unwind-tables -D__COBA$
-Wl,--no-as-needed -Wl,@/usr/xenomai/lib/cobalt.wrappers
-Wl,@/usr/xenomai/lib/modechk.wrappers
/usr/xenomai/lib/xenomai/bootstrap.o -Wl,--wrap=$
(yes I built xenomai for 32 bit userspace apps)

What is the proper way to access errno?

thanks, -C Smith