etags not working properly

[Luca Ferrari]

Hi all,
I'm trying to using etags against a quite larger project, the FreeBSD
source tree. What I've done is as follows:

cd /usr/src && find . -name '*.[ch]' -print0 | xargs -0 etags

After that, if I try to lookup (M-.) any tag, even the simpler and
common one, I got screwed: etags points me to another wrong location
or does not point me anywhere. For instance, "struct vmspace" is not
found, "struct vnode" points me to provnode, "struct rusage" points me
to a completely different #define, and so on.
Am I indexing the source tree in the wrong way?

I'm using emacs 24.3.1 with the associated etags.

Thanks,
Luca