Neil,
The patch version 3 is actually bare minimum but it relay does the
trick. I think it is what I suppose to sent to you at first. Please
forgive my mistakes, but I'm just learning how to cooperate 
with the community.

And I don't like the introduction of a new header file just to store one or 2
definitions.  Just put new definitions in mdadm.h  Keep It Simple.

Sorry but I don't agree. At the moment the source code of mdadm/mdmon is
not simple. It's quite complicated. It is hard to analyze how the
control flows and there's a lot of data and code redundancy in it. In my
opinion "refactorization" is what this code needs. Splitting code into
files would be helpful, too.

I am perfectly happy with making 'buf' larger as appropriate, even making it
PATH_MAX in some cases.
I would be happy with more use of asprinf.
I would be happy adding checks before certain critical sprintf calls that
the result will not exceed the buffer.

I did not use asprintf function, because it's GNU specific. I had an
impression that one may try to build mdmdm using non-GNU toolchain.
---
Artur