The -u switch causes makemap to dump the contents (key and data pairs) of an existing database. The database must exist and be readable for it to be dumped. If you attempt to dump a database for which you lack read access, you will get this message:

% makemap -u hash /etc/mail/privatedata
makemap: error opening type hash map /etc/mail/privatedata: Permission denied

You also must specify a type for the map that matches the type specified when it was made. If you mismatch, you will get an error such as this:

% makemap -u btree /etc/mail/access
makemap: error creating type btree map access: Invalid argument

If all goes well, the contents of the database will be printed to your standard output. Each datum will be separated from its key by a single tab character. Note that the order in which they print will not necessarily match the order in which they appeared in the original source text file.