valid_link - controls the use of link(3)

int valid_link( string from, string to );

The  driver calls valid_link(from, to) in the master object from inside
the link(from, to) efunction.  If  valid_link()  returns  0,  then  the
link()  will  fail.   If  valid_link() returns 1, then the link(3) will
succeed if rename() would succeed if called with the same arguments.

link(3)