a subtle(?) tar extraction permission problem

[Robert P. J. Day]

   say i have a directory structure a/b/c/{f1,f2,f3,...}.  for access 
reasons, i decide to change the permissions on the "c" directory, 
perhaps changing the owner/group, and definitely changing the perms to 
include "setgid".

   later, i get a tarball with contents a/b/c/{something}.  i found out 
that if i extract that tarball while root, and the effect is to add or 
delete files under the "c" directory, the permissions on "c" revert 
back to default values.  how annoying.

   apparently, as long as what i'm extracting is already in that 
directory (so that the directory entries themselves don't change), i'm 
safe.  but if the extraction changes the directory contents 
themselves, i get the owner/group/perms resetting on "c", which i'd
*really* like to avoid.

   i've perused the tar options, and i don't see anything that says, 
"don't mess with existing options on existing directories."  is there 
a standard approach to handle this?

rday